Compression ratio is the ratio of the total cylinder volume (swept + clearance) to the clearance volume alone. It tells you how tightly the engine compresses the air-fuel mixture before ignition. Higher compression ratios produce more power but require higher octane fuel to prevent detonation (knock).
Swept volume (also called displacement) is the volume the piston sweeps from bottom dead center (BDC) to top dead center (TDC). Clearance volume is the remaining space above the piston at TDC, including the combustion chamber. Use a burette and degree wheel for precise measurements on performance builds.
Most stock small engines run 6:1 to 8:1. Performance builds commonly target 10:1 to 13:1 depending on fuel type and intended use.
Generally up to about 9.5:1 to 10.5:1 on 91-93 octane pump gas, depending on the engine design. Higher than that typically requires race fuel.
Higher compression increases thermal efficiency, which means more power from the same amount of fuel. Each additional point of compression adds roughly 3-4% more power.
The engine will detonate (knock), which can damage pistons, rings, and bearings very quickly. Always match your compression ratio to the fuel octane rating.
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