I talked to a friend at work. He's insanely smart. Not sure what his role is here at work, but he knows a lot about engines and physics. Here's our conversation which may better help explain the chart that's been posted. My initial email is in green:
I wanted to ask do you believe that we should be making compression corrections being that we are 2700 feet in the air?
“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”
The way I interpret this is if I get 150 PSI @ 2500 feet, I would multiply 150 x 1.067 to give me sea level compression. That leaves you with about 160 PSI if you were to test it at sea level. Or, do I have it backwards?
His response:
"Or divide it by the factor and save the subtraction problem, e.g. 150/.933 = 160.7. Psig is differential pressure between atmosphere and measured pressure. If you took the altitude compensation all the way into space—essentially vacuum--you’d be reading absolute pressure. So this helps compensate.
If you’re going for accuracy, then yes.
You are going about it backward, though. If the mfgr references sea level for their reading, you’ll take their reading times the factor. Thus, a spec’d 160 psi * .933 = 149.3 at 2,500 feet.
On compression testing, I use both. The reading per cylinder tells me whether the cylinder meets spec. The relative readings between cylinders tell me if I have a problem."
