Turbo Airflow & Compressor Sizing

An engine burns fuel in proportion to the power it makes — that proportion is the brake specific fuel consumption — and it needs air in proportion to the fuel, at the air-fuel ratio it runs. Two multiplications therefore turn a power target into a mass flow of air: power × BSFC × AFR. Mass flow, not cubic feet a minute, is what a compressor map is drawn in, and the two numbers you multiply by are the two that differ most between engines, so neither is assumed here. A boosted petrol engine at full load is thirstier and richer than the same engine atmospheric, and using its naturally aspirated figures understates the turbo you need.

The engine swallows a fixed volume a cycle — displacement × speed ÷ two for a four-stroke, times its volumetric efficiency — so carrying the required mass flow through it sets the density, and the ideal gas law turns density into a manifold pressure. The compressor has to make that pressure plus whatever the intercooler and piping lose, starting from atmospheric minus whatever the filter costs; both losses are charged twice, once as pressure to make and once as pressure the engine never sees. Finally the flow is corrected to the map’s reference conditions, because a map is plotted in corrected pounds a minute and reading raw flow against it sizes the wrong turbo.

Steady state at one engine speed, and the compressor side only. No turbine, no A/R ratio, no wastegate, no spool behaviour, no exhaust back pressure and no efficiency island — this page gives you the point to plot, not the turbo to buy, and the outlet temperature that pressure ratio implies belongs to the boost and pressure ratio page. Air is treated as an ideal gas at R = 287.05 J/(kg·K). BSFC, air-fuel ratio and volumetric efficiency are your inputs from your own engine, and a VE measured on an atmospheric dyno is optimistic here. Nothing leaves the browser; the same four anonymous usage counts as the rest of the site apply.