How boost affects compression
Forced induction packs more air in before the piston ever moves, so the effective compression ratio rises with the pressure ratio:
effective CR = static CR × (boost + atmospheric) ÷ atmospheric
Higher effective compression means more power — but also more heat and detonation risk, which is why boosted engines run lower static compression, intercooling and higher-octane fuel. The pressure ratio (absolute manifold ÷ ambient) is the number you use to read a turbo compressor map.
FAQ
What effective CR is "safe"?
It depends on fuel, cooling and tuning — but very roughly, pump-gas builds often keep effective CR in the low-to-mid teens; E85 and race fuel allow more. Treat this as a planning number, not a guarantee.
Does intercooling change this?
This shows the pressure side. An intercooler lowers charge temperature, raising real air density further and cutting detonation risk for the same boost.