Bepercngee- Design, Testing and Installation o Supercharger Systems by Corky BellSUPERCHARGED! Eg Abbreakdown ofa Vortech system. See Chapter 2 Connecting rod inertial loads. See Chapter 3. ‘The Roots blower. See Chapter 5, The Centrifagal Super- charger. See Chapter 6. ‘The Screw Compressor Supercharger. See Chapter 7. Table of Contents 10 Introduction v The Purpose of Supercharging 1 The Distinction of the Supercharger # Drivabilty * Emissions # Economy * ‘Types of Superchargers + The Torque Curve + Internal Compression Ratio * Blowers and Compressors « The Logic of Supercharging * Selecting Supercharger Acquiring a Supercharged Vehicle 13 OEMSupercharged Automobile» Aftermarket Supercharger Ki Building Your Own Supercharger System The Physics of Producing Power 21 Inertial Loads © Power Loads» ‘The Power Equation * Equivalent Compresion ato « Ailow Rate » Presure Ratio « Densty Ratio» Volumetric Efiency Volumeisc Efficiency Ratio * Drve-Power Los « Estimating Power The Balance of Heat 37 ‘Knock + ‘Temperature Relationships * Heat Made by the Supercharger ¢ Heat Made by the Compression Ratio * Heat Made by the Supercharger and the Compression Ratio * Add the Intercooler The Roots Supercharger 45 ‘Aiermarke Systems » Racing * Construction + Advantages « Disadvanages * Bypas Valve» Selecting the Roots Supercharger » Calculating the Size * Drive Power Pulley Rao Bele Load «Idler Diameter Inercoling © Mounting the Roots The Centrifugal Supercharger 77 Construction + Advantages # Disadvantages * Antisurge Valve # Lubrication # Selecting the Centrifugal Supercharger * Calculating the Size + Drive Power * Pulley Ratio * Bele Load * Intercooling * Mounting the Ceatrfugal Supercharger The Screw Compressor Supercharger 87 CConstrution * Advantages + Disadvantages « Bypass Vibe * Inerooling # Tbriction + Miler Cycle Selecting the Twin-Serew Supercharger * Calculating the Size ® Dive Power « Ply Ratio © Belt Load Mounting the Twin Screw" Intercooling 105 Calculating the Need for an Intercooler # Choosing the Intercooler © Design ofthe Alr/Air Intercooler *Ineernal How Area «Heat Tanser ‘reds Turbutatoe# Sizing the Cores Frontal rea ® Core stcamning * Ducts * Core Thickness * Core Flow Direction » Internal Volume * End-Cap Design + Tube Sizes and Shapes * Hoses and Connections © Postoning the Intercooler * Mounting the Intercooler» Staggered-Core Imercooler* Multiple Intercooler « Design ofthe Airto-Water Intercooler 1 Charge’ Ar Heat Exchanger » Water Pumps * Coolant Reservoir ® Front Cooler © Other Intercooler Types « Intercooler Summary Intake Manifold 137 Inlet Shape « Injector Location * Throttle Bodies # Replacement Manifolds * Runner Length * Plenum Electronic Fuel Injection 145 Principles of EFI + Fuel Injectors and Pulse Duration + Sequential versus Non-Sequential Injection # Flow through the Injector * Modifying Stock EFI Systems + The Heat Benefit of Fuel * Calculating Injector Size + Testing Injectors + Fuel Pump Requirements * Fuel Pressure Regulator * Fuel Rail Size + Fuel Line Size * Fuel Pickup + Programmable Aftermarket Engine Management Systems * Tuning Events in the Chamber 173 ‘Voltage # Spark Plugs * Ignition Timing * Electronic Ignition Retard © Detonation * Knock Sensor * Fuelst t i { t I { 12 13 4 15 16 7 18 19 20 Secondary Components Inlet Air Pickup * Air Filter © Ram Air« Exhaust System * Basic Tube Size + Onygen Sensor Postion » Muller Styles, Sizes, and Number ® Materials dnd Finishes» Hangers» Fasteners nd Gaskets * Flanges Taipipe * Special Requirements for Front: Wheel-Drive Mounting the Supercharger The Large Roots + The Smaller Roots, Centrifugal, and Twin-Screw « Design Criteria » Choice of Materials» Lug Configuration * Fasteners © Corrosion Protection Drive Mechanisms Power Required to Drive the Supercharger # Belts # Drive Power * Duty * Types « Noise « Tension * Orbiting « Pulley Ratio « Pulley Specifications ® Palley Diameter « Pulley Temperature + Pulley Materials « Idler « Fastener Strength + Bending Moment » Pulley-to-Crankshaft Joint © Summary Preparing the Engine ‘Objecuives * Engine Condition + Compression Ratio Preparing the Cylinder Head # Preparing the Cylinder Block * Improving Head Clamp. Up + Pistons # Crankshafis and Bearings * Con Rods and Bolts © Balancing the Assembly « Camshafts « Valves © Related Components # Raiators and Oil Coolers Testing the System Equipment and Tools « Air Filter Conditions * Throttle Flow Loss # Compressor Conditions * Intercooler Conditions + Intake Manifold Pressure * Exhaust Back Pressure * Air/Fuel Ratio * Automotive Data Logger * Gauges The Outer Limits of Power ‘Top Fuel Car # Mustang « Developments in Supercharging * OEMs « Dealing with the Heat * Miller Cycle # Noise « Seeking More Performance ‘The Roots # The Centrifugal « The Twin-Screw * Durability * The Aftermarket # Veracity/Voracity Designing a Supercharger System Setting the Objective + Calculating the Size * Drive Power « Pulley Ratio # Belt Load * Measuring Power Increasing Fuel Flow « Calculating, Implementing the Design Supercharger Poston + Intake Manifold Inrrcoules Postion + Lubrication » Air Filter» Bypass Vale » Testing » The Details » Compressor Rests * Intercooler Results # Exhaust Back Presse © Installing a Supercharger Kit Choosing the System + The Vehicle and System * Preparation # Dissscmbly Preliminary ® Vacuum Hoses * Throttle Body Removal © Vacium Switched Valve « Cables * Connectors * Throxtle Body Diagnostic Plug » Vacuum Lines © Manifold Removal « Front Cover © Drive System « Supercharger and Manifold Insallaion © Brace * Bc Throttle Body and Air Tube installation » Cables » Diagnostic Plug » Adsl Camp © Vactuim Lines » Compleding the Lntllason * Testing Suppliers Glossary Index Art Credi Acknowledgements About the Author 181 189 201 219 241 255 265 281 299 315 319 324 336 338 339 Intake runner shape is, 1c Chapter 9 Supercharged fuel systems need a good, solid fuel pump. See Chapter 10. What can you really do? See Chapter 17. The finished system. See Chapter 19)Bene uber ion of Robert Beni, Cambie, MA 02198 USA Ieormatin that hos frowzotces/erreer4i7 ——thecierenca™ BentleyPublishers ‘Copies ofthis book may be purchased from selected booksellers or dretly from the publisher The publisher encourages comment from the reader of this book. These communications have been and willbe considered in the ton ofthis and other books Please write to Bentley Publishers at the prepa addres listed at the top ofthis page or ema us though our website, Library of Congress Cataloging-in-Publcation Data Bal, Corky, 1941- ‘Supercharged: design, testing, and installation of supercharger systems. pcm. Includes inde. ISBN 0-8376-0168-1 (pbk : alk paper) 1, Automobiles~Motors~Superchargers. I. Tite ‘TH214ss BA 2001, 629.2504-de21 2001037499 Bentley Stock No. GSUP 080705 109876543 “The paper used in ths publication is acid free and meets the requirements of the National Standard fr Information Seenees-Permanence of Pape for Printed Library Materials.) ‘Supercharged, by Corky Bll '© 2001 Corky Bel Robert Bentley, nc. Bently Publishers isa trademark of Roberty Bente, Ine. All ights reserved. Il information contained inthis book is based onthe information available tothe publisher atthe time of eitorial closing. The rights eserved to make changes at any time without notice, No part of this publication may be reprodced, stored in avetival tem, or transmitted jin any form or by any means, electronic, mechanical, photocopying. record: ing or otherwise without the prior writen consent of the publisher. Tis includes text, igure, and tbls Al rights eserved under Berne and Pan America Copyright conventions. Manfactuted in the United States of America Front cover: Background art Ethan Estes Inset photos (clockwise from top let: Randy Anderson, than Eates, Randy Anderoa, Kleeman USA Ine Diagrams: Corky Bell and Bentley Publishers Backcover: Left dlageam: Corky Bell and Bentley Publishes Right images rom top to bottom): Randy Anderson, (Corky Bell and Bentley Publishers, Magnusson Products, aton Corporation WARNING — Important Safety Notice “The information inthis book represents 2 general summary of engineering principles involved in supercharger system design tint constriction and ther application to vehicles, sing xan tn inractions which we Beleve to be accurate. Hower, the ‘amples instructions, and other information are intended solely ts ilostraions and should be wed in any particular application nly by experienced personnel who are trained in the repair and trodietion ofeny rable snd who hee idependeniy ela ted the repair, modifeaton or aceesiry. Implementation of 3 ‘mosificaton or atachment ofan acestory eseribed in this book ‘may render the whic, attachment, or accessory unsalt for use in Do not perform work deserbein this book ules you ae familiar vith atc automotive repair procedures and safe woekshop pe tes, This book ilusuates procedures required for some service Sand modiiition wor its nota subsite for fll and up-to-date {information fom the vehicle manufacturer or ermarket supplier, ‘or for proper taining as an automotive technician. Note hat i i not possible for to anticipate all othe ways or conditions under ‘which vehicles may be serviced or modified or to nrovde cautions {rtoall ofthe pouible hazards that may resale. The vehicle manu factrer and afermarker suppliers wil continue co issue sevice information updates and parts retrofits afer the editorial dosing ofthis ook. Some of hee updates and vewots wll apply to pro sures and specifications inthis book. We regret that We aint Supp updates to purchases of his book. Weave endeavored to ensre he acura ofthe information n his book. ese note, however, tat consdering the quant and the omplexiy ofthe information vod, wecannet wart the acc ‘yor completeness of the information conesned in this book [FoR THESE REASONS, NEITHER THE PUBLISHER NOK THE AUTHOR MAKES ANY WARRANTIES, EXPRESS ON IMPLIED, THAE {Tite HXAMLis, INSTRUCTIONS OR OTHER INFORMATION IN THIS "OOK Ak PREF OF ERRORS OR OMISSIONS, ARE CONSISTENT WITH INDUSTRY STANDARDS, OR THAT THEY WILL MEET THE REQUIKE- MINTS FOR A PARTICULAR APPLICATION, AND WE EXPRESSLY (OF FTINESS FOR A PARTICULAR PURPOSE, EVEN F THE PUBLISHER ‘Tun pontisoten AND AUTHOR ASO DISCLAIM ALL ABILITY FOR. DIRECT, INDIRECT, INCIDENTAL OR CONSEQUENTIAL DAMAGES. ‘TtaT RESULT FROM ANY USE OF THE EXAMPLES, INSTRUCTIONS. ‘OK OTHER INFORMATION IN THIS BOOK. INNO EVENT SHALL OUR Your common sese and good jgment ae crucial safe and sce astomorive work Read procedures through before star ing them. Think about how alr you ae fing and whether the ‘ondionofyour vehicle, yourlevl of mechanical illo your eel fof reading comprehension might rest in or contibue in some ‘ray toan exurrence which might cause you injury, damage your ‘hile, or retin an unsafe repair or modiicaon. Ir you have ‘oubs Tor these or other reasons abou yur ability to perform se ‘work on your vec hae the work dane aan authorized dale ‘or other gualied shop. “This book is only intended for persons who havea great del of perience in epuiring automolies and who are secking specific information about superchargers. Irs not for those who are look ing for gener information on astomotile rept. REPAIR AND "TAKEN WITH FULL KNOWLEDGE OF THE CONSEQUENCES. efor attempting any work on any vehicle read an warnings and ‘tone tht ae provided with your supercharger, and any wath lng or caution thit accompanies procedure or description in this took Review the warnings and sitions cach time you prepare to work on any vehicle,Superchargers run the whole gamut of automotive performance. Supercharged Buicks, Pontiacs, and Joauars provide entertain ing daily transportation, while supercharged drag racing Mustangs provide entertainment itself The great tradition of the neartiftyyearold small block Chevy as a hot rod ‘engine contrasts historically with the newest science of supercharging, the twin screw compressor. This interesting combination of science and tradition still provides a marvelous powerplant Introduction This book offers the performance enthusiast a practical guide to applying the supercharger to the modern automobile. It is not an engineering treatise on the mysteries of the supercharger. A preliminary discussion of theory provides a basic understanding of how the mechanism works and how it is integrated into the sys tem. Once the theory is grasped, the details of design, installation and operation come into perspective oS | er . RACEWAYvi supercharged! The overwhelming spec. tacle ofthe top classes of modern dragsters will toke your breath away. No other automotive event con 0 stun your senses with 1a demonstration of raw speed, noise, power, and controlled violence. Nowhere in this book will you find equations beyond those that can be solved witha fifteen-dollar calculator. The theory is equally simple. Its my sincere hope that the reader will take the time to understand the equations and their real impli- cations for the success of the supercharged automobile, There has never been a substitute for basic engineering in the pursuit of high performance. For a clear impression of what supercharger engineering can do when applied to a compe tition vehicle, I urge you to attend a Top Fuel drag racing event. Witnessing a modern dragster accelerating to 325 mph in 4.5 seconds will forever imprint on ‘your mind the extremes of controlled power and speed available only through an understanding of the engineering needed. Prior to modifying any component of an automobile, check local, state, and federal law regarding that modification’s legality. Any item sold that has received a California Air Resources Board exemption order number is street legal in all states.The Distinction of the Supercharger The Purpose of Supercharging Prior to World War II, Mercedes-Benz fielded some of the most technically interest- ing, complex, and successful supercharged Grand Prix cars ever raced. Just before that, Harry Miller was building the great supercharged Indy racers that were marvels of engineering for their time, and of enduring craftsmanship and beauty. During World War IL, the great and successful aircraft were supercharged. Indeed, the Battle of Britain might not have been won without the supercharged Spitfire fighter planes. Even the Bocing B-29 SuperFortress