Hi, I was wondering if there was a way to figure out approx. what the compression ratio is on my engine by looking at the displacement and the PSI from a compression test? If anyone could help me out I'd really appreciate it, thanks. BTW- it's a 350 and the average is about 180 PSI on the guage.
-Matt-

 

If you give the cam grind and intake system we could probably give you a rough guess at the compression ratio. The cam profile has plenty to do with cranking pressures, if it didn't then it would be possible to calculate a compression ratio from the psi. About the closest for an actual figure I can give you is about 17 psi per point of CR. As the CR goes up this figure goes up. This is because the more you try to compress air the more it heats, which increases pressure. So that would give you about a 10:1-11:1 CR, but it is still higly variable on the cam. I have seen 8.5:1 4cyl engines with tiny cams turn 190psi cranking. I have also seen 13:1 monster cammed engines turn 165psi.

 

not sure ,but my ZZ4 is around 185-190

 

To add to Dr. Pepper's post, also the condition of the piston rings plays a big role. If they are old and wore, the gaps have probably expanded enough to reduce the PSI from say 160 down to 120. Or the flip side is, the rings haven't been seated properly yet and when they settle they could go down to 180 PSI. You can use the PSi to calculate CR, but you need all the variables (is what it boils down to) which vary from engine to engine (and even piston to piston , but only by .01 CR ).

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George P. Lara
1984 Z28 High Output
Proud Member of SCFB
V6 T5 For Sale

[This message has been edited by MRZ28HO (edited July 22, 2000).]

 

I tested my LG4 several years ago when it was bone stock and had only had about 165,000 miles on it; the figures ranged from 195 to 215 psi . Even checked with another guage, they were within 2-3 psi of the first readings.

Pete

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Now at HiOutput.com

Thirdgen Tech Area
Engine buildups and more...

 

Whoa! That must have been a well taken care of engine with a wide-a$$ LSA and short duration. 195-215 PSI!

 

Stock cams tend to have very very little overlap, which can often lead to high pressure measurements. I don't think it would be odd at all to find that a typical stock, low compression engine would generate more PSI than a modified setup with a long duration (more overlap), high static CR rating. Especially since the compression test is being done at less than idle rpms.

 

Get this....the engine hadn't even been started for three weeks before that...can you say dry?

Pete

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Pete's Place
Now at HiOutput.com


83 SC...LG4/5-speed from an 84 Z28, 3.42 one-tire-fryer, T-tops, PW, PH, gray interior from an 86 Firebird SE, seats from an 89 Formula, Wheels from an 83 Z28, springs and rear bar from an 84 Z28, WS6 front bar, Hedman Hedders and y-pipe, Dynomax cat-back, .420" lift cam, Accel HEI Supercoil and foam air filter, custom air induction through Z28 hood.....best ET (so far on paper) is 15.19@94.25