compression test at altitude


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aynew1
Posts: 9
Joined: Sun Jul 04, 2010 10:19 am
Location: Denver, Colorado
Motorcycle: 1980 gl1100 interstate

compression test at altitude

Postby aynew1 » Sat Sep 04, 2010 10:10 am



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?



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WingAdmin
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Re: compression test at altitude

Postby WingAdmin » Sat Sep 04, 2010 10:56 pm

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
Posts: 9
Joined: Sun Jul 04, 2010 10:19 am
Location: Denver, Colorado
Motorcycle: 1980 gl1100 interstate

Re: compression test at altitude

Postby aynew1 » Sun Sep 05, 2010 5:03 pm

awesome, thanks alot


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