bomber was supercharged. ‘After the war, Alfa Romeo emerged with the Type 159 Grand Prix cars and swept all competitors aside, In the late fifties, British Racing Motors built fascinating 90 cid V-16 racer, the BRM, that produced over 600 bhp. Unfortunately, a rule change climinated it from competition. The legendary howling Novi supercharged V-8s, began to appear at Indy and are revered to this day. In the eighties, supercharged Grand Prix ears were once again within the rules. Usually sporting turbis superchargers, these 90 cid vehicles achieved power outputs of 1300+ bhp in their qualifying trims. Never before or since have men been asked to drive such powerful road-racing cars, Imagine, fyou can, a sixteen-hundred-pound road racing machine \with 1300 horsepower! This would be the rough equivalent of placing 3100 bhp in, the late, great Dale Farnhardt’s hands for a Winston Cup stock car race. ‘One day, a whole new generation of “clean-sheet-of paper” engine designs will integrate the supercharger from the outset. Mazda has shown the way with the highly innovative Millenia 2.5-liter V-6 sedan. This V-6 engine /supercharger design is the world’s frst try at an automobile with a Miller cycle engine. (The Miller cycle is discussed in Chapter 7.) [Any device that packs more air molecules into an engine than the engine can breathe for itself whether it’s a compressed air bottle, an army of servants with palm fronds, or a ‘mineshaft air mover—is a supercharger. Technically, this includes turbine-driven super chargers (powered by the vehicl’s exhaust gas), but these are now invariably referred. toasturbochargers. Turbochargersare covered in the present author's Maximum Boost and are not discussed in this book, whichis limited to belt- driven superchargers ‘The choice between supercharging and turbocharging depends on the objec tives and available equipment. Available space is a factor, as are the simplicity of installation and lower weight of existing supercharger kits. Cost is not generally a factor, as the cost of supercharger and turbocharger kts is approximately the same, Superchargers also eliminate the need to deal with the exhaust-gas interruption created by inserting a turbine into the exhaust flow.2 supercharged! Driveability Fig, 1-1: The classic ‘American Hot Red almost hhas to have a Chevy engine and a Roots blower. ‘Turbos are generally quieter, more economical to operate (powered only under boost, they impose less parasitic drag), and produce more power, but centrifugal superchargers do a good job of high-end street performance and stock-car drag, racing, The centrifugal supercharger, with its ability to blow into the throttle, offers the greatest simplicity of al. Fixed-displacement superchargers maximize low-speed boost, and the American hot rodder has long favored the large-displacement Roots blower. Maintaining that tradition has a strong appeal that beckons many builders. Potential supercharger buyers are often concerned about three issues: drive- ability, emissions, and economy. The flawless, smooth driveability of the modern automobile has spoiled us all ‘Today, few are willing to buy megapower at the expense of ease of driving. Nor do we have to, The supercharged engine works perfectly well with what we ‘would call an “economy,” or stock, camshaft. Low-overlap, short-duration, low- lift cams all work well with superchargers. The magic of electronic fue! injection permits programming for maximum-eeonomy cruise and maximum power in the same fuel curve. The sweet eagerness of high-compression engines is completelyEconomy Types of Superchargers chapter 1: the purpose of supercharging 3. compatible with supercharging, provided an intercooler is part of the package. In other words, no contradiction exists between perfect driveability and serious supercharged horsepower. ‘The days of the raucous, lumpy-idling, monster-motored supercar started downhill when environmentalists started pressuring bureaucrats 10 do something about automotive emissions. Automakers, forced by federal law, reduced emissions of the modern automobile to less than that of a 2-horsepower lawnmower. This allows us tostill enjoy the same performance as a raucous, lumpy-idling, monster-morored supercar with the civility of low emissions. ‘The monster motor was given another boot with the “oil crisis” of the early seventies. Real or not, the result was huge federal emphasis on miles per gallon. Corporate Average Fuel Economy, CAFE, laws were a direct result of the oil scares, Look back at your 1966 GTO. Its performance can now be matched by a much smaller supercharged engine that, when not under boost, delivers considerably ‘more fuel economy. Today, you can go faster in a supercharged Mazda Miata and get four times the fuel mileage when not under boost: ‘The above three issues are also reasons to use a supercharger or turbo rather than hop up a normally aspirated engine. Hundreds of mechanisms can move air, and virtually any of them can be made into some form of supercharger. Its not within the scope of this book to discuss every possible mechanism, but only those that have proven their worth with regard to per formance and durability and that are available today. Three types of air movers have ‘emerged as viable: the ever-present old Roots, the centrifugal, and the serew compres- sor, commonly known as the twin-screw. Each is discussed in a separate chapter. ‘All superchargers fall into the category of either fixed-displacement or non- fixed-displacement types. Imagine the fixed-displacement compressor as similar toa piston and cylinder, where the air volume pumped per ¢ the cylinder. Likewise, imagine the non-fixed-displacement compressor as a palm frond that pushes an amount of air along, but with vague boundaries and unde- fined amounts of air that slip aside the frond and go nowhere: Fixed-displacement Fixed-displacement superchargers (like the Roots and twin-screw) pump a specific volume of air per revolution and do not permit reverse flow. Air comes in the inlet por, then that port closes. The air is moved to the outlet por, that port opens, and the ar discharges. Atno time, and regardless of pressure, can air go backward through the supercharger, except, of course, through the clearances between components. Non-fixed-displacement Non-fixed-displacement types (like the centrifugal) push an unspecified amount of air along, much like a fan blade, so air can flow backward if conditions warrant. Nothing closes to block the flow. Only air being pushed by blades, much like a propeller, influences it to travel toward the engine. If boost at the compressor ‘outlet suddenly gets too high for the supporting air low to maintain, as when the throttle (downstream of the supercharger) is slammed shut at high supercharger speed, some of the air can reverse directions and go right back out through the ‘compressor, just as air could flow backward through a propeller ifa great enough force were pushing it4 supercharged! Fig. 1-2: The centrifugal blower enjoys the widest acceptance in the aftermar. ket supercharger industry. Its inherent simplicity, ease of installation, and thermal efficiency will keep it in the forefront of the enthusiast ‘marke! for years to come, The Torque Curve Internal Compression Ratio Whether the supercharger is fixed-displacement or non-fixed has implications for the speed range in which it is used. The fixed-displacement Roots finds its best use at low and midrange rpm. The centrifugal offers its greatest benefit in the higher rev ranges, with littl in the low and middle ranges. The twin-screw can prove a strong benefit to both low: and high-speed torque. Superchargers are also divided into those that do not have an internal compression ratio (like the Roots) and those that do (like the centrifugal and the twin-serew). Without an internal compression ratio, as in the Roots, no change occurs in the volume or pressure of the ar a it passes through the blower. Ifthe blower inhales, 1 cubic inch at atmospheric pressure, it delivers that cubic inch to the discharge port without changing its pressure or volume. Only when the air is forced into the chamber of an intake manifold will it build up pressure. Imagine the situation as similar to an overloaded junk closet. Throw in one more piece of junk, slam the door on it, and it becomes smashed in tight with all, the rest OF he junk. T's compressed int Use Uoset by slamuiang, ke door. A supercharger with an internal compression ratio (twin-screw or centriftgal) is ne in which both volume and pressure vary asthe air passes through. As the bit of air moves through, itis continually forced into a smaller volume, which also increas es the pressure. Upon reaching the discharge port, it’s already compressed. Look at the stuffed junk closet again. Each piece of air out of the internal-com- pression-ratio blower is already wadded up tight and, when released into the closet, is not subject to further crowding (compression). The major advantage to this, as, compared to superchargers without an internal compression ratio i that it permits «greater thermal efficiency, producing ess heat, as described in the next section, In fixed. displacement types with an internal compression ratio (twin-screw), compression is accomplished by the meshing of the rotors. For non-fixed-is placement types with an internal compression ratio (centrifugal), compression is accomplished by the internal shape of the collector/discharge housing, where the air passes through a passage similar to a venturi, It experiences compression entering the narrower passage, then expansion as it moves past the restriction and, out the nozzle.Blowers and Compressors Fig. 1-3: The twin-screw compressor, while appear- ing 10 be a simple machine, is in fact, a complex ther- ‘modynamic problem as well as @ manufacturing headache. However, the effort to make it a viable supercharger has been well worth the trouble. chapter 1: the purpose of supercharging 5 ‘Table 1-1: Comparison of supercharger types ‘Type iued displacement _Internal compression ratio Roos Yes No Centrifugal No Yes Screw Yes Xs Often someone tries to establish that the categories of superchargers are “blowers and compressors.” Although partly true, the historical goof-up lies in the fable that blowers just move air with no heating effect and that compressors compress the air, thus heating it. Neither is true. The next time someone tells you he has 4 blower on his car and that it doesn’t make any heat, sympathize with him for having no boost either. (In this context, a blower is a supercharger that does not have an internal compression ratio, However, the terms “blower,” “compressor,” and “supercharger” are used interchangeably in this book.) ‘Whatever the mechanism, ifit pushes air hard enough against some restriction {intake valve action) so that pressure rises above atmospheric, the air gets hotter, in accordance with thermodynamic law. Traditionally, heat is supposedly made bya compressor (centrifugal or twin-screw) and not by a blower (Roots), but the real situation is almost the reverse. The compressor, due to its internal compres- sion ratio, enjoys an overall thermal efficiency advantage—that i, it produces less heat when it compresses air than a blower does. To understand wiy; lets look at lone discharge event from the rotors into the manifold or intercooler tube when operating under boost. ‘The imaginary wall of discharge, or “port,” at the exit of a Roots is always at a higher pressure in the manifold than in the compressor, even under vacuum,supercharged! The Logic of Supercharging This is sort of like making boost, but the boost would be from, say, 20 inches of vacuum in the throttle area and blower to 15 inches in the manifold. Recall from above that the Roots does not compressairin the blower—air remains at the pressure (or vacuum) aft of the throttle body, and compression occurs only when airis packed into the manifold. So we are attempting to take lower-pressure sir aund force i inty the higher-pressuse manifold. When the discharge pore ope! and the pumped air is suddenly faced with entering this higher-pressure space, a portion of the heated compressed air in the manifold will disperse itself backward along the rotor faces and into the blower cavities. This carries some of the heat in the compressed air in the manifold back into the blower, later to be partially shoved back into the manifold and partially compressed yet again, to an even higher temperature. In the twin-screw or centrifugal superchargers, which do compress air as it passes through them, the packet of air can be compressed to a higher pressure than that in the manifold. When the port opens, the compressed charge leaps out, propelled from the high pressure of the compressor into the lower pressure of the manifold, ike a bullet down a rifle bore. Once past the exit wall of the super- charger, the air packet experiences less intake manifold pressure, thus no opposing force to send it back where it came from. If the supercharger is forced to produce manifold pressure greater than its internal compression ratio, the air packets begin to behave in a manner similar to those in the Roots, but to a lesser extent, because the pressure difference between the manifold and the packet is less. More powerful engines can be created in two basic ways: we can burn more fue! per time, which is comparatively easy, or we can make more efficient use of the fuel being burned, which is difficult. To burn more fuel per time, we can increase the volume (by enlarging the cylinder bore and/or the crankshaft stroke), number of putts (rpm’s), or volumetric efficiency (breathing capacity). ‘These factors are given in Chapter 32s an equation, PLAN, and are discussed in more general terms below. Making more efficient use ofthe fuel being burned would inerease the aver- age downward combustion pressure on the piston (BMEP). This would require improving the engine’s thermal efficiency, to reduce energy from combustion lost as heat, or its mechanical efficiency, to reduce energy from friction lost as heat. ‘Burning mere fuel per time First, big engines make more power than small ones, all other things remaining, ‘equal. If we have a 350 Chevy, its reasonable to assume it would be about twice as