Static ratio from the real volumes, dynamic ratio from the real cam timing
Compression ratio is one division, but the clearance volume in its denominator is five volumes wearing a trench coat: combustion chamber, head gasket, deck clearance, and the piston crown’s own contribution. This calculator itemises all of them from your dimensions (or takes swept and clearance volumes directly), reports the static ratio with the full component breakdown, and then, from your connecting rod length and intake valve closing angle, computes the effective (dynamic) ratio using true slider-crank piston geometry, because the intake valve is still open while the piston rises, and the cylinder only starts compressing when it shuts. A clearly labelled sealed-model ceiling for cranking pressure is included, with its limits stated, not hidden.
Disclaimer: This calculator gives estimates for general information only. It is not financial, medical or other professional advice. Compression targets interact with fuel, cam timing and tuning; engine building decisions belong with the engine builder and the component makers’ specifications.
What this calculator does
Static compression ratio is (swept volume + clearance volume) / clearance volume, and the craft is entirely in the clearance volume, so the page itemises it: chamber volume as measured (cc’d) by the builder, gasket volume as a cylinder of gasket bore and compressed thickness, deck volume as a cylinder of bore and deck height, and the piston crown’s signed contribution. The dynamic block then does the geometry most calculators skip: from the crank radius, rod length and the crank angle at which the intake valve closes, it computes how far the piston has actually travelled up the bore at closing (the slider-crank equation, not a sine approximation), takes that fraction of the swept volume, and forms the effective ratio the trapped charge experiences.
How to use it
- Chamber volume is measured, not read off a casting. Heads are cc’d with a burette during a build; published figures vary by casting and by any milling, and a milled head’s smaller chamber is exactly the change this calculator exists to catch.
- Gasket thickness is the compressed figure. Manufacturers quote it; using the free thickness overstates gasket volume and understates the ratio.
- Sign discipline on the piston. Dishes and reliefs add clearance (positive), domes remove it (negative). A sign error here moves the answer by twice the crown volume, the most expensive typo in engine maths.
- Dynamic needs the seat timing. Use the cam card’s intake closing at the checking clearance the card specifies; advertised and at-0.050-inch timings differ, and the effective ratio follows the one the valve actually obeys.
The formulas
- Swept volume = π/4 × bore² × stroke. Gasket volume = π/4 × gasket bore² × compressed thickness. Deck volume = π/4 × bore² × deck height.
- Clearance volume = chamber + gasket + deck + piston crown (signed). Static ratio = (swept + clearance) / clearance.
- Effective swept volume: piston travel from BDC at intake closing, by the slider-crank relation with crank radius r and rod length l, times the bore area. Effective ratio = (effective swept + clearance) / clearance.
- Pressure ceiling (model) = atmospheric pressure × (effective ratio ^ exponent – 1), gauge, with the exponent you set.
Worked example
A built 2.0 litre four. Bore and stroke 86 mm (swept 499.56 cc per cylinder), chamber 40 cc, gasket 87 mm bore × 1.0 mm compressed (5.94 cc), deck height 0.25 mm (1.45 cc), piston dome of 5 cc (entered as -5): clearance volume 40 + 5.94 + 1.45 – 5 = 42.40 cc, and the static ratio is (499.56 + 42.40) / 42.40 = 12.78:1. Now the cam: rod 143 mm, intake closing 40 degrees after BDC. The piston has covered 91.44% of its stroke by then, the effective swept volume is 456.78 cc, and the effective ratio is 11.77:1, a full point below the static figure, which is the number the fuel actually experiences. The sealed-model pressure ceiling from that effective ratio at an exponent of 1.3 is 23.98 bar; a real gauge reads substantially lower, for the reasons the limitations give, and the page prints it as a ceiling rather than a prediction for exactly that reason.
Reading your results
Read the breakdown before the headline: the component rows show which volume owns the ratio, and therefore which machining decision (milling, gasket choice, piston swap) moves it how far. Read static and effective together: two engines at 12.78 static with different cams are different engines at the plug gap, and the gap between the two figures is the cam’s fingerprint. And read any change against fuel: ratio decisions are octane decisions, and this page prices the geometry while the fuel’s knock resistance sets the budget.
Limitations
- Creases and crevices are uncounted. The top-land crevice volume, ring pack and spark plug threads hold small volumes the five components omit; laboratory-grade work measures total clearance directly with the piston at TDC.
- The pressure figure is a sealed, no-leak ceiling. Real cranking readings are cut by ring and valve leakage, throttling past a partly open throttle, heat loss to cold walls and cranking speed; treat the row as an upper bound and diagnose engines from gauge comparisons between cylinders, not from this model.
- Effective ratio is a geometric construct. Actual trapping also depends on pressure waves, rpm and volumetric efficiency; the dynamic figure is the standard first-order correction, not a simulation.
- Inputs are per cylinder and assume identical cylinders. Real engines vary slightly by bore; blueprint work repeats this per hole.
Frequently asked questions
Why is the effective ratio always lower than the static?
Because compression cannot begin while the intake valve is open: the piston’s first travel up the bore merely pushes mixture back toward the port instead of squeezing it, and only the travel remaining after closing does compression work. Later closing (a longer-duration cam) traps less of the stroke, so the effective ratio falls further below the static figure, which is why long-cam engines are built with high static ratios: the static number compensates for what the cam gives back.
How much does a thinner head gasket change?
Run the breakdown twice: on the worked engine, halving gasket thickness from 1.0 to 0.5 mm removes about 2.97 cc of clearance, and the static ratio rises from 12.78 to about 13.67, a large move from a part swap, which is why gasket choice is a tuning lever and why the gasket’s own volume is itemised instead of buried. (Piston-to-head clearance sets the physical limit; the calculator does not check it.)
Static or effective: which should I build to?
Effective, for anything the fuel and detonation care about, with the static figure as the machining record. Builders quote static because it is measurable on the bench; tuners think in effective because the trapped charge is what burns. This page reports both from the same inputs so the two conversations use consistent numbers, and its worked example shows the size of error (a full ratio point) that ignoring the distinction invites.
Why does my cranking gauge read so much lower than the ceiling?
Because a cranking cylinder is not the sealed adiabatic model: rings leak by design clearances, the throttle restricts the incoming charge, cold walls steal heat from the compression, and the starter turns the engine slowly enough for all three to matter. Healthy diagnosis compares cylinders against each other (a spread beyond roughly ten percent is the classic worry) and against the manufacturer’s cranking specification for that engine, not against a theoretical bound.
Related calculators
Method: static ratio from itemised component volumes; effective ratio from slider-crank piston travel at intake closing; pressure row labelled as a sealed-model ceiling. Last reviewed: 14 October 2026.