How to use this calculator
- Enter cylinder geometry. Enter bore, stroke and cylinder count.
- Enter chamber and piston volume. Use positive piston volume for a dish and negative for a dome.
- Enter deck and gasket dimensions. Add deck clearance, gasket bore and compressed gasket thickness.
- Read compression ratio. Check total clearance volume and static compression ratio.
How it works
Static compression ratio compares the cylinder volume before and after the compression stroke. Swept volume is V_s = (pi/4) x bore^2 x stroke. Clearance volume stacks the combustion chamber, piston dish or dome, gasket volume and deck volume. The final ratio is CR = (V_s + V_c) / V_c.
If you only need displacement, use the bore and stroke calculator. Then use the piston speed calculator to screen the same stroke at your intended RPM.
Do not back-solve static compression ratio from horsepower alone. Use this page for the geometry, then use BMEP, engine airflow and fuel injector flow when the question moves from compression geometry to power output.
Compression Ratio & Horsepower Workflow
A horsepower target can guide the engine package, but static compression ratio still comes from volume geometry. Use this workflow to keep the geometry, cam timing, airflow and fuel checks separate.
| Question | Use this input or page | Decision it supports |
|---|---|---|
| What is the static compression ratio? | Bore, stroke and clearance-volume stack | Set the geometric CR from chamber, piston, deck and gasket volumes. |
| How does cam timing change the effective squeeze? | Dynamic compression ratio | Screen intake-closing timing before judging octane or boost margin. |
| What power level does the cylinder pressure imply? | BMEP | Relate torque, displacement and RPM to an engine-load target. |
| Can the engine move enough air? | Engine airflow | Estimate airflow demand from displacement, RPM and volumetric efficiency. |
| Can the fuel system support the target? | Fuel injector flow | Size injector flow from horsepower, BSFC, injector count and duty cycle. |
Worked example
Verified against the live calculator
An 86 mm bore and 86 mm stroke cylinder sweeps about 500 cc. With 50 cc chamber volume, 5 cc piston dish, a 1 mm gasket and 0.5 mm deck clearance, clearance volume is about 64 cc and static compression ratio is about 8.8:1.
Frequently asked questions
What is static compression ratio?
Static compression ratio is the cylinder volume at bottom dead center divided by the clearance volume at top dead center. It is a geometric ratio.
Is piston dish volume positive or negative?
Enter dish, valve relief and bowl volume as positive because they add clearance volume. Enter dome volume as negative because it removes clearance volume.
Does this calculate dynamic compression ratio?
No. Dynamic compression also depends on cam timing and intake valve closing. This calculator is static geometric compression ratio.
Can compression ratio be calculated by horsepower?
No. Horsepower does not determine static compression ratio. Static compression comes from bore, stroke and clearance volumes; horsepower depends later on airflow, fuel, RPM, combustion efficiency, cam timing, boost and tuning.
Why does gasket bore matter?
The head gasket creates clearance volume equal to gasket bore area times compressed gasket thickness.
Method & assumptions
- Positive piston volume adds clearance; negative piston volume removes clearance.
- Deck clearance is positive when the piston is below deck at TDC.
- Octane requirement and detonation margin depend on cam timing, chamber shape, fuel, boost, temperature and tuning.