powerful if we made it into a 700 Chevy (that is, doubling its displacement or producing a V-16 engine of the same bore and stroke). You can only guess at the enormous cost and time needed to do that. And then the slug would weigh near twice as much. Second, we can run the engine twice as fast (number of putts per minute). In 1995, the dawning of the 18,000 rpm Formula 1 Grand Prix engine took place. It is, amazingly enough, possible to turn the 3-liter, four-stroke-cycle engine that fast: 150 power cycles per cylinder per second. So it is possible to turn that 350 twice as fast, but seriously difficult ‘Third, we can fool Mother Nature. We can beat 100% volumetric efficiency and pack the combustion chamber with more molecules than the engine can breathe ‘on its own. We can pack in three times the number of molecules and make threeSelecting a Supercharger chopler 1: the purpose of supercharging 7 times the power. Is it expensive? No, not very. Is it heavy? No, not at all, Does it require engineering wizardry? No, no! Isit really doable? Yes! Now we are talking. superchargers! ‘Making more efficient use of the fuel being burned With approximately 30% of the energy in gasoline going straight out the tailpipe as lost heat and another 30% winding up in the cooling system, one might think ‘we've got huge dividends to reap. Not so: some miracle of technology will have ‘to come along to substantially improve the efficiency with which a modern engine uses fuel High-performance engines are typically prepared to reduce losses to friction horsepower. This involves such considerations as improving the surface finish on the crankshaft, bearings, and cylinder walls increasing clearances between the bearing sleeves and the shaft to reduce oil shear; low-tension piston rings and one ring instead of two; a chain versus a belt or gears (the most complex and expensive) to drive the camshaft; cam profile that require less spring tension; eliminating the rocker arms; and low-friction valve seals Still, only small gains are made. Each of the three supercharger types discussed in this book enjoys its own particular areas of merit. Selecting one most suited to the user’s needs involves weighing these ‘merits and determining the best combination thereof. Table 1-2 offers some guid- ance. Clearly, ny supercharger application may have a single objective that far out ‘weighs all other considerations. IfSo, the supercharger that best meets that objective is the logical choice. Otherwise, look for the best combination of merits. ‘Table 1-2: Comparison of the Roots, centrifugal and tsvin-srew characteristics a Low-speed boost capability High boost-pressure capability (10+ psi) Fast boost nse Thermal efficiency Volumetric effiieney Ease of intercooling Power loss through supercharger Simplicity of installation Noise Vibration Space requirements Installation convenience Cost B a cl B © © B fc B © © B bw ee oroh ee ee A= Excellent; B = Good; C = Fair; D = Poor8 supercharged! Fig. 1-4: A 535hp 460 motor from a Ford powered motor home with o Kenne Boll Supercharger. Objectives Top Fuel dragsters and record-setting grocery getters do not have much in com. mon. The successful installation must take into account the details that permit an engine to run well within the specified objectives, The size and type of supercharger selected for a given application will strongly influence the degree of success enjoyed by the system. It is not at all a case of only one supercharger working in a specific situation; rather, just one will work best, although several could work well. The trade-offs of boost threshold, he: low-speed torque, and power are the variables in matching the supercharger to the requirements. To optimize the trade-offs, the requirements must be defined first. These requirements can be spelled out by listing the performance objectives for the particular vehicle Power, top speed, and acceleration are measures of performance. Objectives can vary for day-to-day commuter cars, Bonneville maximum- speed cars, drag cars, super-performance street cars, roadracing cars, and even for the outer fringe of Vehicles called pickup trucks. Specific performance objectives will be items such as desired boost threshold, torque peak, and estimated power output. Higher speed vehicles require larger superchargers, street cars respond well to midra smaller superchargers. torque, and low-speed vehicles Compressor A compressor has a particular combination of airflow and boost pressure at which it is most efficient. The trick in choosing optimum compressor size lies in post tioning the point of maximum efficiency at the most useful part of the rev range. 1 part of the rev range is where some judgment needs to Choosing the most use! be exercised. When efficiency drops off, heat produced by the compressor gocschopler 1: the purpose of supercharging up. Ifa compressor were sized such that maximum efficiency occurred at one-third of the rev range, efficiency at or near the redline would taper off to where the charge temperature would be scorching hot, At the other extreme, if maximum efficiency were at the redline, midrange temperatures could get out of hand. This particular size would then be useful only for running flat out at that rpm;¢.g., the Bonneville car. Somewhere in the midalle ofthe useful rev range of the engin lics the best place to locate the maximum efficiency point. Larger or smaller compressors do not have a huge effect on boost threshold. Boost threshold is mostly a function of the supercharger’s speed. Selecting the correct compressor size is not a black art, Rather, a few simple calculations based on the power of the existing engine and the desired power, coupled with one fundamental guideline, and the choice of blower sizes becomes apparent. The guideline is that when faced with two sizes that appear to meet the objectives, choose the larger. Because it will meet the objectives at lower rpm, i will generate lower loads, noise will be less, idler pulleys will turn more slowly, greater future expansion is possible, and usually, but not always, thermal efficiency will be higher. ‘The calculations are basically a comparison of desired power to existing power. If we assume the system will be intercooled, which it generally should be for boost greater than 5-6 psi, the ratio of the two power figures yields a number almost the same as the required boost pressure ratio. Convert the pressure ratio into an airflow number, as shown in later chapters, then set out to find a blower that will produce that much flow at no higher than maximum continuous speed. For Roots and twin-screws, flow is the displacement per revolution multiplied by «pm and volumetric efficiency, For centrifugal compressors, a flow map will -al flow capability. The procedure for calculations is given in the specific chap ters for the different types of superchargers. Regardless of the type of blower, keep the size selected at least 10-15% larger than just meeting the objectives. Boost Rating Don’t automatically take a kit's rated boost figure at face value, This figure is the foundation upon which all of your calculations and expectations rests, s0 it pays to make sure that the criteria by which the rated boost was calculated by the ‘manufacturer equates to the same criteria under your hood. Things to consider are: whether pump-grade gasoline was used, whether octane boosters were uscd, whether detonation was present, the speci package equate to the test unit? Call the manufacturer to get accurate answers to these questions. intake air temperature, and does the Budget Cost out the components and labor to meet the objectives. ‘Ulimately, the value of the equipment selected will not lie just with power, knowledge, thermodynamic factors, or cost. Rather, it will be determined by the way this baby behaves on the road. Is it actually fast, and does it fel fast? Does it fel responsive and eager to run? Is it erisp and sharp? Does it pull smoothly with ease and grace to the redline? Does it make you smile when no one is around to see?10 supercharged! Fig. 1-5: What we have versus what we want. The torque curve we really want Torque (ib) “TWpical torque curve 0100020008000 4060 00060007000 RPM True, this is something you won’t know until the installation is complete. Suppose the answer is “No.” Have you spent al tha time and money for nothing? Hardly. Ie’s seldom the ease that the whole installation is bad and just isn't going to work. The most frequent problems are that the fuel system can’t do the job ‘or that the blower, intercooler, or pulley isthe wrong size. But ifyou follow the guidelines in this book, your chances of getting it right the first time are distincly improved. Establish your objectives, learn the process required to meet those objectives, and keep an element of conservatism in the boost pressure. Try to avoid learning, the lessons of budget, knowledge, and effort the hard way: by a smoking trail of broken parts and wasted dollars.chapter 1: the purpose of supercharging 11 And Furthermore .... How much power can Iexpect from a supercharged engine? The current fuel octane ratings of 91/93 still offer us a potentially large power increase with supercharging. While absolute boost pres- sure is important to power, itis not the sole measure. Bragging rights are not established by boost gauge readings; quarter-mile times and ‘dynamometer numbers are where the real story is. The power one can ‘expect will depend almost completely on how well that boost is used. A low-compression, 20 psi setup with no intercooling and lots of ignition retard won't come close to keeping up with a 10 psi system that’s well tuned and uses all the right stuff. Estimating power output is chancy. For valid numbers, we must resort £0 equations, which appear later in the book.OEM-Supercharged Automobile Acquiring a Supercharged Vehicle ‘The essence of this book is to provide the performance car enthusiast interested in supercharging with a body of information that can be used to evaluate system designs, whether of a factory supercharger system or an aftermarket kit. This book is also intended as a design guide for the hobbyist who wants to build his own supercharger system. Three viable methods exist to acquire a supercharged vehicle: * buy an OEM-supercharged automobile * buy an aftermarket kit, ifavailable, for your specific application ebuild your awn supercharger eystem ‘The rationale behind the decision that suits your needs and requirements best is no more than a logical summary of the following: # What is the intended use of the vehicle? *# Whats the legality with respect to state of the car? * How much power is required? 4 Is fear ofa failure such that a factory warranty is required? # Can you make a reasonable judgment with respect to the engineer ing of an aftermarket kit? * Do you have the skill, time, patience, and equipment to build your own? nd federal law and the year The serious driver of supercharged machinery, of_ess than six-digit means, has not bbeen catered to by the OEMs of today: While performance abounds in automobiles from Camaros and BMWs to Volvos and Vipers, only Mercedes sees fit to produce an affordable supercharged sports car. One can purchase Buicks and Pontiacs and Millenias sporting blowers, but these vehicles do not meet the author's idea of exhilarating motoring. True, there are blown V-8 Aston Martins and Jaguars, but these are largely unattainable vehicles. Therefore, although they are technically interesting, their presence in the motoring world is of no value to us middle-class leadfoots. Unless your tastes lc along the lines ofa mildly rapid family hack, enjoy ing the fun of a supercharged automobile will tend to push you in directions other than the offerings of the OEMs. Ifyou buy a supercharged OEM vehicle and want to go fast, the ist step in pursuing more performance is a complete analysis of the system design. Chapter 1314 supercharged! Fig. 2-1: For decades, the classic blown sirest rod has enjoyed the combo of a B&M Roots blower, Holley carbs, and the smallblock Chovy. Fig. 2-2: The Eaton super- charger integrated into 0 Chevy smallblock intake ‘manifold by Magnuson shows the simplicity of the basic hardware—augment- cd by the selflubricating Eaton blower. 