Compression Ratio Calculator
Static CR from bore, stroke, and chamber volume.
Inputs
Results
Static compression ratio
10.23
Effective CR (est.)
7.92
Swept vol/cyl (cc)716.6
Clearance volume (cc)77.7
Gasket volume (cc)8.65
Total displacement (cc)5,733
Total displacement (ci)349.8
Total displacement (L)5.73
Recommended Octane91 (Premium)
How to Use This Calculator
- Enter Bore (inches), Stroke (inches), and Number of cylinders.
- Set Chamber volume (cc), Gasket bore (inches), and Gasket thickness (inches).
- Adjust Piston dome/dish (cc) as needed.
- Review Static compression ratio and Effective CR (est.).
- Use Swept vol/cyl (cc) and Clearance volume (cc) to inform your decision.
How the result changes with Bore (inches)
| Bore (inches) | Static compression ratio | Effective CR (est.) |
|---|---|---|
| 1.5 | 2.3 | 1.97 |
| 2.75 | 5.36 | 4.27 |
| 4.25 | 11.42 | 8.81 |
| 5.5 | 18.45 | 14.09 |
What each input means
- Bore (inches)
- Cylinder bore diameter in inches.
- Stroke (inches)
- Piston stroke length in inches.
- Number of cylinders
- Total cylinder count.
- Chamber volume (cc)
- Combustion chamber volume from head casting (measured via burette method).
- Gasket bore (inches)
- Head gasket bore opening diameter (slightly larger than cylinder bore).
- Gasket thickness (inches)
- Compressed head gasket thickness.
- Piston dome/dish (cc)
- Negative = dished piston (adds clearance), positive = domed piston (reduces clearance).
What each result means
- Static compression ratio
- Geometric compression ratio: (swept + clearance) / clearance.
- Effective CR (est.)
- Dynamic CR estimate based on typical intake valve closing at 60° ABDC.
- Swept vol/cyl (cc)
- Displacement per cylinder in cubic centimeters.
- Clearance volume (cc)
- Total clearance volume per cylinder (chamber + gasket - dome + dish).
- Gasket volume (cc)
- Volume contributed by the head gasket.
- Total displacement (cc)
- Total engine displacement across all cylinders.
- Total displacement (ci)
- Total engine displacement in cubic inches.
- Total displacement (L)
- Total engine displacement in liters.
How this is calculated
Worked example, using the default values
- Identify Input Parameters4 parametersBore (inches) = 4, Stroke (inches) = 3.48, Number of cylinders = 8, Chamber volume (cc) = 64 = 7 input(s) provided
- Calculate Static compression ratioStatic compression ratio = (sweptVolumePerCylCc + clearanceVolumeCc) / max(1, clearanceVolumeCc)10.23 = 10.23
- Calculate Effective CREffective CR = (effectiveSwept + clearanceVolumeCc) / max(1, clearanceVolumeCc)7.92 = 7.92
- Calculate Swept vol/cylSwept vol/cyl = (π / 4) * pow(boreIn, 2) * strokeIn * 16.387716.6 = 716.6
- Calculate Clearance volumeClearance volume = chamberCc + gasketVolumeCc - pistonCc77.7 = 77.7
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