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Engine Compression Ratio Calculator

Calculate an engine's static compression ratio and displacement from cylinder bore, stroke, cylinder count, and combustion chamber volume.

Compression ratio
12.45:1
Swept volume per cylinder
572.6 cc
Total displacement
2,290.2 cc (2.29 L)

How it works

An engine's static compression ratio compares the total cylinder volume with the piston at the bottom of its stroke to the volume left once the piston reaches the top: CR = (swept volume + chamber volume) ÷ chamber volume. Swept volume per cylinder is the standard cylinder-volume formula, (π ÷ 4) × bore² × stroke. Verified via web search against JE Pistons' own 'How To Calculate Engine Compression Ratio And Displacement' guide and AJ Design's compression-ratio reference.

Bore and stroke get this page's metric/imperial toggle (mm or inches, both genuinely in active use — mm for most modern engines, inches for classic American ones), converted to centimeters internally so swept volume comes out in cubic centimeters. Combustion chamber volume is kept as a plain, un-toggled cc figure: verified via web search that engine builders worldwide — including in the US — measure chamber volume with a cc-graduated burette, so cc is the domain-standard unit for that measurement regardless of which system the rest of the engine is specified in.

This deliberately models only swept volume and chamber volume, the two terms in the basic CR formula. A precise build sheet also nets out head gasket volume, deck clearance volume, and any piston dome (which subtracts from) or dish (which adds to) clearance volume — real, but smaller, adjustments this calculator doesn't attempt, since they depend on parts this calculator has no input for. Fold any of those into the chamber volume figure yourself for a more exact result.

FAQ

What compression ratio is 'normal'?

It varies by fuel and induction type, not a single universal number — naturally aspirated gasoline engines commonly run somewhere around 9:1 to 12:1, high-octane or race-fuel engines can run higher, and forced-induction (turbo/supercharged) engines are usually built lower, often 8:1 to 10:1, to leave headroom before knock under boost. There's no single 'correct' figure this calculator can validate against; it just reports the ratio your entered dimensions produce.

Why does my calculated displacement not exactly match my engine's advertised size?

Advertised displacement is usually rounded (a '2.0L' engine might be 1,998cc), and this calculator's bore/stroke inputs are only as precise as what you enter — a bore given to two decimal places instead of three, for instance, shifts the result slightly since swept volume scales with bore squared. Small differences between a calculated and advertised figure are almost always rounding, not an error.

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