16, Testing the System, is your starting point. With those data accumulated and analyzed and the weak links identified, you can set out to find the necessary com ponents to improve the system. Keep in mind that the issue here is to improve the system’s efficiency, thereby opening up the potential for huge gains in power. Increasing boost pressure is aso a consideration, but without improvements to the systems efficiency, this path to power is fraught with serious mechanical risk. ‘Once the system has been tested and the merit of each feature has been determined, start the improvement process with the weakest link. Here is where foresight becomes important. For example, an intercooler that loses only 2 psi at the factory-rated boost can be judged okay. It is okay, but only for the factory-rated boost. Likely it will lose 3 or 4 psi at any significantly increased airflow. That kind of loss is not acceptable.Fig. 2-3: This Vortech sys fem for the 2valve 96-97 ‘Mustang illustrates the vari: ety of components needed 10 call a kit “complete.” Aftermarket Supercharger Kit Fig. 2-4: The early produc tion Rotrex blowers were specifically for smalls placement engines, lke this fourcylinder, 2.Qliter VW. chapter 2: acquiring a supercharged vehicle 15 Here’s where the action is taking place. Those Camaros, Vipers, BMWs, ‘Mustangs and even little Mazda Miatas have a plethora of kits and components available for mild to wild performance. The ten- and eleven-second street car can be built today from components engineered in the aftermarket. ‘The purchase of an aftermarket supercharger system is an ideal occasion to employ this hook as the guide itis intended to be, An investigation is necessary to determine the system that will meet your needs. Determine your objectives, then decide what type of supercharger most closely meets those needs. Before a reasonable decision can be made, answers to a variety of questions must be both sought and understood. The following samples will get you on the right track Does the system provide a correct air/fuel ratio at all operational conditions? ‘The air/fuel ratio is a basic building block of a supercharger system, It needs to be maintained over the boost range that the manufacturer claims for the kit. It is not to be expected that the air/fuel ratio will stay correct if the system’s design limits are exceeded. In all ci it is necessary to avoid discussing “fuel enrichment.” Either an air/fuel ratio is correct or it isn’t—no “enrichment” required16 supercharged! Fig. 2-5: The Kenne Bell shops produced this very (OfMppearing Ford F150 bwinscrew assembly Does the system provide a margin of safety on detonation? ‘The attempt here is to determine whether the system installed and operated per instructions will yield useful boost and not be subject to detonation problems. Does the system provide the necessary thermal controls to operate atthe stated boost pressures? Ask for a description and explanation of these controls, What efforts are extended toward quality control? Fitand finish are obvious. Material selections, methods of welding, surface finishes, and other fabrication procedures should also be checked out. Do the components carry a reasonable warranty? Although warranties on performance-oriented components are fre quently subject to severe limitations, the buyer cannot be hung out to dry. It is useful to discuss with the kit maker the warranty limitations and procedures necessary to establish the best warranty terms. Are proper instructions offered with the system? Instructions should provide all the necessary information to install, check out, and subsequently operate and service the supercharged vehicle. Will consulting be provided after the sale? This is where the maturity of a supercharger system manufacturer will truly show. If the system is to be used on a public highway, is it designed with all emis- sions-related equipment in proper order, and/or is the system on EPA- or CARB exemption-order status? In all states, the emission question will be the most important one. ‘When the answers to the above questions are satisfactory, it is time to get down to the fun details, such as compressor efficiency with respect to the system flow rates and boost pressures, aFig. 2-6: Centrifugal blow. rs con easily be adapted to older, carbureled ‘engines. This ATI system is intended for blow-through curb applications, ing Your Own Any reasonably able & chapter 2: acquiring a supercharged vehicle 17 Al kit makers will try to represent their systems as the most powerful. The accuracy ofthese claims is efferent story. The advent of the chassis dyno made it look, fora short period, as though manufacturers would publish legitimate power curves. Unfortunately, in some eases, these have evolved into just another way of stretching a point. For example, a dyno graph may show a blower run at 15 psi ‘when the kit is sold to run at 6 psi. And there are many more clever tricks than just running the wrong boost pressure ricator should have no serious difficulty designing and Supercharger System building his own superchargersystem. Forethought, planning, calculating, sketch- Fig. 2-7: This compact design integrates the super charger info the intoke manifold for the sixcylinder eep 4.0, ing, and measuring, all done in considerable detail, will be the keys to the success of the project. Perhaps the single greatest problem facing the do-it-yourselferi avoiding get ting stuck. Getting stuck is the phenomenon of “You can’t get there from here.” For example, you can’t ever hope for an emissions-clean 10-second street car if ‘you build a draw-through carb type. Trying to adapt a used blower from a 3.8- liter Buick to a 454 big-block will decidedly put you in a position where you are stuck. Avoid going down these paths leading to “stuck.” "The first requirement is wo deteriine the power level desired. Translate that necessary to get the job done. That, in itself, will igure into a boost pr determine the equipment needed. The remainder of the project is the sum of the ce contained in this book.18 supercharged! And Furthermore . . . Fig. 2-8: A Dodge V-10 kit from Carroll Supercharging What isa reasonable price to pay for a supercharger system? ‘The lowest-priced system that offers: + a correctly sized supercharger * a correct air/fuel ratio under boost + proper ignition timing + proper thermal controls ‘+ a margin of safety on detonation * quality components ‘Such a system can put together a good argument for being the best value It is popular to believe that you get what you pay for, but there are super: charger kits costing nearly $7,000 that do not have a correct air/fuel ratio. Conversely, well-designed kits are available for less than $2500. A reasonable price? This must remain the prospective buyer's decision, based on a thorough knowledge of what he gets for his money. What paperwork should be included with a supercharger kit? Instructions and warranty are self-explanatory. Cautions and operating procedures must be well detailed and conservative, What are the warranty implications of installing a supercharger in a new automobile? All factory warranty on drivetrain components will he voiced “There are, however, several circumstances to consider. You can purchase an after market warranty to cover your vehicle for all non-supercharger-induced or -related problems, It is currently in vogue to sell these policies with supercharger systems under the intended misconception that your drive: train is warranted against “supercharger-induced” failures. Not so. fone breaks his supercharged engine, itis not going to be paid for by anyone’s warranty—exactly the same situation as waiting until the fac tory warranty expires and then adding the supercharger. Which means that waiting out the factory warranty before installing a supercharger accomplishes nothing, except insuring thar the mechanism is one-third used up pre-supercharger. Furthermore, it eliminates the fan of ever ‘owning a nice new automobile with enhanced power.chapter 2: acquiring a supercharged vehicle Ie is rare for a modern automobile to have an engine/drivetrain prob- lem within the warranty duration. Those problems that do appear are generally minor and will likely cost under a hundred dollars to repair. To preserve the warranty for many thousands of miles to avoid a pos: sible hundred-dollar component failure rather than enjoying the extra performance seems to me the poorer choice. 10 assuage your concerns, call the car maker’s regional office and discuss with a service rep the areas of the drivetrain that have been a warranty problem. Will Ibave to buy anything else to supplement the supercharger? Wow, what a loaded question. What this usually boils down to is the difference between a system that’s “complete” and one that’s “com: prehensive.” In the world of supercharger kits, “complete” means that all the parts arrive in the box, so you don’t have to make any trips to the hardware store or call the maker about missing pieces. You'd be surprised how often this happens. Whether the kit was designed with the components to do the job is another story. “Comprehensive” means that the kit includes a fuel sys- tem, detonation controls, and boost gauge. If a kit advertised as “complete” arrives minus a few pieces, this is a ‘tragedy that can be set right in afew days at most. But ifthe kits poorly designed, having all the pieces and more is no consolation. 19Inertial Loads Fig. 3-1: The relationship of engine loads to engine components has three sig- nificant piston/crankshoft positions. The Physics of Producing Power ngines must endure two types of loads with distinctly different effects: inertial loads and power loads. Inertial loads can be tensile (produced by pulling) or compressive (produced by pushing). Power loads can only be compressive. They must be understood both individually and in their interaction for a clear view of why the supercharger does not immediately send the crank south, An inertial load results from an object’s resistance to motion. To examine inertial loads in an engine, itis convenient to divide the cylinder assembly into an upper halfand a lower half, Imagine the two halves separated by an imaginary line called the center stroke, ‘The piston always accelerates toward the center stroke, even when traveling away from the center stroke. In other words, when the piston is above the center stroke, it will always be accelerating downward, When itis below the center stroke, even at bottom dead center, it will be accelerating upward. Acceleration is greatest at top dead center and bottom dead center, when the piston is actually siting still When acceleration is greatest, the loads will be highes zero and velocity is greatest as the piston passes the center stro} imilarly, acceleration is Top dead center Center stroke Bottom dead center ‘Top dead center Full stoke = “The size of the loads generated by these motions is proportional to the 1pm of the engine squared. For example, if engine speed is increased threefold, the inertial load will be nine times as great. The action of the piston’s being pulled 2122 supercharged! Fig. 3-2: Connectingrod inertial loads. inertial loads applied to the connecting rod are closely approximat ed by the sine wave curve of load versus crank angle. Power Loads (Forced to accelerate) to a stop at top dead center and then pulled down the bore toward the center stroke will put a tensile inertial load into the con-rod/piston assembly. Similarly, as the piston is pushed to a stop at bottom dead center and then pushed back up the bore toward the center stroke, the inertial load will be compressive. Thus, any time the piston is above the center stroke the inertial load will be tensile, and! below dhe center stroke, it will be compressive, Top dead Top dead center contr ‘The largest tensile load induced into a con rod is at top dead center on the ‘exhaust stroke (because at top dead center on the compression stroke, the gas is already burning and creating combustion pressure to oppose the inertial load). ‘The largest compressive load is generally at bottom dead center after either the intake or power stroke, ‘These inertial loads are huge. A large-displacement engine running 7,000 1pm can develop con-rod inertial loads greater than 4,000 pounds. (That's like a Cadillac sitting on your rod bearing.) ‘A power load results from the pressure of the burning gases applied to the pis- ton. An example would be the compressive load put into a connecting rod as the burning gases force the piston down the bore of the cylinder. Pressure created by the expansion of the burning gases applies a force to the top of the piston equal to the area of the bore times the chamber pressure. For example, a cylinder with «bore area of 10 square inches (3.569-inch bore) with 800 psi of pressure would, be subjected to a compressive power load of 8,000 pounds.Fig. 3-3: Burning gos induces a compressive load in the connecting rod. Fig. 3-4: Combined power and inertial loads Note thot power and iner- tial loads generally subiract from one another. chopter 3: the physics of producing power 23 The peculiar relationship of the inertial and power loads is of most interest in the upper half of the power stroke, Here we have the odd circumstance that the two loads acting on the con rod are doing so in different directions. Remember that an inertial load is tensile above the center stroke, while a power load is com- pressive in all cases. Power load peaks at the torque peak and fades alittle as rpm increases but is generally yreater thant the inertial load. The difference between these two loads is the real load in the con rod (Fig. 3-3) Burning gas pressure (">") ll “Top dead center Top dead conter 7 Tensile loads lL ~ Connacting rod eral load wo| ery sax | |e caus Compressive coon we Power load Strom combustion pressure Bottom dead center24 supercharged! Fig. 3-5: Torque input into the crankshaft versus crank angle at approximately two atmospheres of boost pressure, Note that for the supercharged engine, maxi mum pressure occurs ot ‘about 20° ATDC, yet only ‘about 20% of the mixture will have burned. Even with high boost pressures, the ‘small amount burned will not result in large maximum pressure changes. As the burn nears completion, the greater mixture density can double the pressure at crank angles near 90°, auch that torque input fo the crank at that position can be twice as great. Clearly, the inertial load offsets some of the power load. It is further apparent, as indicated above, that on the exhaust stroke, when the con-rod/piston reaches, top dead center and is unopposed by combustion pressure (Iecause both valves, are open), the highest tensile load is reached. This load is the most damaging of all, because tensile loads induce fatigue failure, whereas compressive loads do not. For this reason, when a designer sits down to do the stress analysis on the con rod and con-rod bolts, the top dead center and bottom dead center inertial loads are Virtually the only ones he is interested in knowing. ‘The thought of doubling an engine’s torque (doubling the power at the same rpm) easily gives one the idea that the power load will double. Thank goodness this isnot true. To show how power can double without the combustion chamber pressures doubling is much easier done graphically. Any significant design load ‘changes would be based on peak pressure in the chambers, and it can be seen in Fig. 3-5that with twice the mixture in the chamber, peak pressure is up only about 20%, There are two reasons for this disparity. First, power isa function of the average pressure over the entire stroke of the piston, not just peak pressure. The average pressure can be dramatically increased due to the much higher relative pressures near the middle or end of the stroke, while the peak does not gain significantly Second, peak pressure is generally reached after only 18-20% of the mixture has burned. Ifthe mixture quantity is doubled, 18-20% oft, too, will have burned by the time peak pressure is reached, Since the total chamber pressure consists of the compression pressure plus the burning gas pressure, itis impossible to double the total pressure by doubling only one ofits constituents. 