Ignition Types and Coil

Wiring
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Lou D. 5/16/16 updated 7/2/16

Ignition Coil Background

Most classic automotive ignition coils, whether the
ignition is a Kettering type, capacitive-discharge, or
magneto driven, are very similar to AC transformers. They
typically have a 100:1 turn ratio. The primary side of the
transformer is designed to operate at relatively low voltage
and high current. The secondary side of the transformer is
designed to operate at relatively high voltage and low
current. The primary can be thought of as the input side,
and the secondary can be thought of as the output.
The transformer is usually drawn this way:

The transformer can also be drawn other ways for

convenience:

Primary : Input : Low voltage : High current

Secondary : Output : High voltage : Low current
Note the secondary side of the transformer, in the case of
ignition coils, is drawn with more "loops", in the
schematic, than the primary side. This is just to indicate
which side is the high voltage side (which does actually
have more windings).
It should also be noted that, here, "low voltage" covers a
range of voltages, from that of the battery, which is
usually 6v, 12v, or 24v, but also includes "moderately
high voltage" spikes of up to 600v. "High voltage" is
actually very high voltage, and in the range of tens of
thousands of volts.
There are many different wiring configurations for such
coils. They may have one or two primary terminals (or
wires), and one or two secondary terminals (or wires). On
some coils, the case, or core, or any mounting point, may
be a connection, and may also be a junction for
connections to both the primary and secondary side. This
can lead to confusion on how to test such a coil, since the
connections to the transformer will not be readily
apparent, and may not be independent of each other.
This page will briefly discuss some of the more common
types of ignitions, and how to determine the internal
wiring of some of the different, common, coils.
Being like a transformer, the input and output have a
polarity relationship to each other. For most applications,
the polarity won't be important, but for testing, it may be
relevant.
This page will also discuss how to determine the
secondary (output) polarity relative to the primary (input)
polarity.
Ignition Background
Automotive ignitions come in many types. The following
descriptions are meant only as a brief summary so the
reader can get an idea of what type of ignition he/she is
dealing with. These descriptions are by no means
complete.
Kettering Ignition
The most common type of automotive ignition is probably
what is known as a Kettering ignition. This type uses a
steady DC supply, through a set of points, or a transistor,
to energize a flyback coil. A flyback coil generates a
magnetic field when a current flows through its primary
winding. This field stores energy. When the current is
suddenly interrupted (by points opening), the field
collapses, and the stored energy creates a moderately high
voltage spike on the primary side of the coil. The
condenser, which is just a capacitor, provides a slight
delay in the creation of this spike, thus allowing the points
to get open far enough to prevent the moderately high
voltage from arcing across the points. The coil, acting as a
transformer, then steps up this moderately high voltage
spike to produce a very high voltage spike on the
secondary winding. The secondary winding provides the
very high voltage to the spark plug to cause a spark.
Here is one, simplified example of a Kettering Ignition:

CDI
Another, very common, type of ignition is abbreviated
as CDI . This stands for capacitive-discharge-ignition. A
CDI uses an ignition coil, which can be much like the one
used in a Kettering ignition, but CDI differs from
Kettering in that the spark energy is not stored in the coil.
The spark energy is stored in a capacitor. This is done
when the capacitor is very quickly charged up to several
hundred volts, usually 300 to 600v. The power for
charging the capacitor may come from a charging coil,
near a rotating set of magnets on the crankshaft, producing
moderately high voltage pulses. Alternatively, the power
may be generated in a solid-state device producing
several, moderately high voltage pulses. Each pulse raises
the total voltage stored on the capacitor. This moderately
high voltage, stored on the capacitor, is then suddenly
discharged, usually through a silicon-controlled-rectifier
(SCR, which is a type of thyristor), which causes a large
spike of current to flow through the primary side of the
coil. The coil, in this case, is simply used as a step-up
transformer . The coil "steps up" the voltage to produce a
very high voltage on the secondary winding. This very
high voltage results in a spark at the spark plug.
Here are four, simplified examples of CDI systems:

The Magneto
Another type of ignition is called a magneto ignition. A
magneto is simply a coil which generates a single, brief,
precisely timed, pulse of AC current when a magnet
passes by it. (The charging coil, seen above in the CDI
ignition, can even be considered a magneto, although it
actually produces several pulses.) The magneto can be
thought of as the power generating portion of the magneto
ignition. Ignitions using magnetos can be categorized into
two different groups, described below.
Flyback Magneto Ignition (Integrated Magneto Coil and
Ignition Coil)
Some magneto ignitions use a coil as a flyback coil. In this
type, the ignition coil is integrated as part of the magneto.
Normally, the pulse of current produced would be AC, but
the current, in this ignition, is switched off by a set of
(opening) points or a transistor, just as the current reaches
a peak value. Thus, it's really a short DC pulse, which gets
cutoff abruptly, similar to that in a Kettering ignition. The
flyback effect produces the spark similarly to that in a
Kettering ignition. This type of magneto is often found on
lawn mowers where there is no battery.
Integrated magneto/ignition coils are usually easy to
identify by the curved portion of their core laminate. It is
curved to match the curved flywheel/rotor containing the
permanent magnet.
Here is an example of an integrated magneto/flyback
ignition coil showing the curvature of the extended portion
of the core:

Here is one, simplified example of a magneto system

using an integrated, magneto/flyback ignition coil:

Stepped-Up Magneto Ignition (Discrete Magneto Coil and

Ignition Coil)
Some magneto ignitions use the ignition coil only as
a step-up transformer. In this type, another, separate coil is
actually the magneto. This magneto coil is the one that has
a permanent magnet passing by it, and generates a pulse of
current. Like in the flyback type of magneto ignition, a set
of (opening) points or a transistor, switch off the AC pulse
at the right time, thus truncating the pulse into a brief DC
pulse. This abrupt interruption of the current in the
magneto coil has a flyback effect, producing a small pulse
of several hundred volts on the magneto coil. However,
the magneto coil does not step up the voltage to make a
spark. Instead this pulse is fed to the ignition coil, much
like how the CDI ignition operates, to step-up the voltage
to produce very high voltage at the spark plug. This type
of magneto ignition is often found on small motorbikes
where the igniton can operate without a battery.
Here is one, simplified example of a magneto system
using a discrete magneto coil and discrete, step-up ignition
coil:

Note on spark voltage, for all of the above ignitions:

After the spike in primary and secondary voltage, when
the spark begins to initiate, both primary and secondary
voltages drop down to an intermediate voltage for the
remaining duration of the spark discharge. This
intermediate voltage may be in the range of about 1/10 of
the initial spike, in open-air tests.

Internal Coil Wiring Configurations

Ignition coils come with many different internal wiring
configurations. Each configuration is not necessarily used
on only one type of ignition system. Some ignition
systems can use multiple configurations, or one
configuration may be used on multiple ignition types.
Note: The following configuration-type designations are
only valid for the discussions on this web page, and
related pages on this website. They are not an industry
designation of any kind. Designations found elsewhere
may be completely unrelated or contradictory.

Two-Primary / Two-Secondary ("Type A")

The type of coil that most resembles a transformer in
terms of wiring, is the Kettering type designed to fire two
spark plugs simultaneously. This type of coil has two input
terminals (or wires) and two output terminals (or wires).
Here is a typical example from a classic, inline-four
motorcycle. The motorcycle would employ two of these
coils to provide spark for four cylinders. This type of coil
is easy to identify since it has obvious connections to two
spark plugs, and two primary connections (or wires).
For the sake of this discussion, let's define this as "Type
A". That is, the the type having two primary terminals (or
wires) and two secondary terminals (or wires).
Type A :
P1 and P2 are primary connections. Both are terminals (or
wires).
S1 and S2 are secondary connections. Both are terminals
(or wires).
There should be no continuity from any primary
connection to any secondary connection.
There should be no continuity from any connection to the
case, core, or mounting points of the coil.
P1 to P2 will show continuity and will have very low
resistance.
S1 to S2 will show continuity and will have very high
resistance.
These coils are most often found on Kettering ignitions
where a distributor is not used. Traditionally, due to the
fact that points were a switched connection to ground,
flyback coils, like this example, must have both primary
connections isolated from ground, so they will always
have two, independently accessible, primary terminals (or
wires), neither of which will have an internal connection
to the case, core, or mounting point of the coil. Likewise,
since spark plugs are in contact with the engine, which is
usually grounded, the spark plug connections on the coil
must also be isolated from ground, and thus will not have
an internal connection to the case, core, or mounting
points of the coil. The secondary must also be isolated
from the primary since a connection to the primary is,
effectively, a virtual connection to ground. This is because
the voltage on the secondary is so high that any voltage on
the primary side will appear insignificant, and thus will
appear as a direct connection to ground, (to the spark
voltage).
These coils can also be used on CDI ignitions or even
magneto ignitions, as long as the resistance values are
appropriate. For use on those type of ignitions, either one
of the primary connections would simply be permanently
grounded, externally.
The connections for a Type A coil are easy to identify, and
relatively self-explanatory.
P1 and P2 can be chosen somewhat arbitrarily. unless
polarity is a concern.
S1 and S2 can be chosen somewhat arbitrarily, unless
polarity is a concern.
See the discussion below for details on polarity.

