I had recently done a compression test on my 94. I thought I had good results at 150 psi on all 6. I have heard of others reporting results around the 200 psi range. I am around 5000 feet but would that make for a 50 psi difference? Engine feels strong to me. It is my first 80 though. It does have 215,000 on it so I would assume a little wear. What are some typical results for compression tests?
compression question.
- Thread starter rtjones
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I could very well be wrong but I would not think altitude effects compression. It certainly effects horse power with a decrease of about 20% at 6K' due to lack of air. At least that what my mechanic tells me. I would think lower compression is due to blow- by past the compression rings and worn valves.
I can't answer your question about normal compression though. It should be easy to find the range in a manual somewhere.
Pressure at 5000 feet is 17% less than sea level. This means that there is 17% less air in the cylinders and the pressure will be 17% lower. So, if you're reading 150 psi at 5000 feet the sea level reading would be about 180psi. Im not sure if the compression gage measures pressure relative to the ambient or has some internal reference but the difference would only be about 2psi. Various threads here are suggesting (as far as I can gather) compressions of 150-200 psi with the high people saying high is better and the low people saying the higher pressures are do to carbon buildup. Your adjusted pressure of 170 seems to fall nicely in the middle. It would be nice to see the readings of each cylinder to 3 sig figs but if they are all 150 (or close) it would seem that you have a very healthy engine!
Tahoe: The purpose of the compression test is to measure the amount of leakage past the rings and valves but at higher altitude you'll have a lower maximum value. For example, if you had perfect sealing by the valves and rings at sea level you might have, say 200 psi. leakage past the valves and rings could reduce that to 180 psi. Take the same engine to 5000 feet and suddenly the perfect sealing case only generates about 166psi.
Tahoe: The purpose of the compression test is to measure the amount of leakage past the rings and valves but at higher altitude you'll have a lower maximum value. For example, if you had perfect sealing by the valves and rings at sea level you might have, say 200 psi. leakage past the valves and rings could reduce that to 180 psi. Take the same engine to 5000 feet and suddenly the perfect sealing case only generates about 166psi.
Thanks for that info! I did not know that altitude actually brings down the actual compression. I thought it only effected the fuel mixture. That also must be why you can bump up the timing to 9 degrees at and get no pinging at 7K'--though it can burn out the converter per my mechanic. No wonder my truck (with 33's) sometimes needs to grab second to keep 55 over 7'-8K' passes.