3 8 8 Cylinder pressure (psi e 0 135 jl, ©\ oc f of ‘Ato ‘Supercharged Pressure at 90°chopler 3: the physics of producing power 25 As Fig. 3-5 shows, chamber pressure at crank angles nearing 90° is over twice as gxcat when operating under boost. Thisis the point at which we get the huge torque increases into the erank that create nearly twice the power. Note also that when this hhuge increase occurs, the supercharged burning gas pressure is less than the atmo {8 pressure was tits peak. Therefore, it does not create a damaging load. If physics pe looks at the graph, he will tell you that the area under the respective curves represents power. Thus, the difference in the two areas represents, power gain due to the supercharger. It certainly is a neat deal that we can double the power but not the load! ‘The preceding discussion establishes that the increased combustion chamber pressure duc to a supercharger, and thus the power load, will have only a moderate adverse effect on the structure of the engine, at less than racing boost pressure, of course. The Power Equation Engines make power by combining oxygen with fuel and burning the mixture in some specific period of time. What we want to do is get more fuel and oxygen to the same period of time. Power increase will be proportional to the extent to which we can make that happen. Four variables are involved, expressed in a simple equation: combine Power = Px Lx Ax N + Pis brake mean effective pressure, or BMEP. This isthe average pressure in the combustion chamber pushing down on the piston. # Lis the length of the stroke. This tells you how far the pressure is goings to push the piston, * Ais the area of the bore. This is, of course, the area the pressure has to work on. ‘* Nis the number of putts the engine makes in one minute. This repre sents how fast the engine is running and how many cylinders it has. Fig. 3-6: "PLAN" is the key to the source of all P = Buring gas pressure power output —| = Avea of bore N= Putsimin (om) L= Lonath of stroke26 supercharged! N-number ofelinders x E™ (For a four-stroke-cycle engine, the 1pm is divided by 2 because each cylinder fires only on alternate revolutions.) Now; there are several interesting relationships here! For example, take the Pand multiply by the A and you have a pressure times an area, which is noth- ing more complicated than the average force pushing down on the piston. Now multiply the PA (force) by the length of the stroke, F (distance), and you have a ‘number that represents the work output (torque) of the cylinder. Then take this, figure and multiply by the N (how fast the job is getting done), and the result is, Power, the thing we are realy after. Please note that this means Power = torgueX rpm Since the whole purpose of this exercise is to get more power, let's examine what this PLAN gives us to work with. First, le’s check out what working with the Nan yield. There are two ways to get more putts per minute: add more cylinders or rev the engine higher. Higher revs leaves litte to work with, as the whole field of endeavor known as blueprint- ing (remachining all engine components to get blueprint dimensions exact and all parts balanced) is almost solely for the purpose of allowing higher rpm with some degree of safety. Consider that those nasty inertial loads go up with the square ‘of the rpm increase. Lighter and stronger parts also help raise the rev limit. That means that at 7,200 rpm, the inertial load will be 144% greater than at 6,000 rpm. Wear and tear lies up there. Ultimately, it is neither cheap, pleasant, nor durable long-term to inerease power output by increasing the N. (The Ferrari Formula 1 mentioned in the previous chapter required a complete redesign to rev at 19,000 pm, an increase of only 1,000 rpm.) Since we cannot, for practical reasons, increase power significantly with N, the only remaining choice isto increase torque by doing something with the PLA. So we must go back and look at the PLA a bit more. We can change the A. Bored, it’s called, but how much does it help? Change A by an eighth of an inch and maybe you'll gain 10%. Not worth the trouble. We can also change L. Stroked. Another 10%, maybe. Obviously, then, if we're pursuing real power, the A and the Ldon’t hold much promise. Changing P becomes our only hope. How to successfully change Pis the crux of this book. Two ways exist: raising the compression ratio or using forced induction. The heat ereated increases quickly by higher compression and produces detonation. It therefore mandates high-octane fuel. Moreover, raising the compression ratio only increases the efficiency of burn- ing the same amount of air/fuel mixture. The best to be hoped forin changing the compression ratio is about 4% more power per compression ratio point. Using forced induction, P can be changed by factors of 1.2, 1.5, 2, 3, 4,5 The real potential is not known, since engineer types push the envelope every year. Suflice it to say that doubling the power of a street engine, while not exactly child’s play, is well within our reasonable expectations. It isessential here to make clear that we are dramatically increasing power with- out changing rpm. Therefore, itis torque ( PLA) that we are really changing.Fig. 3-7: Typical exomples of the diflerences in torque curves for atmospheric engines and the three types of superchargers. Equivalent Compression Ratio Airflow Rate chapter 3: the physics of producing power 27 350 al _ Contrtuga “winscrew 3 250] Zz Roots © 200 E 150] 100] 50 o 070002000" s000_ 4000 S000 6000 7o00 RPM A popular notion is that one can create an “equivalent compression ratio” (ECR), which is supposed to represent the compression ratio of a normally aspirated engine that would be needed to achieve the same end-gas temperature as a super charged engine, where end-gas temperature is the result of both heat from the supercharger and the compression ratio. It is incorrectly thought to be the stock compression ratio times boost pressure ratio. In fact, equivalent compression ratio means only a compression ratio that would produce the same peak chamber ‘temperatures and pressures near top dead center asa supercharged engine with a lower compression ratio. Its not possible for ECR to produce anywhere near the same power, because it doesn’t burn any more fuel per time, and fuel per time is where power comes from. Without the extra mixture pushed into the chamber by supercharging, the peak pressure will not stay high for anywhere near as long, and the average pressure (BMEP) will show litte change. (Do not confuse equivalent ‘compression ratio with effective compression ratio, See the glossary and the dis- cussion of the Miller cycle in Chapter 7.) ‘The airflow rate through an engine is usually referred to as cubic feet per minute (cfim) of air at standard atmospheric pressure. The technically correct but less- used term is pounds of air per minute, This book will use the semi-incorrect term “cfm,” because cfin calculations are easier to make and are more prevalent in the literature than pounds per minute, and the error is negligible. To calculate the airflow rate of an engine without a supercharger—ie., no boost cid x rpmx 0.5 x E, Airflow rare~ rs 1,728 Here, airflow rate isin cfin and displacement isin cubie inches. The 0.5 is due to the fact that a four-stroke-cycle engine fils its cylinders only on one-half the revolutions, E, is the volumetric efficiency ofthe engine, and is explained in detail ‘on page 31, The 1,-728 converts cubic inches to cubic feet. Example; In a small-block Ford, let size = 300 cid, rpm = 6,000, and E, = 80%. Then 300 x 6,000 x0.5 x 0.8 1,728 Airflow rate =417 cfm28 supercharged! ‘The flow rate under boost can be determined by multiplying the basic engine flow rate by the pressure ratio, which is explained below. In the small-block Ford. operating at 12 psi boost: Desired airflow rate = basic engine airflow rate x pressure ratio = ofmx 182 = fin (To convert cubic feet per minute to pounds per minute, use the conversion factor of one pound of air equal to 14 cubic feet at sea level on a standard day: # min min nf i To correct for other altitudes, multiply by the ratio of the old air pressure to the new air pressure, using the values in Table 3-1 Example: cfm @ 10,000 fe = 2058 4 417 =287 fn 2992 ‘Table 31: Variation of ar pressure with aide Aditude (.) Ai presue (in. Hs) Sea level 2992 1,000 28.86 2,000 aaa 3,000 2681 4,000 25m 5,000 2490 6,000 neve 7,000 23.09 5,000 m3 9,000 2139 10,000 2038 1,000 1980 12000 903 13.00 1830 14,000 17385 15,000 1699 Pressure Ratio When we pump in more mixture, the measure of the increase isthe pressure ratio, ‘The pressure ratio is the total absolute pressure produced by the supercharger divided by atmospheric pressure. Absolute pressure means the amount of pressure above nothing at all. Nothing at all is zero absolute, so atmospheric is 14.7 psi absolute at sea level. Two psi boost becomes 16.7 psi absolute, 5 psi boost is 19.7 absolute, and so on. Total absolute pressure is then whatever the gauge reads plusDensity Ratio chopter 3: the physics of producing power 29 14.7. The pressure ratio thus becomes a reflection of the number of atmospheres of pressure generated, 147 psi+ boost 147 pai Pressure ratio = Example: For 5 psi boost 14.7 pois psi 147 psi PR- =134 In this example, approximately 34% more air will go into the engine than the engine could consume by itself For 12 psi boost 14.7 pit 12 psi pre PRB gy 147 psi Here, approximately 82% more air will be going through the system. Pressure is also measured in bar, short for barometric (I bar = 14.7 psi). In the above xample, a pressure ratio of 1.82 equates ro an intake pressure of 1.82 bar. ‘A rough idea of the pressure ratio required for an application can be found by dividing the engine’s desired horsepower under maximum boost by the horse power of the stock engine, Keep in mind thar this isan oversimplification and pro vides only a rough estimate of the actual pressure ratio needed. The real pressure ratio, particularly for non-intercooled blowers, may be much different—usually hhigher—because it must take into account several factors discussed below. “To take an example from Chapter 5 (Roots), ifour objective is 320 hp @ 5,500 ‘pm and the stock engine produces 220 hp @ 5,500 rpm, desired horsepower [Peay ete ‘existing horsepower 320 psi - oe It might seem that a pressure ratio of 2.0 would make twice the power. This is ‘not true, as here is where a couple of little things start to go wrong. And those things are not so litle ‘The first thing to go wrong isthe unfortunate thermodynamic fact that when airis ‘compressed to put more molecules ina particular volume, the compressing process hheats the air and causes it to expand back some. Ifair is compressed to a pressure ratio of 2.0, to make it twice as dense, it will expand back approximately 21% of that amount duc to the heating effect, according to the formula for temperature gain as a function of pressure ratio in Chapter 4. That is if the compressor is30 supercharged! Fig. 3-8: Compressor density ratio versus pressure ratio. Density is degraded by temperature; therefore, ‘actual airmass increase is ‘always less than that indi- cated by the pressure ratio. perfectly efficient and makes the air expand back exactly as the science of thermo- dynamics predicts. However, no compressor is perfectly efficient. All compressors, produce a greater temperature rise than the ideal, Thermal effi of how close a compressor comes to the ideal. ameasure _, Compressor efficiency (E,) 7 ad i newer Press. ratio 20) fo 20 as 80 In mechanical terms, efficiency is a comparison of output and input, or final and original, or actual and theoretical. Thermal efficiency is the ratio of actual temperature to theoretical temperature. Efficiencies are almost always less than 1. Mechanical efficiency equal to 1 would be the perpetual-motion machine Volumetric efficiency can be greater than 1 under certain conditions, as with carefill intake and exhaust tuning. An overall system thermal efficiency greater than 1 can be achieved in a supercharger system by cooling the intercooler with a ‘medium, ike ice water, whose temperature is lower than ambient. Ascertaining a ‘blower’s thermal efficiency lets us find out how badly we need intercooling. If the compressor’s thermal efficiency is 50%, the air will be heated twice as ‘much as it theoretically should, and it expands back by 42%, So we may have thought we had 100% more air but netted only 58% more. The greater the ther- mal efficiency, the less ar i lost to expansion, and the denser the charge remains. (The factors that determine thermal efficiency are discussed in the three chapters ‘on the specific types of compressors ) Calculating the supercharger’s compression and correcting for the tendency to expand back creates a valuable number: the density ratio. Density is the number of moleculesin a specific volume—say, pr cubic foot. Density ratio isthe real comparison number we are after to estimate the power outputs for varying manifold pressures. The density of air varies inversely with absolute temperature. If ai is heated 10% on the absolute scale (see glossary), it becomes 10% less dense. The density ratio is the ratio of before-and-after absolute temperatures:chapter 3: the physics of producing power 31 original absolute temperature Density ratio ~ 71 qbsalute temperature To obtain absolute temperature, add 460° to the Fahrenheit temperature. ‘Therefore, ifthe measured air temperature of the compressor discharge is 210°E, the final absolute temperature would be 670° absolute. If we know the tem perature gain through the compressor rather than the compressor discharge tem- perature, we must add the ambient air temperature to it, because ambient is the temperature from which the gain starts. So if we know that the temperature gain through the compressor on a 90°F standard day was 120°E, the final temperature ‘would again be 210°F, or 670° absolute. Ifan intercooler is present, use the compressor inlet temperature and the inter- cooler outlet temperature. If not, use the compressor inlet and outlet temperatures. Density change as a result of intercooling is discussed further in Chapter 8 Volumetric efficiency, or Ey , of an air pump (engine or supercharger) is the actual volume the unit pumps divided