Originally posted by DemonSVT:
Gap size has nothing to do with a plug's heat range.

Decreasing the gap size is a way to increase the energy of the spark kernel which makes it harder to "blow out" under the greater cylinder pressure of forced induction.

The .035 range is a typical gap to start with.



Correct on the first point but.......

Here's my take on the whole thing:
Reducing the gap doesn't increase the energy of the spark, it only allows the dielectric effect to be reduced enough for spark to form again. It actually decreases the ability of the spark to ignite the charge due to the reduced area and will eventually reduce the energy of the spark when you shorten the gap too far. The closer the anode is to the cathode, the less voltage required current to jump the airgap. An extreme example is to bend the prong until it touches the electrode. When it touches it is considered a short...lots of current flows but no spark.

The ignition system can produce some theoretical maximum energy, we'll call it TE for total energy. In this example
TE= Egap + Epres.
Egap varies with distance and Epres (or dielectric const) would vary on charge density in the cylinder.

If you drop one without raising the other you get a lower spark energy. IF you drop Egap too far and raise Edieletric to far, you have too little area for the spark to ignite the charge.

A wider gap is best when possible!

Wider gap requires lots more potential for the current to jump the gap. Air is an insulator (so is fuel) so that also makes it harder for the spark to jump the gap. If you increase the boost you are increasing the insulation effect, added more dielectric material between the anode and cathode. This also requires more voltage to jump the gap. The extreme example in this case is you pack so much air and fuel into a cylinder that it would be the same as sticking a piece of insulation between the electrodes.

Conclusions:
Spark isn't "blown out", it fails to jump the gap. The gap is decreased to compensate for increased dielectric effect under the new cylinder pressure. This in turn results in a smaller area for the spark to ignite the charge and eventually results in a lower energy spark when the gap gets too close.

Best to leave the gap stock unless you misfire, then only decrease it by a few thousandths each time until the misfire goes away.
OR, get higher energy ignition and leave the gap stock.

I have run .045-.050" almost the whole time I have been turbo with the exception of when I first put it together. AT that time I ran .035-.040" under other peoples advice because I had no first hand experience then.
It worked well enough when warm, but a few misfires on cold mornings taught me the lesson that the gap was too short to properly ignite the fuel on cold mornings.

I run .045" now as a rule since our ignition system is quite strong stock. No issues with misfires related to spark.

Former owner of '99 CSVT - Silver #222/2760 356/334 wHP/TQ at 10psi on pump gas! See My Mods '05 Volvo S40 Turbo 5 AWD with 6spd, Passion Red '06 Mazda5 Touring, 5spd,MTX, Black