About Compression Ratio
In an internal combustion engine, compression ratio (CR) is the ratio between the total volume inside a cylinder when the piston is at the bottom of its stroke (bottom dead center) and the volume left when it is at the top (top dead center). It is calculated as CR = (Vd + Vc) / Vc, where Vd is the swept volume the piston displaces on each stroke and Vc is the clearance volume — the combustion chamber space remaining above the piston at top dead center.
The formulas
- Swept volume per cylinder: Vd = (pi / 4) x bore^2 x stroke, using bore and stroke in the same length unit, converted to cubic centimeters.
- Compression ratio: CR = (Vd + Vc) / Vc, usually written as "CR : 1" (for example, 10.5:1).
- Total engine displacement: Vd per cylinder multiplied by the number of cylinders, often reported in liters (1 liter = 1,000 cc).
Practical uses
- Checking a build's compression ratio after changing pistons, decking the block, or swapping a cylinder head with a different chamber volume
- Estimating total engine displacement from bore, stroke, and cylinder count
- Comparing a target compression ratio against the fuel octane and forced-induction plans for an engine build
Frequently Asked Questions
What is the compression ratio formula?
Compression ratio (CR) equals the total cylinder volume divided by the clearance volume: CR = (Vd + Vc) / Vc, where Vd is the swept (displacement) volume the piston pushes through and Vc is the clearance volume left above the piston at top dead center. Vd is calculated from bore and stroke as (pi/4) x bore^2 x stroke.
What is a good compression ratio for a street engine?
Most naturally aspirated gasoline engines run 9:1 to 11:1 on pump fuel. Turbocharged or supercharged engines are typically built lower, around 8:1 to 10:1, because forced induction already raises cylinder pressure. Ratios above about 11:1 usually need premium fuel or knock-sensing tuning to avoid detonation.
Does compression ratio include the head gasket and deck clearance?
A fully precise build calculation also accounts for head gasket volume, deck clearance, and piston dome or dish volume, all rolled into the clearance volume. This calculator uses the standard swept-volume-plus-clearance-volume formula; for engine building, measure or estimate the combined clearance volume (chamber + gasket + deck + piston shape) and enter that total as the clearance volume.
How does compression ratio affect engine performance?
Raising the compression ratio increases thermal efficiency and power because the air-fuel mixture is compressed more before ignition, extracting more energy from combustion. The tradeoff is a higher risk of knock (uncontrolled pre-ignition) on lower-octane fuel, which is why high-compression engines typically require premium gasoline or careful ignition timing.