by the theoretical volume it could pump. Itcan be considered a degree of “inability to pump”—that is, ifan air pump has a volumettic efficiency of 78%, itis missing 22% of its displacement per revolution For a fixed-displacement supercharger, the theoretical volume is the displace- ‘ment volume; itis also the volume of each cavity between rotor lobes times the number of cavities. ‘Some compensation must be made for the supercharger’s volumetric eficieney, 'y 1 pump,” or the application will wind. up with less airflow than expected. If the supercharger’s volumetric efficiency is 87%, for example, the lost airflow can be compensated by turning the blower 13% faster or making it 13% bigger. This is accounted for in the volumetric efficiencies ratio discussed in the next section. Volumetric efficiency is critical information in determining the size blower cone needs, as a supercharger always pumps less than its displacement, (It might seem that the thermal efficiency and volumetric efficiency of a supercharger are related—that a portion of the air expanding back when heated affects how much. air is pumped. However, the relationship between the two is actually remote and complicated.) In fixed-displacement superchargers, volumetric efficiency for a given boost and shaft speed is an elusive quantity to nail down. The reason for this s the two- faced nature of volumetric efficiency: it increases with shaft speed but decreases with boost pressure, However, a number in the range of 92-95% should suffice for most calculations, Examples in this book will use 92%. The practical result of using approximations for engine and blower volumetric efficiencies, however, is that although initial pulley-size calculations may put you in the right ballpark, it is often necessary to try different pulley sizes to get the desired boost. ‘The fact that the centrifugal is a non-fixed- displacement compressor means that it doesn’t pump a specific volume of air per revolution. Because it has no theoreti cal pumped volume, it doesn’t have a volumetric efficiency, which is a comparison of how much a blower actually pumps to how much it could theoretically pump (its displacement). The lack of a defined displacement and volumetric efficiency means we can’t calculate how much a centrifugal flows. All we can do is test it and use the real data,32 supercharged! Fig. 3-9: Compressor dis charge temperature versus pressure ratio. Why one wants fo secure the high- est compressor efficiency possible: the greater the efficiency, the lower the temperature. Volumetric Efficiency Ratio: When we look at a compressor map (discussed in Chapter 6), we sec real data. Although volumetric efficiency can’t be measured separately, it’s part of the flow, pressure, and speed results shown on the map. For the purpose of calculations, the volumetric efficiency of a centrifugal can be considered 100%, As with fixed-displacement superchargers, itis often necessary to try different pulley sizes to get the desired boost. as Pressure ratio 15 foo iso 200-250 ~—~300~=—«50”~=~«OO ‘Compressor cischarge temperatures ("F) “The effectiveness ofa two- pump system, like an engine and supercharger, is related to the ratio of their volumetric efficiencies. A supercharger with greater volumetric efficiency will aid the breathing of an engine more than one with low efficiency. Also, the ability of an engine to accept air pumped in by a supercharger is related to the engine’s own volumetric efficiency. Power output is directly related to how efficiently the air is pumped by the supercharger relative to how efficiently it is, received by the eng supercharger volumetric efficiencies Volumetric efficiencies ratio = MPE™ERINC? OUAMETTIE ITCIENCES “ engine volumetric efficiencies Because power is proportional to this ratio, it might seem at first glance that as engine volumetric efficiency declines, power should go up. Stuffa rag in the throttle body to halve the engine's volumetric efficiency and double the power. But decreasing the volumetric efficiency to gain power doesn’t work ‘out, because the factors surrounding the quality of the mixture burn go haywire at the same time, Supercharger volumetric efficiencies are given in the previous section. Engine volumetric efficiencies vary, but for general purposes itis reasonable to use 80% for two-valve engines and 88% for four-valve engineschopler 3: the physics of producing power 33 Drive-Power Loss Ifthe 42% expansion described carlier occurs, would we get a 58% power increase Fig. 3-10: Where the drive loss goes. with a pressure ratio of 2.0? No, because one more thing gocs wrong, It takes power fo compress the ar, and the only power source readily available is the engine itself. Therefore, the power the supercharger needs to compress the air must come directly from the crankshaft—typically, 5-15% of the engine’s power. Of that per- ‘centage, partis lost in bending the belt around the pulley, partis lost in turning ‘the gears and beatings inside the blower, and part goes into useful work to produce boost and flow. This book will refer to the sum of these three as drive-power loss, We can also look at the bright side and express these losses as efficiencies, which are more practical to work with than losses, by deducting them from 100%. Belt loss Belts require power to bend them around pulleys and, at redline speeds, typically absorb 2-3% of the engine’s power, Belt efficiency is therefore about 97-98%. Examples in this book will use 97%. Adiabatic loss The power lost as heat inside the blower is adiabatic loss. (“Adiabatic” refers to an ideal process that occurs without loss or gain of heat.) Adiabatic loss is usually 35-50% of what goes into the supercharger. What goes into the supercharger is what's left after we get past the belt loss. So if drive-power loss is 10% and belt loss is 3%, what goes into the supercharger is about 7%. Adiabatic loss is 35-50% of that 7%, or about 3% of the drive-power loss. The remaining 4% of the drive- power loss is useful work Examples in this book will use 40% for aiabatic loss, or 60% for adiabatic efficiency, which is a satisfactory approximation if performance charts are not available, Adiabatic efficiency is similar to mechanical efficiency, but mechani- cal efficiency is not generally used when heat and pressure are involved, as with superchargers. Drive-power loss, the sum of belt loss, adiabatic loss, and useful work, is gener- ally about 10% for boost pressures in the vicinity of 10 psi. Drive-power efficiency, therefore, is 90%, which isthe figure used for examples in this book. Drive-power Joss decreases to about 7% for boost pressures near 5 psi and increases to about 14% for boost pressures near 15 psi, producing efficiencies of 93% and 86%, respectively. Caleulation of drive-power loss is presented in Chapter 14. 7.8% Into supercharger ‘about 34% about 94% adiabatic loss useful work ‘going into the supercharger34 supercharged! Estimating Power Applying the above parameters, we can estimate the power resulting from super- charging using the following calculations, Example: Let engine power = 116 hp, boost = 6 psi, engine volumetric effi- ciency (4 valves per cylinder) = 88%, non-intercooled volumetric efficiency = 92%, ambient temperature = 90°F (a standard day), measured air temperature of the compressor discharge = 210°F, and drive-power efficiency = 90%. Then, 147 psi boost 147 pai _ EZ piso pg 147 pst riginal absolute temperature ‘final absolute temperature Density ratio = 460° + 90° ~ G60" 210° “9%? As indicated earlier in this chapter, the initial pressure ratio and density ratio are frequently inaccurate. A detailed example of a sizing calculation that produces accurate results is given in each of the three chapters on the types of supercharg- cers. However, for the purpose of illustrating the procedure here, we will assume the numbers are correct. supercharger volumetric efficiencies Vouameric aiciencis ratio= ne volumetric effsiences 0.92 0.88 08 Applying these corrections to the engine’s original power produces the following: Power = original power x new pressure ratio x density ratio x volumetric efficiencies ratio x drive power efficiency then, 16x 1.41 0.82 x 1.05 x0. Power = = 127 bhp ‘The reaction to this calculation should be twofold. First isthe startling realization that the loss of 18% of the hoped-for power gain (as indicated by the 82% density ratio) is due to intake charge heat. Second is that the pressure ratio gives us a lot to work with if we can figure out how to avoid having it so badly degraded by heat-related density losses.chopler 3: the physics of producing power 35 And Furthermore... Considering the large power: increases offered by the supercharger, what keeps the entive structure of the engine from going south? A proper answer to this question is a complete analysis of the inertial, power, and thermal loads before and after supercharger installation. If this 18 performed, the conclusion will be two interesting bits of information. First, the inertial loads in a modern internal combustion street engine are so large at maximum power that the power component of the total load is oflitle significance, For example, to induce as much power load into a con-rod bearing as the bearing already sees from inertial loads, the actual power of the engine would need to increase approximately 50%. Second, the thermal load in an engine not originally designed for a supercharger will cause an increase in component and cooling-system, temperatures when operating under boost. The components and cool ing system can handle the temperature inerease for a limited period. ‘This is true for Buicks, Porsches, Saabs, Volvos, Nissans, etc. Iti also true for all aftermarket supercharger kits. The time limit is subject to many judgments and conditions. Experience has led me to believe that the time limit at full boost is on the order of 20 to 25 seconds. This is an operational restriction but not one of any consequence. Consider, for example: How fast will you be traveling if you hold full throttle in 1 600 bhp Mustang for twenty seconds? The answer is obviously an impractically high rate of speed. What is the best compression ratio for a supercharged engine? ‘There is no such thing as the best or ideal compression ratio. The simple fundamentals are ‘*the lower the compression ratio, the easier it is to produce a lot of boost with no detonation ‘*the higher the compression ratio, the greater the fuel efficiency and. nonboosted response Suppressing detonation is more difficult with a high compression ratio. For all practical purposes, one is forced to use the compression ratio of the standard engine. Serious efforts with intercooling make this both possible and practical. When should the supercharger start producing boost? ‘This isa decision you need to make based on your own objectives. flow and mid-range boos is your bag, fixed splacement blowers will be appro- priate. IFtop end is preferred, the centrfugals come into their own, How will the supercharger affect driveability? Electronic engine-management systems permit supercharging without degrading the fine running of today’s vehicles. Carbureted blow: through systems can be made to work well, but draw: through systems will suffer many functional disadvantages—cold starts, emissions, and smooth low-speed function will generally prove less than acceptable.Knock Temperature Relationships The Balance of Heat Power output is ultimately limited by knock, and knock is caused by heat Therefore, the ultimate power to be achieved is limited by heat. Success with a supercharged engine depends on many details, but none is as important as manag ing the balance of heat through the system. ‘The three major influences on this heat are boost pressure, intercooling, and the compression ratio. Learning the relationship between these major influences is fundamental to understanding what the supercharged engine can ultimately do. Knock isthe explosion that occurs in the combustion chamber when the air/fuel mixture ahead of the flame front becomes overheated and spontancously com busts before the flame front arrives. (Some controversy exists as to whether knock is the event itself or the noise it produces. For purposes of this book, knock will be considered the event itself.) Knock is different from controlled burning. The heat that spontancously combusts a mixture that produces knock may come from a variety of sources. These sources must be controlled for an engine to reach its full potential Knock is important because the resulting temperatures and forces are fiercely high. The magnitude of the temperature produced by knock weakens pistons much more than does standard operating temperature. Chamber pressure spikes created by knock hit the piston with such a rapid onset of load that they become impact loads. No piston known can withstand this for more than a few seconds. Not only does the piston take a serious beating, but the top con-rod bearing shell «an actually be pounded to the point of malfunction. Knock is virtually always the «cause of failure in a supercharged engine. A blown engine, in supercharger lingo, is an engine with a knock induced busted head gasket, ring lands knocked off the side of the piston, or, in rare cases, a hole in the piston. To understand how temperatures stack up as a result of boost pressure, intercooting, and compression ratio, suppose a magic combustion-chamber temperature exists that isthe maximum possible before onset of knock. Of course, the engine’s susceptibility to knock will vary asa function of fuel octane, chamber design, and other factors, but let’s use an arbitrary temperature of 1075° absolute. Keep in mind through this discus sion that the 1075° figure isan arbitrary number. This represents about the absolute ‘temperature ofa 13-to-1 compression ratio on what is called a “standard day” (90°F, ‘or 550° absolute). Many tradeofis are involved in trying to keep this final tempera 3738 supercharged! Heat Made by the Supercharger ture below 1075° absolute. The fundamental idea is that the more heat we remove from the intake change, the denser it becomes, and the more boost we can use before the temperature reaches 1075°. The whole process isa. juggling match: boost pres- sure and compression