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Unread 03-11-2009, 11:12 AM   #1
Hybrid Jeep
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Atmospheric Pressure effect on Compression Testing

Hey guys.. I just picked up a 74 Wagoneer with a running AMC 360 for the engine to drop in my 76 CJ5.

Anyway.. I'm at about 6500 feet here in Colorado Springs.. does the altitude have an effect on my readings? I've heard that it does have an effect on vacuum readings.

It would make sense to me that an altitude would effect compression as well.

Basically I checked all cylinders and they are at about 110-120 or so. My service manual says 140 is correct. I've heard that at my altitutude (at least for vacuum readings... you will show 83% of the true reading)... so 116ish would be correct for 140 at sea level.

Am I right in my thinking here?

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Unread 03-11-2009, 11:28 AM   #2
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Whoops.. looks like I answered my own question:

Altitude and temperature also affect the compression readings. Manufacturer’s specifications are almost always given at a specific altitude (14.7 psi at sea level), and 59° Fahrenheit. Both temperature and barometric pressure change as you go up in altitude, so you will need to correct your measurements if you wish to compare it with a factory specification. The following chart provides conversion factors for correctly compensating for changes in altitude:

Compression Test Altitude Compensation Factors

Altitude Factor
500 0.987
1500 0.960
2500 0.933
3500 0.907
4500 0.880
5500 0.853
6500 0.826
7500 0.800
8500 0.773
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Unread 03-11-2009, 11:36 AM   #3
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I never thought about air pressure affecting cylinder compression but I guess it would have to. According to this chart, 6000ft is about 80% of sea level. The AMC 258 has a compression ratio of 9.2:1. That works out to 134psi so I don't know where the 140psi comes from. If atmospheric pressure at 6K ft is 11.8 psi that would make a compression of 109psi. So I think you are right.

http://www.engineeringtoolbox.com/ai...ure-d_462.html
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Unread 03-11-2009, 11:39 AM   #4
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Yeah, it's the ideal gas law.
PV = nRT

pressure x volume = constant x temperature

Or pervert = nert as my chemistry prof used to say.
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Unread 03-11-2009, 01:59 PM   #5
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I was thinking, yeah, but the air pressure on the outside of the pressure gauge is also at 11.8 PSI so you would have to take into effect also. But then I thought I was thinking to much so I desided to stop thinking.
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