How this calculator works
A turbocharger raises the pressure in the intake, so each intake stroke traps more air and the engine can burn more fuel. If the engine makes a known power per litre without boost, the ratio of desired power to that naturally aspirated estimate is also the pressure ratio the compressor must supply. Subtracting one atmosphere from the absolute pressure ratio and multiplying by atmospheric pressure gives the gauge boost. Compressor airflow is estimated from power, a rich-mixture brake specific fuel consumption and an air-fuel ratio, which reduces to roughly 0.11 pounds of air per minute for each horsepower. This is a first-pass sizing calculation: compressor maps, pressure drop across the intercooler, altitude and compressor efficiency decide whether a particular turbo is suitable.
Formula
NA power = capacity (L) x specific output (hp/L). Pressure ratio = desired power / NA power. Boost (psi) = (pressure ratio - 1) x 14.6959. Airflow (lb/min) = desired power x 0.55 x 12 / 60.
Worked example
A 2.0 litre engine with an estimated 75 hp per litre makes 150 hp without boost. To reach 400 hp the pressure ratio is 400 / 150 = 2.667, which is (2.667 - 1) x 14.6959 = 24.5 psi of boost. The compressor must flow about 44 lb/min of air, or 332.6 g/s.
Advertisement
Frequently asked questions
Does this tell me which turbo to buy?
It gives the pressure ratio and airflow the compressor must meet, which are the two axes of a compressor map. You still need to check the map for efficiency, surge margin and choke at your operating points, and match the turbine housing to the application.
Why does the calculation need a naturally aspirated estimate?
The pressure ratio is a ratio of desired power to the same engine unboosted. A high-revving engine with good head flow needs less boost for the same target than a restrictive one. Use a known dyno figure if you have one.
Is 24 psi of boost safe?
Safety depends on the engine, compression ratio, fuel, intercooling and tune. This page sizes airflow and pressure only; it does not define a safe boost limit. Have the tune checked by a competent engine builder.
Why is the airflow estimate approximate?
It assumes a fixed BSFC and air-fuel ratio. A richer mixture for cooling, a different fuel, or a more efficient engine changes the airflow for the same power. Treat the number as a starting point for map selection.