Compression Ratio Calculator
Calculate accurate Static Compression Ratio (SCR) for your engine build.
Cylinder Specs
Distance from piston to deck at TDC
Volumes & Gasket
(-) Negative for Dish/Reliefs, (+) Positive for Dome
Static Compression Ratio
0 : 1
Cylinder Volume (Vd)
0 cc
Total Clearance Vol (Vc)
0 cc
Compression Ratio Guide
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).
The Components
- Bore & Stroke: Determine the swept volume of the cylinder.
- Chamber Volume: The volume inside the cylinder head.
- Piston Volume: Domes subtract from chamber volume (increasing CR), while dishes and valve reliefs add to chamber volume (decreasing CR).
- Deck Clearance: The gap between the piston top at TDC and the block deck surface. This effectively adds volume.
- Head Gasket: Its bore and compressed thickness determine the volume it adds to the chamber.
Compression Ratio vs. Octane
| 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 |
FAQ
What is compression ratio?
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.
Should I input dome volume as positive or negative?
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).
What is a good street compression ratio?
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.
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