A .045 gap on a plug with a clean, squared-off center electrode will fire at light throttle at around 6-10 KV. On snap acceleration the firing line on a scope will jump up to around 15-18 KV. As the plug wears and the electrode gets rounded off and the gap gets wider, the firing voltage will go up to around 15-20K during light load operation. When the loaded voltage approaches 30-35 KV the plug will develop an intermittent miss. Leaner than normal mixtures will require higher voltages to fire them. In the late 1980's GM experimented with extremely lean mixtures and a plug gap of .080" in hopes that enough combustible mixture would find its way between the center electrode and ground to light off. Those engines would miss if the gap was set to 35-40 thousandths, but would run pretty good at 80. Firing lines in the high 40KV ranges were pretty common on those engines. Even slightly dirty or worn plugs would miss.
Jerry

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