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Originally Posted by Tireiron
5psi should be fine with the stock heat range of the plugs. If you were up over 10psi I would look towards the next colder heat range. On my S/C setup at 12psi I am running one step colder plugs.
If you find yourself getting pre-detonation under boost than go to a step colder plug which would be a 7 instead of a 6 in the part number.
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A mixture of terms here that is confusing. I think you mean pre-ignition, when the charge lights from the hot plug, before the spark does, no sound, just ends up with a hole in the piston.
Pre-detonation is the start of detonation and the cooler plug won’t help it, just too much pressure/temperature.
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If you find that you are having spark blowout and misfiring under boost than you would want to decrease the gap of the plugs. Again I run a .030" instead of the stock .045". .030" is the gap of the wrx and sti. The OEM coil packs are decent, but they are not as strong as the newer model COP. So far I have not had issues with my ignition with the setup I have, but I also do not have much mileage or run time on the setup as I am still finishing my build.
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I agree with what you are saying here, but there is a reason for those different plug gaps, a stock car needs the wide gaps to run well, but the car with a different fuel map does not need them.
Since the 70s, The Auto Industries have had to use very lean mixtures, especially at low rpm and idling, it is because of this that they have to use the gaps that they do.
Contrary to popular belief the spark does not jump the gap. The hi-voltage that the coil develops across the gap has to ionise the fuel to become conductive, so that the current can flow across the gap. With such a lean, patchy, stratified, mixture in the chamber, the gas is harder to ionise, higher voltages are needed so the gaps are increased so that a higher voltage is developed.
There also has to be a higher level of energy to prolong the flow across the gap, as if the spark is too fast, there may not be any combustible gas between the gap, so the spark has to last long enough for some gas to be lit. This is why a standard car has to run the .40” plug gap.
If you have changed the fuel maps to have ‘normal’ mixtures, then there is no need to use those big gaps. A plug gap of .25” takes about 12K volts to ionise the gas, and only about 2k volts to flow the spark., as a lower voltage will do it, so .25” to .30” will lower the voltage. The coils will have less shorting, and insulation breakdown problems, that they have when they have to run 45K volts on a .40” gap.
Harvey.