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Compression Ratio Calculator

Calculators · Added

Compression ratio is swept volume plus clearance volume, divided by clearance volume — and every mistake happens in the clearance volume, which is four things added together rather than one. This asks for all four, including the head gasket and the deck clearance that get left out, and can also work out the dynamic ratio from the camshaft's intake closing point.

mm
mm
cc

From the head specification, or measured by filling the chamber from a burette.

mm

The gasket's own bore, usually a shade larger than the cylinder's. Leave at 0 to use the bore.

mm
mm

Piston crown below the deck at TDC. Negative if it stands proud.

cc

A dish adds volume and lowers the ratio; a dome does the opposite.

How to use the compression ratio calculator

  1. 1Enter the bore, stroke and cylinder count. Pick millimetres or inches; everything else follows that choice.
  2. 2Enter the combustion chamber volume in cc, from the head specification or measured with a burette.
  3. 3Add the gasket bore and compressed thickness, the deck clearance, and the piston dish — entering a dome as a negative volume.
  4. 4Tick the dynamic ratio box and add the rod length and intake closing angle if you want the figure the camshaft actually gives you.

Examples

A square four-cylinder

Input
86 mm bore and stroke, 50 cc chamber, 1.2 mm gasket, 0.5 mm deck, 5 cc dish
Result
About 8.7:1 from a 1,998 cc engine

Leave the gasket and deck out and the same engine reads well over 9:1 — a whole point of error.

Working backwards to a target

Input
A target of 10.5:1 on the same short block
Result
The chamber volume it needs, and how much has to come off the head

Milling shortens the deck height too, which moves cam timing and valve-to-piston clearance on an overhead-cam engine.

What the cam gives back

Input
144 mm rod, intake closing 45° after bottom dead centre
Result
A dynamic ratio well below the static one, because compression starts late

It is why a big cam and a high static ratio belong together.

About the compression ratio calculator

Why the geometry is worth getting exactly right

Compression ratio decides how much of the fuel's energy the engine can extract, and it does so steeply at the low end: going from 8:1 to 10:1 is worth several percent of thermal efficiency. It also decides how close the engine runs to detonation, where the mixture ignites ahead of the flame front and hammers the piston crown. Those two pull in opposite directions, and the entire art of specifying a build is finding the highest ratio that stays on the safe side of the second one.

That is why a point of error matters. An engine assembled believing it is at 9.5:1 when it is really at 10.5:1 will be tuned for a fuel it is not actually able to use, and the failure mode is not gradual. Detonation destroys pistons and ring lands in minutes at load.

The slider-crank relation, which is where the dynamic ratio comes from

The piston does not move sinusoidally with crank angle, because the connecting rod is finite. Its distance from top dead centre at crank angle θ is r(1 − cos θ) + L − √(L² − r² sin² θ), where r is half the stroke and L is the rod length between centres. That correction term is what makes the piston spend longer near the top of the bore than the bottom, and it is what a dynamic ratio calculation needs.

The intake valve closes some number of degrees after bottom dead centre, so the crank still has 180 minus that many degrees to travel before it reaches the top. Feeding that angle into the relation gives the distance the piston still has to move, and therefore the volume it will actually compress. Divide by the clearance volume as before and you have the dynamic ratio.

Rod length matters here in a way it does not for the static ratio. A longer rod for the same stroke changes where the piston sits at any given angle, which shifts the dynamic ratio slightly — one of several reasons rod ratio gets argued about at length in engine building circles.

What this cannot tell you

It is geometry from the dimensions you type in. It knows nothing about your fuel's octane, your ignition curve, your cooling system, the altitude you run at, or whether there is a turbocharger in the way — and every one of those changes what ratio the engine will tolerate. A forced-induction engine in particular needs a lower static ratio than a naturally aspirated one, and how much lower depends on the boost and the intercooling.

It also assumes the numbers you enter are the numbers in the engine. On any used head, the chamber volume in the catalogue is a starting point rather than a measurement, and a deck height that has been machined is no longer the factory figure. Measure what you can; the arithmetic is only as good as its inputs.

Frequently asked questions

What exactly goes into the clearance volume?
Four volumes. The combustion chamber in the cylinder head is the largest. The head gasket contributes a cylinder of its own bore — not the block's — and its compressed thickness. The deck clearance is the gap left when the piston crown stops below the deck at top dead centre, and it is negative if the piston stands proud. Finally the piston itself: a dish adds volume, a dome removes it. Omitting the gasket and the deck is the usual error and it reads about a point high.
What is the difference between static and dynamic compression ratio?
The static ratio assumes compression starts at bottom dead centre. It does not — it starts when the intake valve closes, which on any camshaft with real duration is well after BDC. The dynamic ratio substitutes the stroke remaining at that point, worked out from the crank throw and the connecting rod length. A big cam holds the intake open longer, throws away more of the stroke, and drops the dynamic ratio, which is why high static ratios and long-duration cams are usually specified together.
Which intake closing figure should I use?
The one measured at 0.050 inch tappet lift, which is what a modern cam card prints and what every dynamic ratio discussion assumes. An advertised duration figure is measured at a much lower lift — often 0.006 inch — and is typically twenty to thirty degrees longer, so using it makes the intake appear to close later than it does and understates the dynamic ratio. If a cam card gives only advertised numbers, the manufacturer can usually supply the 0.050 figures.
How high can compression go on pump fuel?
There is no single number, because the ratio is only one of the factors. Chamber shape, quench, cooling, ignition timing, altitude, cam timing and the fuel's actual octane all move the limit. As rough orientation, a naturally aspirated iron-headed engine on ordinary petrol has historically been kept under about 9.5:1, while a modern aluminium head with good quench and knock-sensor-managed timing runs well above 11:1 on the same fuel. A forced-induction engine has to come down again. Treat this as geometry, not as a tuning recommendation.
Why measure the chamber instead of using the published figure?
Because the published figure is for a new head. Any head that has been skimmed, decked or reworked has a smaller chamber than the specification says, and there is no way to know by how much except to measure it. The method is to seal the chamber with a plate, fill it from a graduated burette through a small hole, and read off the volume. On a rebuild where the ratio matters, this is the measurement worth taking rather than assuming.

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