Maker & Reference
Engine Compression Ratio Calculator
CR = (swept volume + clearance volume) / clearance volume. For a single cylinder. Higher CR = more power + more pump gas.
Compression ratio
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- Swept volume (cc)
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- Clearance volume (cc)
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- Cyl displacement (CID)
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FAQ
What CR for pump gas? +
9.5:1 to 10.5:1 on 91-93 octane. Aluminum heads tolerate ~1 point higher than iron. Turbo/supercharged builds drop CR to 8.0-9.5.
Why piston dish is positive? +
A dish creates extra clearance volume (lowers CR). A dome takes up space in the chamber (raises CR).