Compression Ratio Calculator
Work out the static compression ratio of an engine build from its cylinder and combustion chamber dimensions.
How to use the Compression Ratio Calculator
- Enter the bore and pick its unit.
- Enter the stroke and pick its unit.
- Enter the combustion chamber volume and pick its unit.
- Enter the piston volume (dish/valve reliefs +, dome −) and pick its unit.
- Enter the head gasket bore and pick its unit.
- Enter the gasket compressed thickness and pick its unit.
- Enter the deck clearance (piston below deck +) and pick its unit.
- Press Calculate. The result updates automatically as you type, and you can switch the result unit at any time.
About the Compression Ratio Calculator
Static compression ratio compares the cylinder volume with the piston at the bottom of its stroke to the volume left when it reaches the top. Higher compression extracts more power and efficiency from each combustion event but raises cylinder pressure and the risk of detonation, so it must match the fuel octane, camshaft timing and whether the engine is boosted.
As a rough guide, pump-gas naturally aspirated street engines usually run about 9:1 to 10.5:1 with iron heads, while turbocharged and supercharged builds often run 8:1 to 9.5:1. Measure the chamber and piston volumes with a burette for accuracy; published figures can be a few cc off. A change of just a few cc or a different gasket thickness can move compression by several tenths of a point.
Formula
CR = (Swept volume + Clearance volume) ÷ Clearance volume
Swept volume = π/4 × Bore² × Stroke. Clearance volume = Chamber cc + Piston dish (or − dome) + Gasket volume + Deck volume, where Gasket volume = π/4 × Gasket bore² × Thickness and Deck volume = π/4 × Bore² × Deck clearance.
Example: 727.4 cc swept and 82.7 cc of clearance gives (727.4 + 82.7) ÷ 82.7 ≈ 9.8:1.
Example calculation
Using the default values:
- Bore = 4.03 in
- Stroke = 3.48 in
- Combustion chamber volume = 64 cm³
- Piston volume (dish/valve reliefs +, dome −) = 5 cm³
- Head gasket bore = 4.1 in
- Gasket compressed thickness = 0.039 in
- Deck clearance (piston below deck +) = 0.025 in
the calculator returns:
- Static compression ratio: 9.8 : 1
- Swept volume per cylinder: 727.41 cm³
- Total clearance volume: 82.66 cm³
- Gasket volume: 8.44 cm³
- Deck volume: 5.23 cm³
Frequently asked questions
What is a good compression ratio for 91 octane?
Most cast-iron-head street engines are happy around 9.5:1 to 10:1 on 91 octane. Aluminum heads and modern chamber designs can tolerate about half a point more.
How does head gasket thickness change compression?
A thicker gasket adds clearance volume and lowers compression. On a 4 in bore, each 0.010 in of thickness adds about 2 cc, typically changing the ratio by roughly 0.2.
What is the difference between static and dynamic compression?
Static compression uses the full stroke. Dynamic compression starts counting when the intake valve closes, so it depends on cam timing and is always lower.
Last updated: · Category: Automotive calculators