Calculate accurate Static Compression Ratio (SCR) for your engine build.
Distance from piston to deck at TDC
(-) Negative for Dish/Reliefs, (+) Positive for Dome
The Compression Ratio (CR) is simple mathematics, but critical physics. It defines how much the air-fuel mixture is squeezed before ignition. Higher compression extracts more energy from combustion, increasing efficiency and power, but increases the risk of detonation (knock).
| Comp Ratio | Recommended Fuel (Iron Heads) | Recommended Fuel (Alum. Heads) |
|---|---|---|
| 8.0:1 - 9.0:1 | 87 Octane | 87 Octane |
| 9.0:1 - 9.5:1 | 89 Octane | 87 Octane |
| 9.5:1 - 10.5:1 | 91-93 Octane | 89-91 Octane |
| 10.5:1 - 11.5:1 | Race Gas / E85 | 93 Octane |
Compression ratio is the ratio of the volume of the cylinder and combustion chamber when the piston is at the bottom of its stroke versus the volume when the piston is at the top of its stroke.
Use POSITIVE number for a Dome (reduces chamber volume/increases compression). Use NEGATIVE number for a Dish or valve reliefs (adds chamber volume/decreases compression).
For pump gas (91-93 octane), 9.5:1 to 10.5:1 is a safe range for modern aluminum head engines. Older iron head engines may need to stay below 9.5:1.
Find the right tool for your next calculation
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Convert torque to horsepower
Find optimal carburetor size
Calculate static compression ratio
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Check fitment & clearance
