compression test at altitude

[aynew1]

I just did a compression test on a gl1100 I bought about a month ago, all cylinders where within a couple of lbs of 135. i was thinking this was kind of low, then i was reading an article about octane ratings and high altitude and it dawned on me that it should be lower at 7000 ft but i don't know how much. does anybody know how much difference this kind of altitude would make?

[WingAdmin]

First, you have to find out the pressure for your altitude:

This shows that the atmospheric pressure at 7,000 feet ASL is about 11.25 psi.

Next, use this formula:

measured compression pressure * 14.7 / ambient pressure = corrected compression pressure

So in your case, 135 * 14.7 = 1984.5
And then, 1984.5 / 11.25 = 176.4

So your corrected compression number is 176.4 psi.

[aynew1]

awesome, thanks alot