How compression ratio is calculated
Compression ratio compares the cylinder's volume with the piston at the bottom of its stroke to the volume left at the top. Add up every source of clearance volume at TDC — the combustion chamber, the head gasket, the deck gap and any piston dish (a dome subtracts) — then:
CR = (swept volume + clearance volume) ÷ clearance volume
where swept = π/4 × bore² × stroke per cylinder. Because volumes are what matter, a bigger chamber or thicker gasket lowers CR, while shaving the head, a domed piston or a thinner gasket raises it. This is static (mechanical) compression — cam timing changes the dynamic figure the engine actually feels.
FAQ
What compression ratio can I run on pump gas?
As a rough naturally-aspirated guide on modern pump fuel: up to ~10.5:1 is happy on regular, ~10.5–11.5:1 wants premium, and above ~12:1 you're into E85, race fuel or very careful tuning territory. Chamber shape, cooling, cam and knock control all shift these lines, so treat it as a starting point.
Do I need the dome/dish figure?
For accuracy, yes. A dished piston adds clearance volume (enter it as a positive number) and lowers CR; a domed piston displaces volume (enter it negative) and raises CR. Flat-tops are roughly zero. The value is usually on the piston spec sheet.
Static vs dynamic compression?
This tool gives static (geometric) compression from the physical volumes. Dynamic compression also accounts for when the intake valve closes, so a big cam can make an engine behave like it has lower compression than the static number suggests.