[lust4speed]

The photo is from the MSD Digital 6 Plus installation manual. Only interesting thing here is they only corelate compression levels to recommended spark plug gap. I keep my cast iron headed engines at 9.2 to 9.3 compression and find that .045" gap works best. Basically the wider the gap the hotter the spark and that helps in lower compression engines. My old drag engines in the 80's were over 13:1 and we kept the sparkplug gap at .035". High pressures and wide gaps don't work that well. Note that GM basically went to the .060" gap on their ridiculously low compression engines which were sometimes under 8:1.

Anyone remember the old Victor sparkplug cleaner and tester? The test part was really to sell customers new plugs. The coil in the tester was rather anemic and one could quickly see that the main difference between a plug firing and not firing was how high the adjustable air pressure was set. The mechanic would leave the gap huge on the old plug and would make sure the new plug was gapped tight. Old plug would fail to spark about half pressure while the new plug could fire with full air pressure. Of course you could take the old plug and put a tight gap on it and open the new plug up and get exactly the opposite result. I'm sure that machine sold thousands of new plugs whether they were needed or not.

Anyway, back to gapping plugs. There's something to be said for a wider gap cleanly firing off a low compression mixture -- and also something said about a high compression engine running race gas not needing or wanting a large gap.

Edit: Opps, Vixen and not Victor