Two-Primary / One-Secondary ("Type B" and "Type

C")
Probably, the most common type of coil is that with a
single output for the secondary. The ubiquitous, bottle-
shaped coil, used on classic cars, is one of these. The
bottle-shaped coil is usually used with a distributor to fire
multiple cylinders.

Other common coils, with one output terminal (or wire),

are electrically the same as the bottle coil, though they
may appear physically different. They are often used on
motorcycles to fire a single cylinder. A motorcycle may
have more than one of these, each one to fire one cylinder.

For the sake of this discussion:

Let's define "Type B" to be the type with two primary
terminals (or wires) and one secondary terminal (or wire),
where the other secondary connection is the case, core, or
mounting point.
Let's define "Type C" to be the type with two primary
terminals (or wires) and one secondary terminal (or wire),
where the other secondary connection is connected to a
primary connection.
Type B :
P1 and P2 are primary connections. Both are terminals (or
wires).
S1 and S2 are secondary connections. S1 is a terminal (or
wire).
S2 is the case, core, or mounting point, of the coil itself.
There should be no continuity from any primary
connection to the case, core, or mounting points of the
coil.
There should be no continuity from any primary
connection to any secondary connection.
P1 to P2 will show continuity and will have very low
resistance.
S1 to S2 will show continuity and will have very high
resistance.
Type C :
P1 and PS are primary connections. Both are terminals (or
wires).
S1 and PS are secondary connections. Both are terminals
(or wires).
The primary and secondary windings both share the PS
terminal (or wire).
There should be no continuity from any connection to the
case, core, or mounting points of the coil.
P1 to PS will show continuity and will have very low
resistance.
S1 to PS will show continuity and will have very high
resistance.
S1 to P1 will show continuity and will have virtually the
same resistance as S1 to PS.
Because these are traditionally used on Kettering ignitions,
like the Type A coils, these also must have both primary
connections isolated from ground, so they will usually
always have two, independently accessible, primary
terminals (or wires), neither of which will have a direct
connection to the core, or case, or mounting point of the
coil. The difference, here, is that only one secondary
terminal (or wire) will be used to create a spark. Ground is
used to provide a path for the secondary current. The other
secondary connection, then, must be grounded. In Type B,
the secondary is directly grounded through the case, core,
or mounting point of the coil, being in contact with a
grounded part of the vehicle. In Type C, the PS terminal is
seen by the secondary as a ground. This is because the
secondary voltage is so high, any voltage on the primary,
relatively speaking, will be insignificant, and thus, PS will
appear as a virtual ground.
Like Type A coils, these coils can also be used on CDI
ignitions or even magneto ignitions, as long as the
resistance values are appropriate. For use on those type of
ignitions, either one of the primary connections would
simply be permanently grounded, externally.
A continuity test to the case, core, or mounting points, will
show if the coil is Type B or Type C.
The connections for a Type B coil are easy to identify, and
relatively self-explanatory.
P1 and P2 can be chosen somewhat arbitrarily. unless
polarity is a concern.
See the discussion below for details on polarity.
Type C battery test:
The connections for a Type C coil are not easily identified
with a continuity test. P1 and PS will appear electrically
the same to an ohm meter. To identify P1 and PS correctly
will require a small battery test with a volt meter. First
confirm there is continuity between all three terminals.
Then apply a AA battery to the primary terminals of the
coil. The battery voltage will drop significantly,
immediately, but should last long enough to complete the
test. Connect the voltmeter as shown in the diagram, and
measure the voltage on S1. The results of the S1 voltage
will reveal which primary terminal is P1 and which is PS.
If the voltage on S1 is near 0v, then the positive of the
battery is connected to P1.
If the voltage on S1 is closer to the battery voltage, which
should be somewhere between 0.5v and 1.4v, then the
positive of the battery is connected to PS.
The diagram also shows an "Alternative method". This
should be used to confirm the findings in the first method.

