What this calculator does
Compression ratio is the total cylinder volume at the bottom of the stroke divided by the volume left at the top. It is the swept volume plus the clearance volume, all over the clearance volume.
Clearance volume is the term that does the work and the one that is hard to measure. A 4 inch bore and 3.48 inch stroke gives 716.6 cc of swept volume in every case, but a clearance volume of 50 cc gives 15.33:1 while 90 cc gives 8.96:1. Small changes in the combustion chamber move the ratio a long way.
The formula
The swept volume is π divided by four, multiplied by the bore squared and the stroke, which is the cylinder volume the piston displaces. The compression ratio is that swept volume plus the clearance volume, divided by the clearance volume. Bore and stroke are entered in inches and the volumes come out in cubic centimetres.
| Term | Meaning |
|---|---|
| Swept volume | What the piston displaces between top and bottom of its travel. |
| Clearance volume | What remains above the piston at top dead centre: combustion chamber, head gasket, piston dish and deck clearance combined. |
| Static compression ratio | The geometric figure this calculates, as distinct from dynamic compression which accounts for valve timing. |
| Detonation | Uncontrolled combustion, which becomes more likely as compression rises, and is what limits the practical ratio on pump fuel. |
The inputs explained
| Field | What to enter |
|---|---|
| Cylinder bore (in) | Cylinder bore diameter in inches. |
| Piston stroke (in) | Piston stroke in inches. |
| Clearance volume (combustion chamber + gasket + dish/dome, at TDC) (cc) | Clearance volume in cubic centimetres. This is the sum of combustion chamber volume, gasket volume, piston dish or dome and deck clearance, and it usually has to be measured rather than looked up. |
When to use it
Planning an engine build
Compression ratio determines what fuel the engine needs and how much power it makes, so it is one of the first figures settled in a build.
Assessing a head gasket change
A thicker gasket adds clearance volume and lowers the ratio, which is a common way to make a high-compression engine run on pump fuel.
Checking a claimed specification
Bore, stroke and chamber volume are usually published, and the ratio can be checked against the claim.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How does clearance volume change the ratio?
A fixed cylinder size across a range of clearance volumes.
| Clearance volume | Compression ratio | Swept (displacement) volume | Displacement in cubic inches |
|---|---|---|---|
| 50 cc | 15.33:1 | 716.6 cc | 43.73 ci |
| 60 cc | 12.94:1 | 716.6 cc | 43.73 ci |
| 75 cc | 10.55:1 | 716.6 cc | 43.73 ci |
| 90 cc | 8.96:1 | 716.6 cc | 43.73 ci |
Questions
What compression ratio can I run on pump fuel?
Broadly 9:1 to 11:1 for a naturally aspirated engine on regular fuel, and up to about 12:1 with premium, good chamber design and modern knock control. Forced induction needs considerably lower. Detonation is the limit, and it depends on fuel octane, chamber shape, cooling and timing as much as on the ratio itself.
How do I measure clearance volume?
By filling the combustion chamber with a measured liquid using a burette and a plate, which is the standard workshop method. It is the sum of the chamber, the gasket bore volume, the piston dish or dome and the deck clearance, and each part has to be accounted for separately.
What is the difference between static and dynamic compression?
Static is the geometric ratio this calculates. Dynamic accounts for when the intake valve actually closes, since the piston is already partway up the bore by then. A long-duration camshaft lowers dynamic compression substantially without changing the static figure, which is why high static ratios work with aggressive cams.
Does higher compression always mean more power?
Up to a point, since higher compression extracts more work from the same charge. The gains diminish above about 12:1 and the fuel octane requirement rises steeply. Past the detonation limit, higher compression costs power and damages the engine rather than helping.
For two-stroke fuel mixing, see the 2-stroke fuel mix calculator. For working out fuel consumption, see the fuel consumption calculator.