Compression Ratio Calculator
Calculate compression ratio from swept and clearance volume, or work backwards.
What the Compression Ratio Calculator does
Compression ratio compares the cylinder volume at bottom dead centre with the volume left at top dead centre. Because the clearance volume sits in the denominator, small changes to the combustion chamber or gasket move the ratio far more than you might expect.
Formula
CR = (Swept volume + Clearance volume) ÷ Clearance volumeClearance = Swept volume ÷ (Target ratio − 1)Ideal efficiency = 1 − 1 ÷ CR^0.4
Inputs explained
| Input | Unit | Required | Notes |
|---|---|---|---|
| Solve for | one of 2 options | Yes | — |
| Swept volume per cylinder | cc | Yes | Accepts more than 0. |
| Clearance volume | cc | In some modes | Accepts more than 0. Shown Solve for is Compression ratio. |
| Target compression ratio | number | In some modes | Accepts more than 1. Shown Solve for is Clearance volume for a target ratio. |
How to use it
- Choose Solve for.
- Enter Swept volume per cylinder.
- Fill in the remaining inputs the form shows for your choice.
- Select Calculate.
Worked example
A cylinder sweeping 500 cc with a 50 cc clearance volume.
- Swept
- 500 cc
- Clearance
- 50 cc
(500 + 50) ÷ 50 = 11:1, which normally calls for premium fuel.
Frequently asked questions
Why do turbocharged engines run lower compression?
The turbo already raises cylinder pressure. Combining that with high static compression would cause knock, so boosted engines trade static ratio for boost pressure.
What counts as clearance volume?
The combustion chamber, the head gasket bore, deck clearance above the piston at top dead centre, and any dish in the piston crown — minus any dome.