Boosted / Effective Compression Ratio
Boost raises the pressure the piston starts with, so the effective ratio climbs with the manifold pressure ratio.
| Effective compression ratio | 15.12 | : 1 |
| Manifold absolute pressure | 24.7 | psi |
| Pressure ratio | 1.680 | × |
- effective CR = static CR × (boost + atmospheric) ÷ atmospheric
When you need it
When choosing a static compression ratio for a supercharged or turbocharged build, and when comparing what different boost levels do to a given short block.
What this number is, and is not
It scales the static ratio by the pressure ratio across the engine, which makes it a clean way to compare combinations on the same basis.
It is not a detonation predictor. Charge temperature after the compressor, intercooler efficiency, fuel, cam timing, ignition timing and chamber design all move the real limit enormously. Two engines with the same effective ratio on paper can behave completely differently.
Practical compression for boost
The old rule of dropping to 8:1 and below came from carbureted blower engines on pump gas with no knock control, and it is still sound in that context.
Modern turbocharged engines run considerably more static compression than that, because good intercooling, E85 or direct injection, and knock sensing changed what is survivable. If you are running E85 and a proper intercooler, the combinations this calculator makes look alarming are often entirely workable — but that is a decision to make with a knock strategy and exhaust gas temperatures in front of you, not a spreadsheet.