Compression Testing Issue

[Jack Klarich]

Rod, mine is an older model with a release on the gauge

 

[Bodeen]

Come on guys, I qualified my post recognizing that the comparison between hydraulic and air pressure was illustrative of a principal and not a direct comparison. While it is true that air is compressible, it is until it isn't. Not sure how volume has anything to do with measuring pressure. Make the line pumping oil to the pressure gauge larger to pump more volume but the gauge will read the same pressure.

CCP is what it is and the distance between the spark plug hole and the gauge does not change CCP.

I agree completely, I was just responding to this part of the post.

But the fact remains: the readings were about 30 psig different and the only tangible explanation is the larger volume of air trapped in the 18" hose adapter compared to the shorter metal adapter.
What else could it be?

Proven of course by Jack finding zero difference in different lengths of hoses...