Compression Ratio Calculator
Static compression ratio from bore, stroke and combustion chamber volume, with the swept volume.
Results
Compression ratio
10.083
Displacement per revolution
454.17
cm³
Displacement per revolution
1816.7
cm³
Ratio
0.9535
What this tool does
The chamber volume is the whole clearance space at top dead centre: cylinder head chamber, head gasket thickness, piston dish or dome and deck clearance all belong in it. Raising compression gains efficiency until the fuel starts to knock, which is why octane rating and compression ratio go together.
Formula
CR = (Vd + Vc) ÷ Vc
Variables
| Symbol | Meaning | Unit |
|---|---|---|
bo | Bore | mm |
st | Stroke | mm |
cv | Combustion chamber volume | cm³ |
nc | Number of cylinders | — |
CR | Compression ratio | — |
VD | Displacement per revolution | cm³ |
VT | Displacement per revolution | cm³ |
RS | Ratio | — |
Worked example
- Bore82 mm
- Stroke86 mm
- Combustion chamber volume50 cm³
- Number of cylinders4
- Compression ratio10.083
- Displacement per revolution454.17 cm³
- Displacement per revolution1816.7 cm³
- Ratio0.9535
Limitations
- The result is an estimate based only on the values you type. Real situations often include factors this calculator does not know about.
- For work that must comply with a standard or be signed off, check the result against the applicable code and have it reviewed by a qualified engineer.