spark plug gap?

[guest]

Mike, dumb ? here. If you have a good ignition system is it better to run the spark plug gaps a little wider? I normally run mine at .040 but was thinking maybe a little wider like .044 would get a better burn. This is for a mild build TC95 with DTT ignition. thanks

[Mike]

Plug gap is mostly a function of cylinder pressure when everthing else is equal (available spark energy). As the gap gets wider, more of the spark kernel is exposed to the cylinder charge but less energy is available. A closer gap provides more concentrated energy but a smaller kernel. The energy must be high enough to overcome the resistance created by the cylinder pressure for the gap being used. The higher the cylinder pressure the more energy required to fire across the gap. Current thinking is that you want to run as wide a gap as possible. With the spark energy and cylinder pressures in an average street TC95 a gap of .040-.042 seems to work pretty well. In a 14:1 comp motor a gap of .025-.030 may be more appropriate. Fuel quality also plays a role, among other things.
Thanks, Mike

[guest]

Mike, thanks.

[pwr96]

Hi Mike,I heard about shimming spark plug so as to have gap opening toward intake valve also filing about half of the metal back above the electro tip will give a better spark.What do you think?Thanks for you time and knowledge.

[Mike]

These things help as compression goes up but is difficult to measure on the dyno. Many plugs feature the pointed electrode and so I would reccomment one of them vs modifying (Bosch Platinum Plus). Indexing can't hurt but I've seen three theories on the correct orientation, leading me to believe that it really has a small affect. If you were to index, pointing the open to the center of the combustion chamber seems to be the most common recommendation and this is how we've done it over the years.
Mike