One Primary / One Secondary ("Type D" and "Type

E")
Coils designed specifically for CDI and magneto driven
ignitions are very common, though they are more likely
found on smaller motorcycles and scooters, and especially
on lawn-care equipment. The notable difference, with
regards to coil wiring, is that the primary side operates
with one connection grounded. This means CDI/magneto-
specific coils will only have one primary terminal (or
wire).
For the sake of this discussion:
Let's define "Type D" to be the type with one primary
terminal (or wire) and one secondary terminal (or wire),
where the other primary connection, and other secondary
connection, are the case, core, or mounting point.
Let's define "Type E" to be the type with one primary
terminal (or wire) and one secondary terminal (or wire),
where the other primary connection is the case, core, or
mounting point, and the other secondary connection is
connected to the first primary connection.
Type D :
P1 and PS are primary connections. P1 is a terminal (or
wire).
S1 and PS are secondary connections. S1 is a terminal (or
wire).
PS is the case, core, or mounting point, of the coil itself.
The primary and secondary windings both share PS
P1 to PS will show continuity and will have very low
resistance.
S1 to PS will show continuity and will have very high
resistance.
S1 to P1 will show continuity and will have virtually the
same resistance as S1 to PS.
Type E :
PS and P1 are primary connections. PS is a terminal (or
wire).
P1 is the case, core, or mounting point, of the coil itself.
PS and S1 are secondary connections. Both are terminals
(or wires).
PS to P1 will show continuity and will have very low
resistance.
S1 to PS will show continuity and will have very high
resistance.
S1 to P1 will show continuity and will have virtually the
same resistance as S1 to PS.
It should be pointed out that even though a CDI/magneto
coil, normally, can't be used for a flyback ignition without
making special provisions, a flyback coil can be used for
CDI/magneto ignitions, provided the resistance and
impedances are appropriate. It is just a matter of
grounding one of the primary terminals of the flyback coil.
However, this does not necessarily imply that flyback
coils and CDI/magneto coils have the same electro-
magnetic characteristics, even if their resistances and turn-
ratios are the same. One general difference is the physical
size. CDI/magneto coils tend to be smaller and lighter
since they are not required to withstand long periods of
active current flowing in their primary windings, as might
occur in some Kettering ignitions.
Since some magneto ignitions, usually found in lawn
mowers, integrate the magneto coil with the ignition coil,
(as explained in the integrated magneto ignition
description), it should be noted that even though the
physical structure of that type of coil appears unique, the
behavior of the coil is not very different from one with a
more conventional appearance. As a matter of fact, that
type of coil can be used with a CDI ignition, or with
discreet magneto ignitions where the ignition coil and
magneto coil are separate.
Here, again, is that example of this type of integrated,
"lawn mower", coil.

If special provisions are made to insulate the frame or core

of these types of coils, and it is deemed the electrical
characteristics are appropriate, even these coils can be
used in a conventional Kettering, ignition. They can also
be bench tested this way by setting the dwell
appropriately. Here is a photo of the lawn mower coil
being tested using a transistorized Kettering ignition
system.

The spark jumps from the plug wire to the frame (lower
left corner of the laminated frame in the photo). The wire
is held near the frame with a ziptie just to control the gap,
which is about 3/8" to 1/2". This tester uses a 12v supply
and a very short dwell. That black wire with the alligator
clip is not a permanent ground, it is the switched ground,
controlled by the tester circuit, to produce the appropriate
dwell. Details for the tester can be found elsewhere on this
site.
Type D, Type E, battery test:
A continuity test cannot easily be used to determine if the
coil is Type D or Type E. P1 to S1 will appear to have the
same resistance as PS to S1. To identify the coil type will
require a small battery test with a volt meter. First confirm
there is continuity between all three terminals. Then apply
a AA battery to the primary terminals of the coil. The
positive battery connection is connected to the primary
wire or terminal, and the negative battery connection is
connected to the case, core, or mounting point, of the coil.
The battery voltage will drop significantly, immediately,
but should last long enough to complete the test. Connect
the voltmeter as shown in the diagram, and measure the
voltage on S1. The results of the S1 voltage will reveal if
the coil is Type D or Type E.
If the voltage on S1 is near 0v, then the coil is Type D.
If the voltage on S1 is closer to the battery voltage, which
should be somewhere between 0.5v and 1.4v, then the coil
is Type E.
The diagram also shows an "Alternative method". This
should be used to confirm the findings in the first method.

