Compression Ratio
Static compression ratio from bore, stroke, chamber volume, head gasket, deck clearance and piston dish or dome — every volume shown, nothing assumed.
The swept and clearance volumes of one cylinder, the static ratio they give, the total displacement, and how a gasket, deck or dome change moves the ratio.
Example: 86 × 86 mm with a 48 cc chamber, 1.0 mm gasket at 87 mm bore, 0.5 mm deck and a 3 cc dish: 499.6 cc swept over 59.8 cc clearance = 9.35 : 1.
Swept plus clearance,
over clearance.
How the static ratio is built from measurable volumes, what each of the gasket, deck and piston terms does, and why the dynamic ratio is a different question.
The definition
Compression ratio is the total volume above the piston at bottom dead centre divided by the volume at top dead centre: (V_swept + V_clearance) ÷ V_clearance. Swept volume is π/4 × bore² × stroke. Clearance volume is everything left at TDC: the chamber in the head, the volume inside the gasket bore over its compressed thickness, the deck clearance (piston crown below the deck, or above it as a negative), and the piston’s dish (positive) or dome (negative).
Measured, not assumed
Chamber and dish or dome volumes are best measured by filling (cc’ing) — catalogue figures for heads and pistons are nominal. Gasket thickness must be the compressed figure. Small volumes matter: on the example engine a 6 cc dome instead of a 3 cc dish moves the ratio from 9.35 to 10.82.
Static against dynamic
This is the static (geometric) ratio. The effective ratio while running depends on when the intake valve closes — a long-duration cam lowers it — and on boost, which raises the effective pressure ratio. Neither is computed here; the safe static ratio for a fuel, cam and boost level is the engine builder’s judgement.
What the number does not settle
Piston-to-valve and piston-to-head clearance, quench distance, octane requirement and detonation margin are not outputs of this page. Nothing here is a tuning recommendation. Inputs stay in your browser; the same four anonymous usage counts as the rest of the site apply.