ratio add the heat, and the intercooler removes some oft. This relationship can be expressed as follows: Without an intercooler Temperature added by boost pressure + temperature added. by compression ratio must total less than 1075° absolute, or Tye + Toy < 1078° With an intercooler Temperature added by boost pressure — temperature removed by intercooler + temperature added by compression ratio must total less than 1075° absolute, or Tyg Tig+ Ter < 1078" ‘The presumption is thar whatever combination of boost pressure, intercooling, and compression ratio we choose will be workable ifthe sum of the temperatures isles, than 1075° absolute. (Remember that 1075° absolute is only 615° Fahrenheit.) {tis both illustrative and fundamental to see how much a psi of boost will cost us, in terms of temperature. This can be caleulated as follows: (PRO28 X Trig) ~ Tay where PR= pressure ratio Tyjg= ambient temperature on the absolute scale. (Examples in this book will use the temperature on a “standard day,” which is 90°F = 90° + 460° = 550° absolute.) ‘The exponent 0.28 in the numerator is determined by the gas constant, a ‘number that indicates the extent to which a gas heats up when compressed.,The XY key on the fifteen-dollar calculator mentioned in the introduction will allow us to find the value of PRO28, To raise a number to the 0.28 power, enter the ‘number in the calculator, press the x¥ key, enter 0.28, and press the equal sign. For example, 2028 = 1.21419, ‘What this formula does is multiply the absolute ambient temperature by a pressure-ratio factor (PR028) to find the temperature to which the charge will be raised, Subtracting the absolute ambient temperature gives the temperature ain. For example, the temperature may increase from 550° to 700° absolute, but the temperature gain would be 150°, Another way to express the temperature-gain formula is (PR028 1) x Typ which is the form used in this book,Heat Made by the Compression Ratio chopler 4: the balance of heat 39 Using the above formula, we can determine how much a 1 psi boost will raise the temperature. Temperate gin [ (mae ieee tet a J x 550° = 10.2° absolute This 10.2° san ideal temperature gain only. The real temperature gain is the ideal number divided by the thermal efficiency of the air pump making the boost. Table 4-1 gives the thermal efficiencies (E,) of typical air pumps. ‘Table 4-1: Thermal efficiencies of typical ar pumps ‘Type Ee(%) Roots 5 Centrifugal 75 Twin screw 70 “Turbocharger (typical) 75 Using these figures, we can calculate the real temperature gains for 1 psi of boost: 10.2° 0.55 10.2° 0.75 Typical Roots temperature gain ~ = 18.5° absolute ‘Typical centrifiyal temperature gain = = 13.6° absolute 10.2° 0.70 ‘Typical srew compresor temperature gain = 4.6° absolute Keep in mind that these are typical numbers, to give an idea of the relative size of the increases. The real temperature gain per psi decreases with boost pressure because of the 0.28 exponent in the formula and the fact that the blower’s effi- ciency is not linear. For example, the temperature gain per psi at 15 psi boost is about two-thirds of the gain at 1 psi boost. Examine Fig. 4-1 for an idea of how serious these temperatures become. (The graph is calculated with compressor efficiency remaining constant, regardless of boost. This is not quite correct but is acceptable for illustration. All numbers are calculated from the above figures.) ‘The compression ratio does virtually the same thing the blower docs: i raises the temperature of the air charge based on the pressure ratio. Of course, here the pressure ratio is called the compression ratio. One huge difference, however, is that air compressed in the chamber does not experience a density loss, because it’s in a scaled container, and the air molecules can’t get out. Therefore, unlike with the supercharger examples above, this number does not need to be degraded by engine efficiency.40 supercharged! Fig. 4-1: Compressor discharge temperature gain a8 a function of boost. Roots 220 180 160 140 Centtugal “Temperate gain (*F) Fie rarer C In aa S MNO NC OME icin amc) Boost (psi) ‘Temperature gain caused by the compression ratio is calculated with the same equation as for gain through the compressor, except that the compression ratio replaces the pressure ratio: ‘Temperature gain = (CK928 ~ 1) x Tay Example: Let the compression ratio = 9 to 1 on a standard (90°F) day. Then, Temperature gain = (9°28 — 1) x 550° = 468° absolute A gain of 468° absolute over an ambient temperature of 550° is 1018° absolute. To ealeulate the chamber temperature directly, rather than finding the temperature sain and adding it to ambient, we would not subtract the absolute ambient tem perature (or, in this form of the equation, the “1”). The formula then becomes ‘Temperature = CR8 x Tay = 9028 550° = 1018°Fig. 4-2: Chamber tem perature gain as a func- tion of boost for @ 910-1 compression ratio. These temperatures are found by Using the numbers from Fig. 4-1 (and ombient tem perature of 90°F) in the femperature formula. Heat Made by the Supercharger and the Compression Ratio chapter 4: the balance of heat 4l 1450 Roots 1350 ‘Screw compressor \ 1300 1250 Centritugal 1200 1150 (Chamber temperature ("abs) 100 23 45 6 7 8 © 1011 1213 14 15 Boost (ps) Finding the temperature in the combustion chamber after the mixture is squeezed by both the supercharger and the compression ratio requires more than just add- ing the temperature gains of the two squeezes. This is because the compression ratio squeeze starts from the temperature produced by the supercharger squeeze, not ambient temperature. Therefore, instead of using ambient temperature in the compression ratio formula, we use ambient plus the temperature gain created by the supercharger. Example: In an engine with a 9-to-1 compression ratio and a Roots blower, let boost pressure = 10 psi. From Fig. 4-1, we can see that 10 psi boost from a Roots blower produces a temperature gain of 156°F. Because the gain starts from ambient, the temperature out of the blower is 90°F + 156°F = 246°, or 706° absolute. Then Temperature = 9°28 x 706° = 1306° Since the three types of superchargers have different thermal efficiencies, boost limits dictated by the temperature limits will be different. Any combination of boost pressure temperature rise and compression ratio temperature rise that cre- ates less than our gain limit of 1075° absolute will work. For example, Fig. 4-242 supercharged! Add the Intercooler Fig. 4-3: Compressor discharge temperature gain afer intercooling, as a func- tion of boost shows that at 1075°, we could get about 2 psi out of the Roots and 3 psi out of the centrifugal and the twin-screw. These are only approximate figures—the real ‘numbers would be somewhat higher. It’s clear, however, that unless we do some- thing to reduce the chamber temperature, we can’t apply much boost before we hit the 1075° ceiling, ‘The intercooler can rescue this bad temperature situation. Let’s ook at the whole picture again when we place an intercooler between the supercharger and the engine. Assume the intercooler will remove 85% of the temperature put in by the supercharger. Fi 4-2 akes the numbers in Fi. 4-1 and applies an 85% reduction to the compressor discharge temperature gain, yielding 15% of the original figure. To calculate the final intercooled chamber temperatures, we use the same procedure as in the previous section, inserting the intercooled temperature gains from Fig. 4-3in the temperature formula for a 9-to-1 compression ratio, giving us, Fig. 44, This shows that with intercooling, we ean run substantially higher boost before reaching the arbitrary 1075° limit: about 10 psi for the Roots and 13 psi for the centrifugal and twin-screw. Keep in mind that the 1075° limit is nota real ‘number but an approximate one. Although this exercise in numbers is all of approximations, it’s easy to show, for example, that you can run 40 psi boost intercooled at the same chamber tem perature as 8 psi non-intercooled. This is an impressive argument for the benefits and urgency of keeping temperatures in check, = ‘Screw compressor i 24 € \ © 20 Centitugat & § : 16 Pe ote se FO T BRN 13 4 15 Boost (psi)Fig. 4-4: Chamber tem perature gain after inter- cooling, as a function of boost for a 910-1 compres sion ratio. These tempera- tures are found by using the numbers from Fig. 4-3 (and ambient temperature of 90°F) in the temperature formula. And Furthermore . . . chapter 4: the bolonce of heat 43 1100 eke suew i compressor B 1000 3 Contig § 1040 i & 1020 1000 ote ee SOT RRND 15 Boost (osi) What docs compressor efficiency mean, and why is it important? Compressor efficiency means nothing more than the real temperature of the air coming out of the supercharger under boost relative to a calculated number based on thermodynamic equations. Calculate one, measure the other, divide the calculated by the measured, and you have compressor efficiency. Matching a compressor’s efficiency curve to particular engine is important, in that getting maximum efficiency somewhere near the power peak or maximum rpm means that the com- ;pressor has induced the lowest possible thermal load. “Highly efficient” is a goofy expression invented by casual writers about superchargers to ‘mean nothing more than that whatever vehicle a supercharger is on gets boost at low speeds.Fig. 5-1: These rotors ore the heart of the Rootsstyle blower. In their simplicity cand beauty, they resemble a modern sculpture. The Roots Supercharger The Roots supercharger has the odd distinction of being the oldest of the super- charger designs, yet subject to the most modern research. The Roots was origi nally conceived as an air mover in nonautomotive applications. Until recently, its construction was based on the abilities of machinery available at the beginning, of the twentieth century, In the 1 jon undertook design studies to create a modern Roots supercharger. Their R&D program produced remarkable results. Not only were they able to dramatically improve thermal efficiency of the classic Roots, the noise characteristics were also brought within tolerance. It represents a remarkable engineering job, probably not feasible before computer analysis and computer-controlled production machinery. Eaton perfected the twisted-rotor design, then reconfigured the discharge ports based on noise characteristics and noise suppression. They thus created a Roots supercharger worthy of consideration for a vehicle from the drawing boards of Mercedes Benz: the SLK sports car, {-seventies, Eaton Corpor: The T-Bird Super Coupe was the first modern application of the Roots to a high-volume production vehicle. Pontiac and Oldsmobile followed soon after. All have used the Eaton blower, but only Ford saw fit to add intercooling to the ‘equation. The Eaton will see wider application in the future and will likely spawn a few new Roots blower designs by other major companies, 4546 supercharged! Aftermarket Systems Fig. 5-2: Perhaps the high est science and greatest pro- duceabily ever designed into 0 Rootsstyle blower cre combined in the Eaton supercharger. Racing Popularity of the Roots supercharger among street rodders has grown to such a point that tradition almost demands a “real street rod” be blown, and blown by a Roots, This tradition has been built by a long and prosperous relationship between Roots manufacturers and the American aftermarket. A wide variety of kit and component makers have successfully filled the needs of the performance enthusiast. Many of these companies have risen to the challenge of producing their own blowers. Although tradition presents a picture of a huge Roots blower siting atop an equally huge V-8 engine, the Roots supercharger is atts best when applied to small and medium-sized engines, Large engines already enjoy more torque than tires can withstand at low speeds and can find large performance benefits only in the upper gears, Smaller engines struggle everywhere for acceleration, which is exactly where the fixed-displacement, low-speed pumping capabilities of the Roots can shine. ‘Through this inherent nature of the Roots to perform well at low engine speeds, it is now possible to have a small engine become a “low-speed torquer.”| Top Fuel ‘The Roots supercharger is currently hing of the hill in Top Fuel drag racing. 