Compression ratio calculator
Enter the bore and stroke of one cylinder plus the volumes that remain when the piston is at the top: the combustion chamber, the head gasket, the gap between piston and block deck, and any dish or dome in the piston. The calculator adds them up and gives the static compression ratio.
Runs in your browser: nothing you enter is sent anywhere.
How to use it
- Choose metric or inches and enter the bore and stroke of one cylinder.
- Enter the combustion chamber volume in cc.
- Enter the head gasket bore and its compressed thickness, and the deck clearance.
- Enter the piston dish as a positive volume or the dome as a negative one, then read the ratio.
How it works
- Swept volume = π ÷ 4 × bore² × stroke.
- Gasket volume = π ÷ 4 × gasket bore² × compressed thickness.
- Deck volume = π ÷ 4 × bore² × deck clearance.
- Clearance volume = chamber + gasket + deck + dish (or minus dome).
- Compression ratio = (swept + clearance) ÷ clearance.
Worked example
A cylinder with an 86 mm bore and 86 mm stroke sweeps 499.6 cc. A 45 cc chamber, a 88 mm × 1.0 mm head gasket (6.1 cc) and a 0.5 mm deck gap (2.9 cc) with a flat piston give a clearance volume of 54.0 cc. The ratio is (499.6 + 54.0) ÷ 54.0 = 10.25 : 1.
Tips
- Use the compressed gasket thickness, not the thickness of a new gasket out of the box.
- Measure the deck clearance with the piston at top dead centre; it is the largest unknown after a rebuild.
- Higher is not always better. Fuel octane and the engine’s design limit the usable ratio, so follow the builder’s specification.
Frequently asked questions
How is the compression ratio calculated?
Ratio = (swept volume + clearance volume) ÷ clearance volume. Swept volume is π/4 × bore² × stroke. Clearance volume is the chamber plus the head gasket, the deck gap and the piston dish, minus any dome.
What is the difference between dish and dome volume?
A dished piston has a recess, which adds to the clearance volume, so enter a positive number. A domed piston fills part of the chamber, so enter a negative number. A flat piston with valve reliefs usually has a small positive volume; check the piston maker's data.
How do I get the chamber volume?
It is printed in the cylinder head specification or measured by filling the chamber with liquid from a burette, called cc-ing the head. The value here is for one chamber with the valves closed and the spark plug fitted.
Is this the same as the cranking pressure or the dynamic ratio?
No. This is the static ratio from geometry. The dynamic ratio also depends on when the intake valve closes, and the pressure you read with a gauge depends on cam timing, leakage and cranking speed.