Dynamic Compression Ratio Calculator
Static compression only counts from BDC. The intake valve is still open there, so real cylinder filling starts when it closes. This uses the effective stroke from that point.
| Dynamic compression ratio | 7.03 | : 1 |
| Static compression ratio | 8.46 | : 1 |
| Effective stroke | 2.8128 | in |
| Stroke actually used | 80.8 | % |
| Effective swept volume | 579.23 | cc |
| Clearance volume | 96.02 | cc |
- effective stroke = piston position at intake valve closing, measured from TDC
- x = r + L − ( r·cos θ + √(L² − (r·sin θ)²) ), r = stroke ÷ 2, L = rod length, θ = 180° − IVC
- DCR = (effective swept volume + clearance volume) ÷ clearance volume
When you need it
When matching a cam to a compression ratio, or working out why a high-compression engine with a big cam feels soft and will not pull below three thousand.
Why static compression lies
Static compression assumes the cylinder starts compressing at bottom dead center. It does not. The intake valve is still open well past BDC, and nothing is being compressed until it closes — some of the charge simply gets pushed back up the port.
The bigger the cam, the later the intake closes, and the shorter the stroke that is actually doing any compressing. That is why an 11:1 engine with a large cam will happily run on pump gas while a 10:1 engine with a tight stock cam rattles under load. The dynamic figure is the one that tracks how the engine behaves.
What to aim for
Roughly 7.5 to 8.5:1 dynamic is the pump-gas window on iron heads. Aluminum heads, a tight quench and disciplined ignition timing let you work at the top of it.
Below about 7.5 the engine goes lazy — soft throttle response, poor idle quality and low cranking pressure. If you land there, the cam is too big for the compression you built, and the fix is more static compression rather than less cam.
Use the right closing figure
The intake closing point must be the seat-to-seat number, normally quoted at 0.006 inch tappet lift, not the 0.050 inch figure used for comparing cams. The two can differ by twenty-five or thirty degrees, and using the wrong one will throw the answer off by a full point or more.