Altitude and Compression

[ColoradoSteve]

Just did a compression test and all four cylinders said 120. I can't find anything to tell me what the compression should be, but I thought it ought to be 150 or higher? I'm at an altitude of 8300', is that a factor? The car runs great and doesn't burn any oil so I'm not concerned, just curious.
Steve
'83 944

[ERAU944]

correct, you will have less pressure in the cylinders as a result of being at that altitude. the simple explanation is that there are less molecules of air to compress per volume, so you're going to see lower numbers.

ISA (International Standard Atmosphere) is 29.92 inches of mercury (or 1013.2 millibars) at 15 degrees celcius/59 farenheit at sea level. there's gotta be a reference somewhere that'll tell you approximately what ratio you should be seeing at that altitude... but 120 sounds like it should be about normal to me... the important thing is that your cylinders are even in compression, which they are!

iirc at 10000 ft the millibar level is approximately 700 or so...

[alfadoc]

In Denver we add 20% to compression test and vacuum readings in order to equal sea level. We also subtract 20% from published horsepower figures. :-(