What each input means
- Bore and stroke: the cylinder diameter and how far the piston travels. Together they give the swept volume.
- Combustion chamber volume: the volume of the chamber in the cylinder head, in cc, with valves and spark plug fitted. Head makers list it, or you can measure it.
- Dish, valve reliefs or dome: a dish or valve pockets add volume above the piston, so they lower compression. A dome fills part of the chamber, so it raises compression. Pick the type with the toggle and enter the volume as a positive number.
- Head gasket bore and compressed thickness: the gasket forms a thin ring of volume between block and head. Use the compressed thickness, not the thickness out of the box.
- Deck clearance: how far the piston top sits below the block deck at top dead center. If it sticks out above the deck, enter a negative number.
The compression ratio formula
The static compression ratio compares the volume above the piston at bottom dead center with the volume left at top dead center:
swept volume = π ÷ 4 × bore² × stroke
gasket volume = π ÷ 4 × gasket bore² × gasket thickness
deck volume = π ÷ 4 × bore² × deck clearance
clearance volume = chamber + dish (or − dome) + gasket + deck
compression = (swept + clearance) ÷ clearanceWith dimensions in inches the volumes come out in cubic inches; multiply by 16.387 to get cc. The calculator works in millimetres internally and gives every volume in cc.
Worked example
Bore 4.000 in, stroke 3.480 in, 64 cc chamber, 5 cc dish, head gasket 4.100 in bore and 0.041 in thick, piston 0.025 in below the deck.
- Swept volume: 716.62 cc per cylinder (349.8 ci for eight cylinders).
- Gasket volume: 8.87 cc. Deck volume: 5.15 cc.
- Clearance volume: 64 + 5 + 8.87 + 5.15 = 83.02 cc.
- Compression: (716.62 + 83.02) ÷ 83.02 = 9.63:1.
To reach 10.5:1 with the same piston, gasket and deck, the head needs a 56.4 cc chamber. A 0.051 in gasket instead of 0.041 in drops the ratio to 9.41:1.
The deck volume is treated as a plain cylinder of bore diameter. The small crevice between the piston and the bore above the top ring is left out, as most hand calculations do.
Static and dynamic compression
Static compression is pure geometry: the full stroke counts. In a running engine the intake valve closes after bottom dead center, so the piston only starts compressing once the valve shuts. Dynamic compression uses that shorter effective stroke, so it depends on the cam's intake closing point and on rod length. This page does not calculate dynamic compression. Use the static figure to compare parts, and take the cam into account before you settle on a combination.
Questions
How does head gasket thickness change compression ratio?
A thicker gasket adds clearance volume and lowers the ratio. In the example on this page (4.000 in bore, 3.480 in stroke, 64 cc chamber, 5 cc dish), going from a 0.041 in to a 0.051 in gasket drops compression from 9.63:1 to 9.41:1, and a 0.031 in gasket raises it to 9.86:1.
Does a domed piston raise compression?
Yes. A dome takes up space in the chamber, so it subtracts from the clearance volume. In the same example, swapping the 5 cc dish for a 5 cc dome raises compression from 9.63:1 to 10.81:1. A true flat top (0 cc) gives 10.19:1.
How do I measure combustion chamber volume?
With the head on a bench, valves and spark plug fitted, seal a clear plate over the chamber and fill it through a small hole with fluid from a graduated burette. The volume you pour in is the chamber volume in cc. Pistons are measured the same way in the bore, a known distance down.
What is the difference between static and dynamic compression ratio?
Static compression uses the full stroke and only depends on the parts. Dynamic compression starts counting when the intake valve closes, so it depends on the camshaft and rod length and is always lower. This calculator gives the static ratio.