| Combustion Chamber | Compression Ratio |
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What Compression Ratio Means
Compression ratio compares the cylinder volume when the piston is at the bottom of its stroke to the volume when it is at the top. A 10:1 ratio means the air-fuel mixture is squeezed into one tenth of its original space before ignition. Higher ratios extract more power and efficiency from the same fuel, up to the limit set by knock.
What Goes Into the Calculation
Static compression ratio depends on five volumes: the swept volume set by bore and stroke, the combustion chamber in the head, the head gasket, the piston-to-deck clearance, and the dish or dome of the piston. Add the four small clearance volumes together, compare them to the swept volume, and you have the ratio. Shrinking any clearance volume raises compression.
Tuning Compression for Your Fuel
Pump premium gasoline typically supports around 9.5 to 11:1 on a naturally aspirated engine, while race fuel or E85 allows much more. Forced induction needs lower static ratios because the boost adds its own compression. Use the target tool above to find the chamber volume that lands you exactly where you want.
Static vs Dynamic Compression
This calculator gives static compression, the pure geometric ratio. Dynamic compression also factors in when the intake valve closes, which a longer-duration cam delays, effectively lowering compression. Big-cam builds often run a higher static ratio to keep dynamic compression healthy.
Frequently Asked Questions
How do I lower compression?
Use a larger combustion chamber, a thicker head gasket, a dished piston, or more deck clearance. Each adds clearance volume and drops the ratio.
What does milling the head do?
Milling shaves the head surface, shrinking the combustion chamber and raising compression. Even a couple of cc makes a noticeable difference.
Is higher compression always better?
No. Too much compression for your fuel causes knock, which can destroy an engine. Match the ratio to your octane and whether you run boost.