How to Use This Tool
Enter bore, stroke and cylinder count. Add an overbore if the block has been machined, and set either a target compression ratio or a measured chamber volume.
The formula, and why bore matters more
A cylinder is a circle swept along a line, so its volume is:
π/4 × bore² × stroke, multiplied by the number of cylinders.
Bore appears squared and stroke does not. Starting from 86 × 86 mm in four cylinders — 1,998.2 cc — adding a millimetre of stroke gives 2,021.5 cc, while adding a millimetre of bore gives 2,045.0 cc. Twice the gain from the same millimetre.
That asymmetry is why rebuilds talk about overbores in fractions of a millimetre: +1.00 mm on a four-cylinder recovers worn bores and adds about 47 cc almost incidentally.
Oversquare, square, undersquare
The bore-to-stroke ratio describes the shape of the cylinder and hints at the engine's character:
- Oversquare (bore > stroke, ratio above 1). Shorter stroke means lower piston speed at a given crank speed, so it can rev higher. There is more room in the head for larger valves. Typical of high-revving petrol engines.
- Square (ratio 1.00), such as 86 × 86 — a common compromise.
- Undersquare (bore < stroke, below 1), like 84 × 90 at 0.933. A longer stroke gives the connecting rod more leverage on the crank, favouring low-speed torque, and limits revs because piston speed rises faster. Typical of diesels and traditional truck engines.
These are tendencies rather than rules. Valve timing, port design, forced induction and fuel injection all shift an engine's character far more than the bore-stroke ratio does — it is a hint about intent, not a prediction of behaviour.
Compression ratio is not displacement
Displacement is how much air the pistons sweep. Compression ratio is how much that air is squeezed:
CR = (swept volume + clearance volume) / clearance volume
Clearance volume is everything left above the piston at top dead centre — the combustion chamber in the head, the head gasket bore, the piston-to-deck gap, and the piston's own dish or dome.
For one 499.56 cc cylinder at 10.5:1, the clearance works out at 52.58 cc. Take one cc away and the ratio becomes 10.68. Because the clearance volume is the small number in the fraction, tiny changes move the ratio a lot — and it is why engine builders measure chamber volume with a burette instead of trusting a figure from a catalogue.
What this cannot tell you
The calculation here treats clearance volume as one number. A real measurement adds up several contributions, and the piston dish or dome can be either sign. It also says nothing about dynamic compression, which depends on when the inlet valve closes and is what actually decides whether an engine will detonate on a given fuel.
Raising static compression without accounting for cam timing, fuel octane and knock control is how engines get damaged. Treat this as arithmetic for planning, not as a build specification.