We run all of our race engines and high performance street engines equipped with MSD at .035". Reason for this is as follows:
Plug gap is not just a function of how much voltage you have available from the ignition source (MSD, HEI, or your points/coil). It's a function of how much resistance there is across the plug gap: As cylinder pressures get higher (from improved volumetric efficiency and the compression ratio of a performance engine), the resistance across the plug gap gets higher. Once resistance across the gap goes above a certain point, the plug will stop firing and the spark will jump the next lower available resistance gap, which is usually inside the distributor cap or off the plug wires/boots. It doesn't matter how good your ignition system is - the spark will quit firing the plug and start firing your plug wires.
I saw an excellent demonstration of this while doing dyno tuning at the Westech Performance facility in LA - they have a distrubutor machine set up with a pressurized spark plug tester so you can actually see the plugs firing (...and NOT firing) with varying cylinder pressure. Using an MSD distributor with MSD box, I ran the machine while cranking up the pressure on the spark plugs. At .060" gap, the plugs stopped firing at 180 psi cylinder pressure and the wires started arcing from the available MSD voltage. By decreasing plug gap to .035", cylinder pressure could be increased to over 250 psi without the plugs misfiring.
If you have a real performance engine, run plug gaps at .035". If your engine has only modest compression and does not have good volumetric effeciency, you can run wider gaps since you don't have enough cylinder pressure to cause a problem. GM ran wide plugs gaps on their HEI cars because the compression ratio on those cars was down in the 7.5:1 range. Just because you have HEI or MSD does not mean you can run .050-.060 gaps on a 10.5:1 engine with much success...
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