Today
CalcMotor ES

Engine displacement and compression ratio calculator

Enter bore, stroke and number of cylinders to get displacement in cc and litres, then calculate the compression ratio from the chamber volume or the chamber you need for a target ratio.

Published: · Last reviewed:

Engine displacement
Compression ratio

The chamber volume must include everything above the piston at top dead centre: chamber, head gasket, piston crown and deck clearance. Using the chamber alone gives a ratio that is too high.

Result

Formulas

Displacement per cylinder = π/4 × bore² × stroke

Total displacement = displacement per cylinder × number of cylinders

Compression ratio = (swept volume + chamber volume) / chamber volume

Chamber volume for a target ratio = swept volume / (ratio − 1)

Worked example: 86 mm bore, 86 mm stroke, 4 cylinders gives π/4 × 86² × 86 × 4 ≈ 1,998 cc. With a 50 cc chamber on a 499.6 cc cylinder the ratio is (499.6 + 50) / 50 ≈ 11.0 : 1.

Limitations

  • This is the geometric compression ratio. The “chamber volume” must include the head gasket, the piston crown shape and the deck clearance, not just the cylinder head chamber. Using the head chamber alone gives a ratio that is too high.
  • The effective ratio depends on valve timing and is lower than the geometric one. This tool does not model it.
  • Do not use it as the only basis for machining or tuning decisions. Measure the real chamber volume (for example by cc-ing the head) and follow the manufacturer’s limits.

Sources

All formulas are covered by automated tests with hand-calculated reference values. Found an error? See the about page.

More tools