I've seen aftermarket "high voltage" coils that output 40kV, but what is the output voltage for the standard OEM coil pack?

I want to use a spark tester with an adjustable gap to check the strength of the spark, but I need to know the rated voltage.

EDIT March 19, 2016:

There was an insert with the spark tester which had voltages for various Ford, GM and Chrysler systems listed.

The two Ford systems listed were between 28-34kV. I figured since Mazda's and Fords used to use the same ignition systems a lot of the time, I'd set mine to approximately 30kV and see what happened.

At a 30kV gap, I got what looked like weak thin orange sparks.

I was doing this at night in the shade of the engine hood. Of course, I've never done this before and don't know if this is the right voltage or what a healthy spark should look like.

  • Could you measure it with a DMM? There are some that will read that high. Don't use the standard one from the auto parts place I suspect that's way above it's rated voltage. The duration is short but there are also DMM that will record the max. But now you're talking about a more expensive DMM.
    – cdunn
    Commented Mar 18, 2016 at 14:20
  • set the tester to around 25kv and go from there? Commented Mar 18, 2016 at 16:09

1 Answer 1


So I found a really cool page on the Megasquirt site ( they sell hobbyist EFI systems ) which let's you calculate the breakdown voltage needed to jump the gap of a spark plug in the combustion chamber.

They list a whole bunch of calculations and assumptions that go into their breakdown voltage calculator, which I can't copy to here, but the calculator takes four inputs: plug gap in inches, intake pressure in kPa, the compression ratio and the intake air temperature.

So, the compression on the European version of the 2L Mazda FS engine used in the 626 is 9.7, I'm using 1.1mm gapped plugs which is 0.043 inches, and at idle my engine draws 18.5 inches of mercury which comes out to about 62.65 kPa. My ignition system is wasted spark, so it has to fire two plugs at once, one on the compression stroke and one on the exhaust stroke. The voltage needed to fire the exhaust stroke plug is about 2kV according to what I've read.

So at idle on a warm day, say 35*C air intake temp the breakdown voltage will be about 14kV plus 2kV for the waste spark which gives about 16kv total.

However, those are the easiest conditions. If I look at wide open throttle ( atmospheric pressure of 101.c kPa ) on a below freezing day of say -5*C then the number jumps way up to 22kV + 2kV giving about 24kV.

Now these are the absolute minimum needed to initiate breakdown and ionization of the AF mixture across the gap so current can start flowing to ignite the mixture, but what about things like quenching?

So to account for the extra energy needed to avoid quenching I'm magically going to pick a number out of the air and say let's add 25% to that minimum number of 24kV and arrive at 30kV, which means if I see any spark at all on my adjustable gap spark tester when it's set at 30kV then I'm guessing I'm probably going to be OK.

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