Displacement & Compression Calculator

The displacement formula

Each cylinder's swept volume is a simple cylinder of metal-free space: V = π/4 × bore² × stroke, with bore and stroke in millimetres giving cubic millimetres, divided by 1000 for cc. Multiply by the cylinder count for total capacity. The defaults loaded here are an OM642's 83 mm bore and 92 mm stroke across six cylinders, which lands on its familiar 2987 cc.

Bore versus stroke, and why the ratio matters

Two engines of identical capacity can behave completely differently depending on how the volume is split. An undersquare engine (stroke longer than bore, like the default here) has a long lever on the crank and lower piston speed limits, favouring torque at low rpm, the classic diesel layout. Oversquare engines (bore larger than stroke) allow bigger valves and higher revs before piston speed gets dangerous, the petrol performance recipe. The bore:stroke ratio falls straight out of the two inputs above.

How compression ratio is defined

Compression ratio compares the cylinder's total volume at bottom dead centre to its volume at top dead centre: CR = (swept + clearance) ÷ clearance. The clearance volume is everything left when the piston is at the top: the combustion chamber in the head, the head gasket's compressed thickness, any piston dish or minus any dome, and the tiny crevice above the top ring. Measuring it properly means cc-ing the head with a burette, which is why builders treat published CR figures as nominal.

What changes it in a build

Skimming the head or block removes clearance volume and raises compression; a thicker head gasket or dished pistons lower it. Small volumes move the ratio a lot: on the default figures, removing just 2 cc of clearance volume raises the ratio by roughly a full point. That sensitivity is why decking a block or switching gasket thickness has to be calculated, not guessed, especially on forced-induction engines where the ceiling on compression is set by boost and fuel quality together.