Originally Posted by F100cleveland
I'm running a full MSD ignition, Billet dizzy, Digital 6 plus box, and HVC coil. I have about 10.8:1 compression and run an Autolite AR-25 plug. I had the gap set at .060" and it was fine, didn't get extinguished on the dyno, that I know of, but I decided to put it down to .045" after reading about different gaps. Well, it idles a lot worse now with the smaller gap than it did with the bigger gap. Do I go back to the 0.060" or can I go bigger or ????
Here's why your idle is better with the larger gap. At lower RPMs, your ignition system fires multiple times in a very short period. The larger gap slows that down somewhat by taking longer to arc across the poles. That "huge" spark ignites the mixture at lazy RPM and the multiple spark discharges "keep" the spark there for a rather long time. What you didn't say was where you have your timing set for initial advance. By closing the gap, you would likely need to retard your initial somewhat to bring back the idle quality.
You also didn't say where you have your total advance set and at what rate you have it coming in by...so there's really no way to get an idea of how much gap you can live with, however, I'd think that 60 is enough. I usually run 50-55 and I'm using a 7-series box.
Plug gap is just part of ignition timing. It is often overlooked because the time that electricity takes to bridge a gap of .010 versus even 1 inch is so minute that we need exponential notation to even calculate it. However, on MSD systems, the spark "remains" bridged across the gap of the plugs for a very long time (relative to a single spark system, like common points type ignitions with standard coil and no "black box") and that spark "face" greatly aids in low speed combustion by creating and keeping a solid flame front in the chamber.
Every serious racer should be indexing their plugs and gapping for their timing conditions.
...and, "bigger" probably isn't the way to go, especially at higher RPMs where the MSD "effect" deteriorates into a single spark. Too big and you'll have problems, especially unburnt fuel and even possibly predetonation and/or multiple flame fronts in the chamber (pinging under load) that will cause more problems than would make sense with a reasonable gap. Don't get me wrong, there could be something to be learned in experimenting with gaps in your combiation...but, if you do not start with your plugs indexed, then what's the use in even exploring that path? To begin experimenting, you need a reasonable starting point that is consistently reproducible. You can't obtain that without some forethought. You also need some manner of measuring results whether negative or positive...the seat of your pants is probably not going to get it very accurate and it could be possible to introduce some harm to your engine without recognizing it until that all too famous "too late" occurs.