Q: I have a question about engine compression. What would the following compression ratios be in terms of pounds per square inch (psi) of cylinder pressure?
- 8:1 compression
- 9:1 compression
- 10:1 compression
- 11.1 compression
The reference engine is a small block Chevy 350 bored .030 inches over.
A: Cylinder compression and cylinder pressure are not directly related to one another.
An engine’s compression ratio is based on cylinder volume. The volume of the cylinder with the piston at top dead center (TDC) is compared to cylinder volume with the piston at bottom dead center (BDC).
For example, if cylinder volume with the piston at BDC is 10 times greater than the volume with the piston at TDC, the compression ratio is 10.0:1.
Cylinder cranking pressure is controlled by four factors:
- Piston ring seal
- Valve seal on the seat
- Head gasket seal
- Camshaft overlap (The time the intake and exhaust valve of one cylinder are both open.)
The more overlap, the less cranking pressure you will have.
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Hot rod just did a 3 cam test all the same except lob separation. the cam with the most overlap made the most cranking pressure.
Yes I saw the article in Hot Rod also. I have a compression tester gauge that ‘DOES’ correlate compression ratios to compression/cranking PSI. It is reasonably accurate. I wouldn’t use it for detailed info, but it is a good predictor of compression ratios. The person asking the question probably knows that their are a lot of variables but just wants a rough estimate.
This helps me out some, im building a 84 LT9 and i know the compression is a garbage 8.3:1
but im still unsure how to make that better.
Absent changing the parts to achieve a higher mechanical compression ratio, use a camshaft that closes the intake valve sooner. This captures the charge in the cylinder rather than pushing it back out the intake valve.
I have a new 2018 Chevrolet CRATE ENGINE ZZ 383 SP deluxe fitted into 1970 C-3 chassis. Miles drive 3,400. Confirmed it turns out 335-HP, 380-Torque w/ Tramec 5-speed transmission, quick fuel 750 carburetor, running 3:55 gears, and OEM rated 9.6-1 compression heads. After first pull, tuned, and installed smaller fuel jets. Aluminum heads are – bee hive style aluminum Chevrolet stock heads.
I’m looking for a total of 395-405HP NET Chassis Dyno verified HP and additional Torque numbers. Do you any suggestions to obtain HP goal.
I’ve heard 1 extra lb. of Compression adds approximately 25 gross HP. More aggressive Cam add about 20-25 hp, Port & polish heads and Edelbrock air gap manifold add, around 30-HP. Do you have any recommendations to help me achieve net HP goal?
And is 20-25 hp gain w/ 1-lb of additional compression realistic?
I think you’re confusing points of compression ratio and pounds of compression