Compression ratio calculator

Combine swept volume and the entered clearance volumes to calculate a theoretical static compression ratio.

Formulacompression ratio = (swept volume + clearance volume) ÷ clearance volume

Every clearance volume belongs in the denominator

The chamber, gasket, deck clearance and piston dish add to clearance volume. Enter a dome as a negative piston volume because it displaces space from that total.

Static ratio is not cylinder pressure

Cam timing, sealing, altitude, temperature and cranking conditions affect dynamic and measured pressure. This page only evaluates the geometric static ratio.

A worked example

An 86 mm bore with an 86 mm stroke sweeps 499.6 cc per cylinder. With a 40 cc chamber, an 88 mm gasket bore compressed to 1.2 mm, 0.5 mm of deck clearance and a 3 cc piston dish, the clearance volume adds up to 53.2 cc: 40 for the chamber, 7.3 for the gasket, 2.9 for the deck and 3 for the dish. That gives 10.4:1.

Change one item and watch how little it takes. Skimming the head to a 38 cc chamber lifts the ratio to about 10.8:1, while a 1.5 mm gasket instead of 1.2 pulls it back under 10.2:1. This is why engine builders measure each volume rather than trusting the catalogue.

Frequently asked questions

How do I find my chamber volume?

Measure it rather than look it up. The head is levelled, a plate is sealed over the chamber with grease, and fluid is run in from a burette until the chamber is full; the reading is the volume in cc. Published figures are a starting point, but any head that has been skimmed, ported or valve-seated is no longer at its catalogue number.

Do I enter a piston dome as a positive or a negative number?

Negative. A dish adds space above the piston, so it is a positive volume in the clearance total. A dome occupies space that would otherwise be clearance, so it is entered as a negative figure. Getting the sign backwards on a 5 cc dome swings the answer by more than a full ratio point.

What is the difference between static and dynamic compression?

Static ratio is pure geometry: swept plus clearance over clearance, which is what this page calculates. Dynamic compression accounts for the intake valve closing after bottom dead centre, so part of the stroke is spent pushing mixture back out. A long-duration camshaft can leave an engine with a high static ratio behaving like a much milder one.

Is a compression test the same thing?

No. A gauge on a cranking engine measures pressure in psi or bar, and the number depends on cam timing, ring and valve sealing, throttle position, altitude and cranking speed. It is a diagnostic for engine condition and a comparison between cylinders. It is not a way to read off the geometric ratio.

What ratio can I run on ordinary pump fuel?

That is a build question rather than an arithmetic one, and the honest answer is that it depends on the fuel available to you, the chamber design, the head material, the camshaft, boost if any, and how the ignition is mapped. Two engines at the same ratio can behave completely differently. Take your measured figures to the engine builder or the manufacturer’s specification.