acalculator

What is my compression ratio?

Enter the bore, the stroke and the clearance volume of one cylinder to get the static compression ratio. Or enter a target ratio to find the clearance volume, bore or stroke.

Your numbers

Units
Compression ratio
10.99

A 3.386 in bore and 3.386 in stroke with 50 cm³ of clearance volume gives a compression ratio of 10.99:1.

Swept volume per cylinder
499.6 cm³
Cylinder volume at bottom dead center
549.6 cm³

Compression ratio: 10.99. A 3.386 in bore and 3.386 in stroke with 50 cm³ of clearance volume gives a compression ratio of 10.99:1.

How to calculate

Computes the static compression ratio of a piston engine from the bore, the stroke and the clearance volume, or any one of them from the other three.

Example with the default inputs (Bore 3.38582677165354 in, Stroke 3.38582677165354 in, Clearance volume 50 cm³): A 3.386 in bore and 3.386 in stroke with 50 cm³ of clearance volume gives a compression ratio of 10.99:1.

Formula: CR = (Vs + Vc) ÷ Vc, with Vs = π ÷ 4 × bore² × stroke (Heywood 1988).

  • This is the static (geometric) compression ratio. The dynamic ratio, which depends on when the intake valve closes, is lower.
  • The clearance volume is the whole volume above the piston at top dead center: the combustion chamber, the compressed head gasket, the deck clearance, and the piston dome (subtract) or dish (add).
  • The cylinder is a round bore of the given diameter. Every cylinder of the engine is the same, so the ratio for one cylinder is the ratio for the engine.

Machine-readable copies: Markdown, JSON.

Worked examples

Each example is checked against the calculator on every build.

  1. Bore 3.386 in, Stroke 3.386 in, Clearance volume 50 cm³ gives Swept volume per cylinder 499.6 cm³, Compression ratio 10.991144.Source: hand calculation in content.mdx: Vs = π ÷ 4 × 8.6² × 8.6 = 499.56 cm³; CR = (499.56 + 50) ÷ 50 = 10.99 (Heywood 1988, chapter 2)
  2. Bore 4 in, Stroke 3.48 in, Compression ratio 10 gives Clearance volume 79.62 cm³.Source: hand calculation in content.mdx: Vs = π ÷ 4 × 4² × 3.48 = 43.731 in³ = 716.62 cm³; Vc = Vs ÷ (10 − 1) = 79.62 cm³
  3. Stroke 3.543 in, Clearance volume 60 cm³, Compression ratio 9.5 gives Bore 3.344 in.Source: hand calculation in content.mdx: bore = √(4 × (9.5 − 1) × 60 cm³ ÷ (π × 9 cm)) = √72.15 cm² = 8.494 cm = 84.94 mm

How it works

The static compression ratio compares the volume of one cylinder with the piston at the bottom of its stroke (bottom dead center) and at the top (top dead center):

  • swept volume Vs = π ÷ 4 × bore² × stroke
  • compression ratio CR = (Vs + Vc) ÷ Vc

Vc is the clearance volume: all of the space left above the piston at top dead center. Rearranged, the same equation gives the other values:

  • Vc = Vs ÷ (CR − 1)
  • stroke = 4 × (CR − 1) × Vc ÷ (π × bore²)
  • bore = √(4 × (CR − 1) × Vc ÷ (π × stroke))

Lengths are converted to meters and volumes to cubic meters first (1 in = 25.4 mm, 1 in³ = 16.387064 cm³, 1 cc = 1 cm³ = 1 mL). The calculator also shows the swept volume and the total cylinder volume at bottom dead center.

To build the clearance volume from parts, add:

  • the combustion chamber volume (measured in cc),
  • the head gasket volume: π ÷ 4 × gasket bore² × compressed thickness,
  • the deck clearance volume: π ÷ 4 × bore² × distance from the piston top to the deck,
  • the piston dish or valve pocket volume, or subtract the dome volume.

Assumptions

  • This is the static, geometric ratio. Valve timing does not enter.
  • Every cylinder is the same, so one cylinder's ratio is the engine's ratio.
  • The compression ratio is above 1 and at most 1,000.

Worked examples by hand

An 86 mm × 86 mm cylinder with 50 cc of clearance volume. Vs = π ÷ 4 × 8.6² × 8.6 = 0.7854 × 73.96 × 8.6 = 499.56 cm³. CR = (499.56 + 50) ÷ 50 = 10.99:1.

The clearance volume for 10:1 with a 4.00 in bore and 3.48 in stroke. Vs = π ÷ 4 × 4² × 3.48 = 43.731 in³ = 716.62 cm³. Vc = 716.62 ÷ (10 − 1) = 79.62 cm³.

The bore for 9.5:1 with a 90 mm stroke and 60 cc of clearance volume. bore = √(4 × 8.5 × 60 ÷ (π × 9)) = √(2,040 ÷ 28.274) = √72.15 = 8.494 cm = 84.94 mm.

Other questions people ask

How do I calculate an engine's compression ratio?

Divide the cylinder volume with the piston at the bottom by the volume with the piston at the top: CR = (Vs + Vc) ÷ Vc. Vs is the swept volume, π ÷ 4 × bore² × stroke, and Vc is the clearance volume above the piston at top dead center.

What goes into the clearance volume?

Everything above the piston at top dead center: the combustion chamber in the head, the volume of the compressed head gasket (π ÷ 4 × gasket bore² × gasket thickness), the deck clearance (π ÷ 4 × bore² × deck height), and the piston dish or valve reliefs. Subtract the volume of a domed piston.

How do I raise or lower the compression ratio?

Make the clearance volume smaller to raise it (a thinner head gasket, a smaller chamber, a domed piston, or a milled head or deck) or larger to lower it. A longer stroke or bigger bore also raises it, because the swept volume grows while the clearance volume stays the same.

What is the difference between static and dynamic compression ratio?

The static ratio is pure geometry and is what this calculator gives. The dynamic ratio counts only the part of the stroke after the intake valve closes, so it is always lower, and it depends on the camshaft.

What is a typical compression ratio?

Heywood gives 8 to 12 for spark-ignition (petrol) engines and 12 to 24 for diesel engines. The right value for your engine also depends on the fuel octane, boost and cam timing.