Things to consider when Doing Your compression test Anywhere but sea level

[imdrax]

ok today i was searching around and couldnt find anywhere that had an acurate way to adjust your sompression numbers according to your current elevation. i stumbled upon a very informative little tidbit on another forum somewhere so i thought i would share with everyone. :

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.

 

[PieEyedPiper]

While an interesting point, you should probably be more concerned with relative compression to the rest of your cylinders than an exact, pinpointed, altitude corrected, specific value.

 

[imdrax]

yes obviously but i hate it when i see posts like "oh no my compression #'s are below service limit!!!!!!!!!!!!!" and there in like colorado or something. i just wanted to post this so that anyone at high altitudes would be able figure out what there service and factory numbers are. but you are correct they must more importantly look at variance in the #'s.

 

[psychlow]

To give another example, my compression numbers are 120PSI across the board at 6000' elevation, which is around 80% of the atmospheric pressure at sea level.

Correcting for altitude, that would be around 140-150 PSI at sea level.

 

[imdrax]

To give another example, my compression numbers are 120PSI across the board at 6000' elevation, which is around 80% of the atmospheric pressure at sea level.

Correcting for altitude, that would be around 140-150 PSI at sea level.

Well using the method i posted above, the factory spec for a 1g turbo is 134.5 psi and the service limit is 98.6 psi. So you are within range. Your readings would be equivalent to about 147.4 psi at sea level.