My understanding is that a compressed fuel/air mixture has a significantly higher resistance than open fresh air at atmospheric pressure. I do not know what the numbers are, but the 1/4" gap is supposedly on the conservative side of a good spark for a typical magneto ignition to fire under compression. It is a rule of thumb that seems to work for me. I have had a few engines that would make a plug spark in open air and look OK but would not fire at all under compression. If it will jump a 1/4" gap in open air and the plug is good it has always been able to fire under compression for me so far. I
I would have assumed that the fuel mixture in the compressed air would have also increased the resistance beyond what simply compressing the air would. I do not know for a fact what effect the fuel mixture has on the resistance across the plug gap though. If the subject ignition tester actually tells a person anything, the fuel must not have much of an effect.
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