I have told the story several times about running a glow plug K&B 4011 on Davis 1/2 A fuel. Started with the glowplug lit, took the battery off after maybe 20 seconds. One engine ran fine, other undercompressed, but would run fine with glowplug lit. Unless the unlit glowplug was contributing some heat (don't think so, but don't know) the glowplug compression was fine for that fuel.