Tuning
Calculate effective compression ratio under boost, target intercooled charge temps, and density ratio for any altitude. Mandatory math for turbo and supercharger builds.
A boost and effective compression calculator shows what your static compression ratio becomes under gauge boost, using atmospheric pressure and optional intercooler efficiency. Turbo and supercharger builds need this before choosing pump gas, E85, or race fuel — geometric CR alone misleads once pressure rises.
Street tuners use it when debating 9.5:1 pistons at 8 psi vs 10.5:1 at 6 psi. It is a planning worksheet, not a knock predictor — timing, IAT, fuel quality, and dyno tuning still decide safety.
Pair with static compression ratio math and injector sizing for fuel delivery headroom.
Boost is gauge PSI — what a typical boost gauge reads above atmosphere.
Static CR should come from measured geometry or a trusted build sheet, not a guess.
Intercooler efficiency: use a lower value for small street intercoolers in heat-soaked traffic; higher for efficient air-to-water setups.
Knock on the street means pull timing or fuel — do not rely on effective CR alone at high boost on 91 octane.
Pressure ratio = (Boost + 14.7) ÷ 14.7
Effective CR ≈ (Static CR − 1) × Pressure ratio + 1
14.7 PSI is sea-level atmospheric pressure. Example: 9.5:1 static + 8 PSI boost → ratio = 22.7/14.7 ≈ 1.54 → Eff. CR ≈ (8.5 × 1.54) + 1 ≈ 14.1:1 (plan fuel/octane accordingly).
Pressure ratio = (Boost + 14.7) ÷ 14.7. Effective CR ≈ (Static CR − 1) × Pressure ratio + 1 (simplified model).
Example: 9.5:1 static at 8 psi → ratio ≈ 22.7/14.7 ≈ 1.54 → effective CR roughly 14.1:1 — plan fuel and timing accordingly.
Altitude changes atmospheric pressure — sea-level 14.7 psi is the default reference in this tool.
GM’s LS9 crate engine is a supercharged 9.2:1 compression platform rated 650 hp from the factory blower. For custom turbo builds, owners often start near 9.0:1–10.5:1 static and model boost in this calculator before choosing octane.
A common garage scenario: 9.5:1 static with 8 psi boost on pump gas. Effective compression jumps into the mid-12s to low-14s depending on intercooler efficiency — the reason E85 or lower timing is discussed before adding psi.
Calculator effective CR is geometric + boost pressure — heat soak, bad gas, hot IAT, and timing tables cause knock before math says you are safe. Intercooler efficiency matters.
Dynamic compression on a big cam can be harsher than static CR suggests at cruise.
Depends on timing, IAT, and fuel — many street turbo setups stay roughly mid-high 12s to low 14s effective on 91 octane with good intercooling; always tune on a dyno with knock sensors.
This tool uses an efficiency factor on boost — heat-soaked intercoolers raise IAT and knock risk without changing geometric CR.
Automotive boost gauges read gauge pressure above atmosphere — the formula adds 14.7 psi sea-level atmospheric.
Pair this calculator with these related tools in the garage — same session, no signup.