One Primary / Two Secondary ("Type F")

This is another possible configuration, designed
specifically for CDI and magneto driven ignitions.
However, this one is designed to be used with two spark
plugs to be fired simultaneously.

For the sake of this discussion:

Let's define "Type F" to be the type with one primary
terminal (or wire) and two secondary terminals (or wires),
where the other primary connection is the case, core, or
mounting point.
Type F :
P1 and P2 are primary connections. P1 is a terminal (or
wire).
P2 is the case, core, or mounting point, of the coil itself.
S1 and S2 are secondary connections. Both are terminals
(or wires).
There should be no continuity from any primary
connection to any secondary connection.
There should be no continuity from any secondary
connection to the case, core, or mounting points of the
coil.
P1 to P2 will show continuity and will have very low
resistance.
S1 to S2 will show continuity and will have very high
resistance.
Electrically, it is very similar to types D and E described
above. The defining difference is that the secondary is
completely isolated. Other than that, it can be tested
similarly to the other CDI/magneto coils.
The connections for a Type F coil are easy to identify, and
relatively self-explanatory.
S1 and S2 can be chosen somewhat arbitrarily, unless
polarity is a concern.
See the discussion below for details on polarity.

Ignition Coil Polarity

The primary side of a transformer has a polarity relative to
the secondary side. Looking at the ignition coil simply as
an AC transformer, it too, has a polarity relationship
between the primary and secondary windings. The internal
wiring connections of the ignition coil were determined by
the above discussions. It will now be shown how to
determine the polarity relationship of those connections.
The polarity relationship can be defined different ways,
but there is a traditional convention. The normal
convention is; when an AC voltage, (of appropriate
frequency), is applied at the input of the transformer, and
the polarity of the output is chosen such that the positive
portion of the input signal produces the positive portion of
the output signal, and the negative portion of the input
signal produces the negative portion of the output signal,
the input terminals and output terminals are of the same
polarity.
Because transformers are AC devices, their polarity is not
usually labeled with + and -. Instead, a small circle or dot
is used to indicate the terminals with matching polarity.
(Note the polarity of the sinewave icons.)

Note: because transformers are inductive devices, their

input to output phase relationship is affected by frequency,
particularly by low frequency. This can alter the apparent
polarity if an AC signal is used and that signal is low
enough in frequency to cause a phase shift between input
and output. It is important to use test frequencies high
enough so that phase shifting won't occur (by any
significant amount), when using AC signals. A sinewave
with frequency above 200 Hz or so should be adequate for
most typical ignition coils.
Using Step Response to Determine Polarity
Applying a signal to the input of a transformer, and
viewing the input-to-output relationship on a scope is a
nice way to determine the polarity, but there is a simpler
way for the specific application of ignition coils. That is to
simply apply a DC voltage briefly to the input side of a
coil, and view the voltage polarity, as measured by a
voltmeter, on the output side.
When a DC voltage is suddenly applied to a circuit, the
way the output of that circuit reacts to this voltage is
called its "step response". If the step response voltage
were to be viewed on a scope, it might look similar to this:

