Compression Test Adjustment for Altitude?
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Compression Test Adjustment for Altitude?
Hi,
I've got a '91 Legacy with the 2.2 L. I've heard varying instructions on adjusting the compression numbers for altitude. I'm currently at 8,000 ft. so should I add 16 psi (2psi/1,000ft.) to the end result on a cylinder or just read the numbers at that height? If I use the compression ratio of 10:1 for the engine (I think that's right) then 1401b/sqin should be about right given atmospheric pressure is about 14 1b/sqin, or do I have it all wrong?
Thanks,
Mike
I've got a '91 Legacy with the 2.2 L. I've heard varying instructions on adjusting the compression numbers for altitude. I'm currently at 8,000 ft. so should I add 16 psi (2psi/1,000ft.) to the end result on a cylinder or just read the numbers at that height? If I use the compression ratio of 10:1 for the engine (I think that's right) then 1401b/sqin should be about right given atmospheric pressure is about 14 1b/sqin, or do I have it all wrong?
Thanks,
Mike
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The pressure in your area will be less than 14 psi.
In the "International Standard Atmosphere", the pressure at sea level is 1013 millibars (mb) and the temperature is 59 degrees F. Here a properly calibrated altimeter should read zero altitude. At 5000 feet above sea level the pressure has decreased to 842 mb and the temperature is 41 deg F. At 10,000 feet the pressure is 698 mb and the temperature is 23 deg F. From sea level to 10,000 feet, the rate of pressure change averages out to a drop of 1 millibar for about every 32 feet of elevation gain.
Where 1000 millibar = 14.504 psi = 29.529 inHg
8000'/32 = 250 mBar
1013 millibars - 250millibars = 763 millibars
As a percentage:
763 millibars/1013 millibars = 75%
So take what the "normal/expected" value & multiply by .75 to determine the altitude corrected value.
Ouch, that hurt.
Paul
In the "International Standard Atmosphere", the pressure at sea level is 1013 millibars (mb) and the temperature is 59 degrees F. Here a properly calibrated altimeter should read zero altitude. At 5000 feet above sea level the pressure has decreased to 842 mb and the temperature is 41 deg F. At 10,000 feet the pressure is 698 mb and the temperature is 23 deg F. From sea level to 10,000 feet, the rate of pressure change averages out to a drop of 1 millibar for about every 32 feet of elevation gain.
Where 1000 millibar = 14.504 psi = 29.529 inHg
8000'/32 = 250 mBar
1013 millibars - 250millibars = 763 millibars
As a percentage:
763 millibars/1013 millibars = 75%
So take what the "normal/expected" value & multiply by .75 to determine the altitude corrected value.
Ouch, that hurt.
Paul
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