• NAXJA is having its 18th annual March Membership Drive!!!
    Everyone who joins or renews during March will be entered into a drawing!
    More Information - Join/Renew
  • Welcome to the new NAXJA Forum! If your password does not work, please use "Forgot your password?" link on the log-in page. Please feel free to reach out to [email protected] if we can provide any assistance.

What's the max cylinder pressure in compression test?

@dam

NAXJA Forum User
It seems like, with a 9.2:1 compression ratio, it'd be 9.2x atmospheric pressure. 9.2 * 14.7 psi = 135. That seems low though. Someone on another thread said 175 psi, but wouldn't that require a much higher compression ratio? How would you get more than 9.2x of whatever you have to work with?

I did a compression test today on my 1995 4.0 with 233k miles. Engine had been driven about 3 hours before, so it was getting cool. My numbers were...

123
115
123
120
127
123

Some people were saying these are bare minimum- ready for a rebuild. However, I'm at nearly 6000', and it was a warm day. Altitude alone has a .836 correction factor, which would give me...

147
138
147
144
152
147

So, those alone bring me up to above my 137 theoretical max. Two questions:

1) How do you get cylinder pressures greater than compression ratio times atmospheric pressure?

2) Does my engine look healthy to you guys, given that it was tested not cold, but pretty cool?
 
Wait...I think I figured it out. It has to do with the specific heat of air.

P1*V1^y = P2*V2^y where y is the ratio of specific heat for air.

So, (P1*V1^y)/V2^y = P2

y = 1.4
P1 = 14.7 psi
V1 = 43.48 ci
V2 = 3.48 ci

So 509 psi is the maximum theoretical pressure in an ideal, perfectly sealed, adiabatic system. Lots of margin there...seems too high even. Perhaps I ran the numbers wrong.
 
I believe static compression ratio vs dynamic running compression is not directly comparable because of cam timing and what not. Your numbers are OK. As long as your rings are still decent you'd probably see an improvement by getting a valve job but I'm not convinced its worth it. I'm at 6500 and my compression numbers were 157-166 on an engine with 330,000 miles with a fresh valve job.
 
Back
Top