What it does
This calculator works out an engine's static compression ratio from the parts that set it: bore, stroke, combustion chamber volume, head gasket bore and thickness, deck clearance and the piston's dome or dish. It shows the swept volume and the clearance volume of one cylinder, and how the ratio changes with different head gasket thicknesses, so you can see what a part swap will do before you buy.
How it's calculated
The compression ratio compares the cylinder's volume with the piston at the bottom of its stroke to the volume left with the piston at top dead center (TDC):
Swept volume is the cylinder the piston travels through. Clearance volume is everything above the piston at TDC:
clearance = chamber + gasket + deck + dish (or − dome)
gasket = π ÷ 4 × gasket bore² × gasket thickness
deck = π ÷ 4 × bore² × deck clearance
Lengths in inches give cubic inches. Multiplying by 16.387064 turns cubic inches into cc, since an inch is exactly 2.54 cm. With lengths in millimeters, the calculator divides by 1,000 to get cc.
Worked example
A small-block V8 with a 4.030 in bore and a 3.480 in stroke, a 64 cc chamber, a 4.100 in gasket bore with 0.041 in compressed thickness, 0.025 in deck clearance and 5 cc of valve reliefs:
- Swept: π ÷ 4 × 4.030² × 3.480 × 16.387064 = 727.41 cc
- Gasket: π ÷ 4 × 4.100² × 0.041 × 16.387064 = 8.87 cc
- Deck: π ÷ 4 × 4.030² × 0.025 × 16.387064 = 5.23 cc
- Clearance: 64 + 8.87 + 5.23 + 5 = 83.10 cc
- CR = (727.41 + 83.10) ÷ 83.10 = 9.75:1
With a 5 cc dome instead of 5 cc of reliefs, the clearance volume drops to 73.10 cc and the ratio rises to 10.95:1.
Effect of head gasket thickness
For the example engine above, changing only the gasket's compressed thickness:
| Gasket thickness | Compression ratio |
|---|---|
| 0.027 in | 10.09:1 |
| 0.030 in | 10.01:1 |
| 0.039 in | 9.80:1 |
| 0.041 in | 9.75:1 |
| 0.051 in | 9.53:1 |
| 0.060 in | 9.34:1 |
Limitations
- This is static compression. Dynamic (effective) compression also depends on when the intake valve closes, which this calculator doesn't model.
- Use measured or published volumes. A chamber or piston volume that's off by a few cc changes the result noticeably, which is why engine builders measure them with a burette.
- Use the gasket's compressed thickness, not its thickness out of the package.
- Rotary engines and two-strokes with exhaust ports aren't covered by this formula as written.
Sources
- Engineering ToolBox: Piston engines, compression ratio CR = (Vd + Vc) ÷ Vc
- JE Pistons: How to calculate engine compression ratio (gasket, deck, dome and dish volumes)
- NIT Srinagar, Mechanical Engineering: Engine design and operating parameters (compression ratio as maximum ÷ minimum cylinder volume)
- NIST Handbook 44 Appendix C: 1 inch = 2.54 cm exactly