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.

measuring piston travel with a dial gauge on a small block chevy v8 engine
(Image/Jeff Smith)

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:

  1. Piston ring seal
  2. Valve seal on the seat
  3. Head gasket seal
  4. 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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