Then post references on how pressure or vacuum affect arc flashes
Vacuum??? When does a running engine ever have vacuum during the spark-plug firing? Even if there's vacuum in the intake manifold, the cylinder has pressure as the compression stroke takes place and the plug fires.
Compression is easy. In simplest terms, air is an insulator. When you compress air, you're packing more insulation between the two electrodes. Around here, 150-ish psi cranking compression is pretty normal for older V-8 engines. Normal barometric pressure is on the order of 14.7, so cranking compression would be about ten times that amount.
You'd need to open the gap ten times farther to require the same voltage to ionize the air. This is why they sell spark testers with enormous gaps. There's many designs for spark testers, my favorite looks like a spark plug, but with no side electrode and a ground clamp welded to the side. (see below)
Here's a proper gap for an HEI ignition--although they sell similar spark testers for ballast-resisted ignitions. The difference is that the HEI-style has a recessed center electrode which increases the gap even more.
https://www.amazon.com/OTC-6589-Ele...Spark+tester&qid=1568047819&s=gateway&sr=8-12
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Below is a photo from a troubleshooting guide I wrote for HEI ignitions. The guide references the older 4-pin module "large cap" style of HEI, but most of the troubleshooting tips can be applied to the small-cap TBI HEI.
https://www.chevelles.com/techref/ftecref5.html
HEI style on the left, ballast-resisted style on the right. Note recessed center electrode on HEI style.
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