Notice, the input step is a positive DC voltage suddenly

applied to the primary winding. The output "response",
when the output polarity matches the input, has a large
positive pulse, followed by a smaller negative pulse,
followed by a smaller positive pulse, and so on. If the
polarity of the output is reversed, the initial, large pulse,
will be negative. When a DC voltmeter measures this
response, the polarity of the largest pulse will determine
the polarity registered by the meter. Because DC
voltmeters measure average voltage, and the positive and
negative pulses, in the decaying sinusoidal reponse, nearly
cancel each other out, the DC voltage reading will be
pretty low, possibly on the order of 0.1v to 1.0v or so
(positive or negative). But because the initial pulse is the
largest, that will determine if the meter registers a positive
or negative voltage.
To determine the polarity, a AA battery can be used. Some
coils have a direct connection between the primary and the
secondary, so that should be used as ground, for
simplicity. Connect the battery's negative post to ground.
Then apply the battery so the positive can be connected to
the other open primary connection. Connect a meter so the
black lead is connected to ground, and the positive is
connected to the open secondary connection. Set the meter
to read about 1 to 10 volts max, and make sure the
negative sign can be seen easily in case the result is
negative. Right when the battery positive is connected, see
if the meter registers a positive voltage or a negative
voltage.
If the meter registers a positive voltage pulse, then the
primary connected to the battery's positive post, and the
secondary connected to the red meter lead, are of the same
polarity.
If the meter registers a negative voltage pulse, then the
primary connected to the battery's positive post, and the
secondary connected to the black meter lead, are of the
same polarity.
Note: opening the switch will have the opposite effect, so
that can be used as an alternative method as well. In other
words:
If closing the switch gives a positive voltage pulse on the
meter for a given polarity relationship, opening the switch
should give a negative voltage pulse, (confirming the
polarity relationship).
If closing the switch gives a negative voltage pulse on the
meter for a given polarity relationship, opening the switch
should give a positive voltage pulse, (confirming the
polarity relationship).
As noted in the diagram, the results are the same
regardless of the switch position, as long as the battery
polarity does not change. This means, in
a Kettering ignition, the polarity seen on the meter when
the test switch opens, is the same polarity that will be seen
when the coil is in operation when the points open, or the
transistor cuts the primary current, (as long as the vehicle
battery is applied to the coil with the same polarity as the
AA battery in the test).
Also note: there may be a small DC voltage registering on
the meter if the battery is left connected. It can be positive
or negative, depending on the wiring configuration, and
may indicate some dirt is on, or in, the coil causing a small
amount of continuity. Shorted windings in the coil could
also cause this, but the coil would likely have other
symptoms if this was the problem. This voltage should be
small enough to be ignored. This should not affect the test
because this value should be much smaller than the pulses
created during the test. Thus the test will show correct
positive or negative voltage readings, and will still be
valid.

Primary / Secondary Current and

Superposition
Among the different configurations for the coil wiring,
there are four cases where the primary current and the
secondary current actually flow through the same
conductor. And in two of those cases, the secondary
current actually flows through the primary winding. In
some cases, due to coil polarity, the secondary current, or
secondary voltage, oppose those of the primary, and in
some cases they enhance it.
With that in mind, it raises the question of how does this
affect the spark. It is beyond the scope of this page to
detail the concept of "superposition", but the
Superposition Theorem does explain how two
simultaneous currents in a single conductor can be
resolved to a single current, or two voltages across a single
resistance can be resolved to a single voltage. But that
may not necessarily predict how significant the effect will
be in practical terms.
In order to investigate the effect, the four different circuits
were built, all using the same parts. The coil (Dodge Neon
coil-pack) was a dual output, low resistance coil, which
could be wired in each of the four different configurations.
Results
There was no difference, in the sparks, which could be
detected. Using 5/8", and later 3/4", gaps, the brightness
and loudness of the sparks seemed apparently the same.
When measuring the voltage on the primary side, using an
oscilloscope, the estimated, average discharge voltage, and
average discharge duration, was the same for each
configuration. The measurements were not extremely
precise, as sparks are generally somewhat erratic by
nature, but in observing the duration and voltage, none of
the four configurations showed any observable difference
from any of the others. That is not to say that there is no
difference, but that the difference could not be detected. A
difference would be expected. A more precise experiment
may be required to detect it.

(The diagrams are shown using points for illustration

purposes only. The actual test igniter was an IGBT
device.)
With a 5/8" gap, spark duration was about 973 usec, and
primary discharge voltage was about 37.8v.
With a 3/4" gap, spark duration was about 780 usec, and
primary discharge voltage was about 45.0v.
The primary discharge voltage is the approximate, average
voltage on the primary winding, after the initiation of
spark, during the remainder of the spark. The primary
voltage just before the spark initiates will be several
hundreds of volts.
For reference, this is a very rough sketch of the primary
signal for all four configurations, with a 5/8" spark gap.

These results are almost anecdotal since they are only for
one set of conditions on one specific coil type. But it does
show, at the very least, for one instance, the different
configurations have negligible difference on spark voltage
and duration.
It should be noted, however, that polarity, at the spark
plug itself may show some differences in actual use on a
vehicle. This is because, in use, the temperature difference
between the center electrode, versus the ground electrode,
may result in less resistance to arcing if the polarity is set
so the center electrode (hotter) emits electrons. With
regards to the polarity determination test, seen above, this
means the polarity of the spark plug wire should be such
that it gets a negative pulse when the primary current is
cut.