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Engine airflow in CFM equals displacement times RPM times volumetric efficiency divided by 3456 (for 4-stroke engines). Turbo engines often achieve 100–120% VE under boost.
CFM = (CID × RPM × VE) / 3456Pressure ratio equals absolute outlet pressure divided by absolute inlet pressure. 15 psi boost at sea level: (14.7 + 15) / 14.7 = 2.02 pressure ratio.
PR = (Baro + Boost) / BaroHorsepower correlates with air mass flow. At a given BSFC and AFR, required CFM scales linearly with target HP.
HP ≈ CFM × 0.12 (approximate for pump gas)Updated: July 2026
Required airflow ~52 lb/min (~720 CFM corrected). At 22 psi boost (PR 2.5), plot point on compressor map to verify efficiency island above 70%.
Each turbo handles ~400 HP (~52 lb/min each). Single turbo would need 104 lb/min — verify compressor can flow at 2.8 PR without surging.
At 5000 ft elevation, barometric pressure is ~12.1 psia. Same gauge boost (15 psi) gives higher pressure ratio than at sea level because starting pressure is lower.
Boost pressure alone does not determine power — airflow (CFM/lb-min) matters. A small turbo can make 25 psi on a large engine but runs off the efficiency map and produces less power than a properly sized turbo at 15 psi.
At altitude, atmospheric pressure is lower, so the same gauge boost produces a higher pressure ratio. A turbo runs harder at altitude for the same boost reading — monitor charge temps and knock.
Turbo sizing requires matching compressor airflow and pressure ratio to target horsepower and engine displacement. This calculator estimates boost pressure, mass airflow (CFM), and pressure ratio to plot operating point on a compressor map.