1 believe this is due to four fundamental reasons: Boost rise is quick. ‘+ The Roots is virtually all these racers have used. ‘© The class racing rules prohibit other equipment. * Scrious power can be obtained from racing fuel at low boost levels. When your vehicle can top 300 mph in less than 5 seconds and can smoke the tires even at that speed, horsepower may not be your major concern. These types of supercharged vehicles are discussed further in Chapter 17. Bonneville Many of America’s legendary hot rodders have run and set records at Bonneville with engines assisted by Roots superchargers. Such characters as Mickey ‘Thompson, Ed Iskenderian, and the Summers brothers created some of the most colorful and interesting times in the history of Bonneville Salt Flats.chapter 5: the roots supercharger a7 Fig. 5-3: Note the strength and craftsmanship Kuhl Supercharging puts info their big block blowers. Fig. 5-4: This Kuhl mani fold is typical of the hard- ware needed to place a oir of throttle bodies or carburetors atop the big Roots blowers. Construction The Roots supercharger is composed of two meshed, lobed rotors. Geared together and rotating, in opposite directions, the rotors mave the air charge around an outside path in the cavities between lobes and the interior wall. The volume in each cavity times the number of cavities is the amount of air the Roots ‘can move per revolution. The Roots has its niche in medium- to low-pressure supercharging, where thermal efficiency is not so important, and quick response and low-end torque are qualities most sought after. Keep the Roots in this terri- tory and it remains a happy mechanism. Ignore the vision of Roots applications, for nitro-burning Top Fuel dragsters—they’re a completely different world. They run for only a tenth of a minute and burn a cousin of dynamite for fuel. Fig. 5-5: Air enters a Roots Inet blower at one side or at the end, circles around the inside wall of the rotor hous- ing, and exits from a port at the side. outlet48 supercharged! Fig. 5-6: A cross-section and side view of a Roots. Advantages Disadvantages Image courtesy Eton Corporation low-speed boost ‘The great advantage of the Roots blower isits ability to make excellent boost at low ‘engine rpm, This is because its pumping capability doesn’t change much with rpm. Volumetric efficiency may fluctuate, but in general, flow is proportional to rpm. At very low speeds—say, 1,000 rpm—the Roots has substantial leakage past the internal clearances. This is because leakage is in part, a function of time, so a slow moving rotor allows more time for the leakage to occur. This explains why maximum hhoost may nor he achieved until about one-third of the engine’s redline speed. Simplicity ‘The simplicity of the Roots is a strong point. Its few moving parts and relatively low rotational speeds lend a high degree of reliability. No surge Unlike a centrifugal compressor, the Roots does not experience surge, because the throttle is generally mounted at the compressor inlet. Although this has its own shortcomings, surge is not one of them. Without surge, the additional complexity ‘ofan antisurge valve is nor needed (although its close relative, the bypass valve, is usefull addition), Surge is explained in Chapter 6. Heat One of the consequences of no internal compression ratio is the production of ‘more heat. Heat, a this book emphasizes, is the archenemy of power. The modern, Roots blowers created by Eaton have brought thermal efficiency into an acceptable range, although i still lags the twin-screw and centrifugal ‘The Roots pumps air in specific amounts based on its clearance volume. The «clearance volume of a compressor is the open space between the rotor lobes minus a small quantity called “carryback volume.” Carryback is a void at the pocket of the recess between two rotor lobes and the meshing rotor. This small void does. not get completely discharged when that segment of the rotor is exposed to the discharge port. Rather, it takes in some of the heat created by pushing air into the manifold and “carries it back” to the intake. This heat slightly increases the temperature of the incoming air.Fig. 5-7: Clearance volume isthe space remaining inside the housing afer subtracting the volume occupied by the rolors and ports. The rotors pick up cir at the top, camry it ‘around the perimeter, and dis- charge itat the bottom. The three cavities on each rotor make this circuit once per revolution. Therefore, clear ‘ance volume is also equal fo six times the shaded area. Fig. 5-8: Carryback vol ume is the pocket between cone rotors tip and the other's cavity. This pocket is created by the necessity for different radii of curvature botween the two surfaces. chapter 5: the roots supercharger 49 eo Throttle placement Because the Roots is a fixed-displacement supercharger, the throttle is customar- ily mounted at the compressor inlet. Without the design complexity of a bypass valve, this becomes a necessity. If the throttle were downstream, closing it would ‘cause a buildup of pressure between it and the supercharger, forcing it to a stop and wreaking havoc with the belts. A technique for blowing into the throttle with fixed-displacement superchargers has been developed but 1s not yet available in the aftermarket. Certainly, in many ciccumstances, including a bypass valve will be easier than relocating the throttle. However, the closer the throttie is to the intake valves, the ‘more crisply and responsively an engine will run. Itis currently popular to use the bypass, yet leave the throttle in front of the blower. Noise The clearance volume in a Roots is released twice per revolution in a two-lobe rotor and three times per revolution for a three-lobe rotor. This quantized dis- charge creates a pulsing sound under boost. The two-lobe rotor tends to accentu- ate the noise, because of its lower pulsation frequency (two pulses per revolution rather than three) and greater discharge volume per pulse. ‘Noise is also proportional to the number of molecules being moved per revolu- tion, Ieis easy to imagine that there would be no noise ifthe rotors operated in a complete vacuum. Therefore, throttling the air charge entering the blower forces, it to operate at the same part-thrortle vacuum as the intake manifold. Thus, with50 supercharged! Fig. 5-9: Representive of «@ large-engine Roots super charger kits his assembly from Weiand Fig. 5-10: . and this cone from B&M, Fig. 5-11: The fundo- 7 Blower mentals ofa Roots blower MMOL pas not considered & AW oO Intercooler Vacuum operated bypass valveBypass Valve Fig, 5-12: The byposs valve permits “breath- ing oround! or "venting around!” the supercharger, depending on operating conditions, Lubrication chapter 5: the roots supercharger 51 few molecules present, itis quieter. Only when full throttle is applied does the blower revert to full song. Eaton Corporation created a twisted rotor that reduced the rate of opening of the discharge port and thus the suddenness with which air is discharged into the port. Varying the shapes of the discharge ports further reduced this suddenness. “These features and heavy-wall tubes have reduced overall operating noise within modern NVH (noise, vibration, and harshness) tolerances. Cruise conditions and other normally aspirated operational modes can suffer from ‘undesired pumping losses. When cruising at approximately 13 inches of vacuum, the system without a bypass valve will create about 20 inches of vacuum between, the throttle and blower, because the Roots is pulling from the throttle plate. This “boosting” from 20 inches to 15 is a constant and unnecessary waste of power and also produces a small amount of heat. ‘Although the heat is not too harmful, the bypass valve fixes this situatio allowing the Roots (or any other blower mounted after the throttle) to “free- wheel” by pumping back into its intake side, balancing the pressure between the manifold and the throttle body. When the engine is not under boost, the bypass valve is held open by intake manifold vacuum. Depending on its size and throttle opening, up to half the airflow can pass through the valve rather than through the blower. (Although it might seem that on each cycle the valve would send. progressively hotter air into the supercharger, this doesn’t occur, because a Roots has no internal compression ratio.) When throttle position permits vacuum near atmospheric pressure in the intake manifold, the bypass valve closes, and the boost heads for the engine. Vacuum level required to close the valve can be regulated by an adjustment and/or spring setting, ‘The lubrication requirements of the Roots blower vary with the manufacturer and unit. Bearings pressure-fed by engine oil are the norm. The unique Eaton supercharger, with an onboard reservoir, offers permanently lubricated bearings that need no further attention. Both methods provide long-term durability52 supercharged! Selecting the Roots Sclecting the appropriate blower size starts, as always, with determining one’s Supercharger performance objectives. Each model has a specific pumping capability and a rev Fig. 5-13: The Roots is not exclusive to large-displace- ment engines. The 1.éditer Mazdo Miata onjoys @ substantial benefit kom the small Eaton blower limit that determine the amount of power it can produce. This is the ideal power it could produce in a perfect world. Reality, however, requires the ideal power to be degraded by the density decrease due to the temperature rise and by power lost to volumetric (in)efficienty and in turning, ue blower. These factors combine to indicate a specific size and rpm for a given application All efficiency factors must be for the specific supercharger involved. The ‘numbers can vary tremendously from an antique 6-71 GMC blower to the latest design from Eaton. Volumetric efficiency The Roots is a fixed-displacement supercharger. This means it will theoretically pump a specific volume of air (its displacement) per revolution. As discussed in Chapter 3, the actual volume pumped divided by the theoretical volume it could Pump is the volumetric efficiency, or E,, Eis usually represented on a graph versus pressure and flow, all of which are determined in lab tests. ‘Volumetric efficiency varies from make to make and model to model and can range from a low of 30% to highs in the 90s. The higher the pressure the Roots is asked to produce, the less the volumetric efficiency, as greater leakage is forced past the rotor tips. Interestingly, the faster the rotor turns (within reason), the greater the volumetric efficiency, as tip leakage has less time to occur. Volumetric efficiency numbers are available from supercharger manufacturers or suppliers. Fig. 5-H4illustrates two characteristics of fixed-displacement blowers: volumetric efficiency increases with shaft speed but decreases with boost pressure. At 8,000 rpm, raising the boost from 0.34 bar (5 psi) to 0.69 bar (10 psi) causes a loss of 4%. Thermal efficiency Pethaps the weakest characteristic of the Roots supercharger is its propensity for ‘making heat along with boost. Two factors contribute to the heat: lack of an inter nal compression ratio, discussed in Chapter 1, and, toa lesser extent, cartyback of a portion of the compressed air chary c, discussed earlier in this chapter.Fig. 5-14: The hwofaced nature of volumetric effi ciency: it increases with shaft speed but decreases with boost pressure. This is for an Eaton model 90 Roots blower. Fig. 5-15: Thermal eff- ciency of the Roots varies with size and design but generally does not exceed 60%. Fig. 5-16: Typical Roots compressor boost capability versus rpm, as set for 8 psi chapter 5: the roots supercharger 53 0 oe = z BG Poo 3 B70 5 s a 50. 2000 4000 6000 -—«6000-—«10000~—«12000~—«¥4000 ‘Suoercharaer speed (rm) 260 PR+1.69 ee Temperature gain F) 240 220 200 40 160 140 120 100 20 0 40 20 0, ‘2000 4000-6000 6000 10000 «+1200 —14000, ‘Supercharger speed (pm) mage courtesy Eaton Corporation ‘Boost pressure (psi) Yoo 20003000 4000-000 -—«6000~—«=7000, ‘Supercharger speed (rpm)54 supercharged! Fig. 5-17: This cutaway ‘shows the transfer gear and the twolobed rotor shope. Calculating the Size Another heat-related problem is that high boost pressures, say 12+ psi, can ‘cause thermal expansion of the rotors. To cool the rotors at these pressures, it is often necessary to pass the air/fuel mixture through the rotors and housing. This, climinates any possibility of meeting emissions requirements, because raw fuel clings to the rotors, requiring a very rich mixture just to get enough fuel through the supercharger to run the engine. Low-speed engine smoothness will sutfer as well, ‘Thermal efficiency varies with size and design. The large, classic Roots blow- ers are pressed to achieve a thermal efficiency of 40%, With substantial effort and ‘considerable “enginuity,” the Eaton Corporation has raised the thermal efficiency ‘of some ofits recent Roots designs to the mid-60s, Finding the size of the supercharger needed to do the job requires knowing your conditions and objective. Conditions are the engine parameters: stock horsepower, redline rpm, and volumetric efficiency. The objective is the power desired To find this power, we need to take into account, as indicated above, what \we're gaining from the supercharging and losing through breathing restrictions, heat, and friction. Recall the formula for engine power from Chapter 3, Desired power = stock power x pressure ratiox density ratio X volumetric efficiencies ratio x drive power efficiency Here we have the small dilemma that we don’t really know the exact pressure ratio, because it depends on the density ratio, which depends on the pressure ratio, and so on and on, This is a case of two unknowns in one equation, which presents no easy math solution. Although a little bit like the chicken and the egg deal, it’s satisfactory to make an educated guess at one number—the pressure ratio being the easiest—and solve for the other, the density correction, then use successive iterations to home in on a more accurate pressure ratio. When the results of two successive iterations produce boost pressures within 1 psi of each other, it's time to stop. These calculations are correct only if the engine configuration remains the same. They don’t work if modifications have been made to the compression ratio, camshaft, or redline speed. Correction factors for those conditions are discussed later in this section.chapter 5: the roots supercharger 55 Example: Stock engine: 302 cid, 220 hp @ 5,500 rpm, volumetric efficiency (Ev) = 80% ‘Objective: 320 hp @ 5,500 rpm (fa specific quarter-mile time is your objective, see the “racer math” formula in Chapter 18.) Asindicated in Chapter 3, we'll use 92% for supercharger volumetric efficiency and 10% for drive-power loss, which means 90% drive-power efficiency. From Chapter 4, we find that the typical thermal efficiency of an Eaton Roots is 55%. Using the formula from Chapter 3, supercharger volumetric efficiencies Volumetric efficiencies ratio = pmcercelencies ra ‘engine volumetric efficiencies 92% - = 115% 80% With the parameters established, we can begin the sizing calculations. First calculation: Estimate the pressure ratio, Using the formula for pressure ratio from Chapter 3, desired horsepower PR- existing horsepower 320 psi -— = os 220 psi ‘The pressure ratio can tell us the boost pressure needed, as a pressure ratio of 1.45 means that the boost pressure is 45 of an atmosphere above atmospheric pressure of 14.7 psi: Boost = 0.45 x 14.7 psi = 6.6 psi Second calculation: Find the density ratio. ‘To find the density rat to know how much the temperature rises. As indicated in Chapter 4, we have to divide the ideal temperature gain by the thermal efficiency of the blower. Using the formula for temperature gain from Chapter 4 and dividing by 0.55, ), we need (PR028 ~ 1) x Taig ‘Temperature gain = oe (1.45028 — 1) x 550 Pe 0F 0.5556 supercharged! ‘The density change due to heating isthe ratio of the absolute temperatures before and after the blower, A temperature rise of 110°F on a standard day of 90°F will produce a discharge temperature of 200°F from a non-intercooled blower. Using the formula for density ratio from Chapter 3, Denso arg ~