What is compression ratio?
The static compression ratio compares the volume in a cylinder with the piston at the bottom of its stroke to the volume left when it is at the top. Higher compression squeezes the air-fuel mixture more, which makes more power and efficiency, but raises the risk of knock (detonation) on lower-octane fuel.
Compression ratio formula
Clearance volume = chamber + gasket volume + deck volume + piston dish (or − dome)
Compression ratio = (swept volume + clearance volume) ÷ clearance volume
Worked example
A 4.00 in bore, 3.48 in stroke cylinder sweeps 716.6 cc. Clearance volume is the 64 cc chamber, plus 8.65 cc for a 4.100 in × 0.040 in gasket, plus 2.06 cc for the piston sitting 0.010 in below the deck, plus a 5 cc dish = 79.71 cc. The compression ratio is (716.6 + 79.71) ÷ 79.71 = 9.99:1.
How to raise or lower compression
- Smaller chambers (milling the heads or different heads) raise compression.
- Thinner head gaskets raise it; thicker gaskets lower it, a common trick for boosted builds.
- Domed pistons raise it; dished pistons lower it.
- Zero-decking the block removes the deck volume and tightens quench.
Need the engine size? Use the engine displacement calculator.
Frequently asked questions
How do you calculate engine compression ratio?
Add the swept volume of one cylinder to its clearance volume (chamber, gasket, deck and piston volume) and divide by the clearance volume. Use the same units, usually cc, for all volumes.
What is a good compression ratio for a street engine?
Many naturally aspirated street engines run about 9:1 to 10.5:1 on pump gas. Modern direct-injection engines often run 11:1 to 13:1 thanks to precise fuel control. Turbocharged engines usually run lower static compression.
Does a thicker head gasket lower compression?
Yes. A thicker gasket adds clearance volume. On a 4.1 inch gasket bore, every extra 0.010 inch adds about 2.2 cc per cylinder.
What is the difference between static and dynamic compression?
Static compression uses the full stroke. Dynamic compression accounts for the intake valve closing after bottom dead center, so it is lower and depends on the camshaft.
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Last updated October 2026.