[mwebb]

since most people are not familiar with running compression waveforms ...

there is three stacked up the bottom one is "good" relative to the upper two , it shows 720 degrees of engine rotation and what pressure or vacuum is present in the combustion chamber at what degree of engine rotation

720 degrees is two complete turns of the crankshaft

there is no spark , there is a pressure transducer in the spark plug thread connected to a digital storage oscilloscope .

below zero is vacuum , above zero is pressure , in the combustion chamber that fluke pv350 pressure transducer gets zero set to local ambient barometric pressure prior to capturing a waveform which is zero on the screen s