Download An explanation of standard vs high performance ignition coils

Standard vs High Performance Ignition Coils
A standard 12V coil has a primary resistance of about 1.5ohms which can be verified with a
DMM. This coil typically produces about 18,000V of output, depending on the size of the spark
plug gap. If the spark plug gap is too small, the plug will fire at a lower voltage, producing a
cooler spark, and if the gap is too large, the coil may never reach sufficient voltage to fire the
plug causing a rough running engine. In the mid 70’s when the government was requiring
additional emission controls, Triumph engineers found that the engine became hard to start as a
combination of engine friction, and voltage drop caused by the starter motor. During this crucial
starting period, a stronger spark was needed. They came up with a clever solution, that has
confounded owners, and mechanics ever since. The solution was to switch back to a 6V coil
(1.5 ohm prim resistance with 18,000V output) which had been used years earlier in the 6V
cars. Here comes the really clever part. They connected two wires to the positive side of the
coil. The first was a normal wire which was connected to the starter relay, and only provided
voltage while the starter was engaged. This voltage was the standard 12V. What happens when
you run a 6V coil at 12V, the output voltage doubles to 36,000V. A nice hot spark, just when you
need it most and if the battery voltage dips while driving the starter, there is plenty of margin
above the nominal 18,000V required to fire the plugs.
Why not just run it this way all of the time? The coil was not designed to run at this higher input
voltage all the time, and it will eventually overheat and fail. How did they prevent this? The
second wire connected to the positive side, was not a standard wire, it is called a resistance
wire, or ballast resistor. This wire produced a resistance of about 1.5 ohms. This resistance in
series with the primary resistance of the coil, creates what engineers call a voltage divider
circuit, and the coils sees only about 6V. So, once the starter disengages, the 12V connection
opens, and the coil now goes back to it’s standard 6V mode producing 18,000V, and it lasts for
years.
You are probably asking “why is this of interest to me?” Well it may not be, however, if you have
any interest in installing a sport coil as I did, this is all very relevant. Sport coils are designed to
produce 40,000V all of the time, giving us a much hotter spark, and presumably a better and
more complete burn in the cylinder. The Sport coils typically are rated for continuous
operation12V systems. So what happens when we simply install one in our Triumph using the
original wiring? During the start cycle, it sees the 12V input and produces the 40,000V like the
original, and after the start cycle, it sees 6V input, and produces 20,000V like the original, not
much of an improvement.
“Oh I see”, my original coil was a sport coil. Not quite. The original coil could tolerate the 12V
input for short periods like during starting, but could not tolerate it for very long. The sport coil
can tolerate the 12V input, all day long. “Why does it behave the same in my car” you ask.
Because of the resistance wire. The resistance wire is dropping the input voltage to 6V when
the engine is running, and the sport coil does not require this, in fact, it diminishes the benefit of
the sport coil. How can we fix this, and get the most from our sport coil? Several ways, replace
the resistance wire (pink on our cars) with a standard wire. Run a second wire from a voltage
source to the + terminal on the coil (make sure that voltage source turns off with the ignition key,
otherwise the coil will be on all of the time), or simplest of all, the other wire on the coil which is
white with a yellow stripe(W/Y), is connected to the starter relay which is right next to the fuse
panel. Remove the W/Y wire from terminal C4 of the starter relay, an plug it onto the unused
terminal next to the white wire on the fuse box. If this is occupied, find another terminal that
switches on/off with the ignition switch. This causes the coil to be powered by 12V all of the
time, and will not harm the resistance wire which remains in place in the event, you ever want to
bring the car back to it’s original configuration.
Standard vs High Performance Ignition Coils
The second thing you need to do when installing a high voltage coil is to increase the spark plug
gap. The standard plug gap of 0.025” is based on the standard coils output voltage of 18,000V.
If you leave the standard gap, as soon as the new coil’s output reaches 18,000V, the plugs will
fire resulting in little to no benefit. In order to take advantage of the higher voltage, you will need
to increase the spark plug gap to 0.30- 0.35”. Some experimentation will be necessary, but you
are looking for the largest gap that still gives you a smooth idle.
Now, before you run out and buy a sport coil, there are still a few more things you need to know.
The standard coil has a primary resistance of 1.5 ohm puts out 18,000V and is designed for 6V
systems even though our cars have 12V systems (this is not a problem due to the ballast wire).
One very important consideration is your ignition system, points or electronic. Points can
tolerate more current than an electronic ignition module. The standard coil draws 6V/1.5 =4A
while running. The ignition modules prefer to be exposed to less than 5A, so the standard coil
works fine. The module can tolerate the slightly higher currents for relatively short periods of
time such as during starting.
Here is where things begin to get a bit confusing, not all Sport/High Voltage coils are created
equal, in fact some can damage your Ignition Module. They produce the high voltage output, but
require different amounts of input current. The table below contains a few examples; As you can
see, the MSD will overheat your Ignition Module in short order even if you leave the ballast wire
in place, running it at 6V. The Accel coil is safe if you leave the ballast wire in place, but
bypassing the ballast wire increases the current to a dangerous level of 8.5A which will shorten
the life of your Ignition Module, perhaps substantially. Since the Accel is rated at 12V, it should
be safe to run with the ballast wire bypassed in a points based system, but I found it ran very
hot, and was concerned about premature failure. In addition, I did not feel it contributed
significantly to the car’s performance.
I later decided to add the Electronic Ignition Module which forced me to restore the ballast
resistor (back to original condition) in order to prevent the coil from overheating and damaging
the Ignition Module. The Pertronix coil seems to be the only one that can be used safely with the
Ignition Module, and still produce the higher voltage. What have I learned from this experience?
In my humble opinion, the Ignition Module made a far more significant improvement to the car’s
performance than the high voltage coil did. The engine idles much more smoothly, and seems
to have much more power throughout the range. The new Accel coil is running at 21,000V
which is a nominal improvement over the stock coil, but that unit was quite old, and was
probably not running at peak performance any longer. I am not typically a big believer in
performance upgrades, but the Electronic Ignition has earned my highest recommendation.