I've always been told to disregard the number on the compression gauge, and just look for no more that 10% variation between the high number and the low number. Someone once explained why the actual psi number can't be trusted, but it has done slipped my mind.
As an aside for gizmo heads, large diesel engines often have pressure taps so that you can attach a gauge and check cylinder pressure while running. Really big engines (the three-story tall ones that power ships) have cylinder pressure gauges permanently hooked up.
In the good old days they had a gizmo like a recording barograph (revolving drum with paper on the outside, pen connected to pressure bulb, so the pen moves with pressure, and marks it on the moving paper) anyway, one of those, connected to the combustion chamber, and would make an "indicator card" showing the profile of pressure rise with compression, then pressure spike with injection/combustion, then pressure fall as the cylinder stroked, then pressure drop as the exhaust valve opened. One boat I worked on had an english Sun-Doxford engine, with all this gizmo, and in the engine manual there were indicator cards from the engine's acceptance trials.
Now they do this with an ultra-sound thing that bounces sonic beams through the engine, and instantly plots out a pressure curve on the display of your PC. The theory being that air under pressure is denser, therefore transmits sound faster, while the rest of the iron, coolant, etc, transmits at the same rate. For something like $15,000 you could be the first kid on your block to have one, and amaze your friends.
What will they think of next.
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