Turbo Size Calculator (compressor airflow lb/min & pressure ratio for a target hp)

Understanding the turbo size calculator — concept and principle

Choosing a turbocharger starts with two numbers: how much air the engine needs at the target power, and what pressure ratio the compressor must make. This calculator follows the turbo makers' matching method to estimate both, so you can plot the point on candidate compressor maps.

Plot 36.9 lb/min at a pressure ratio of 2.81 on candidate compressor maps.

How it is calculated

Airflow Wa = HP × A/F × BSFC ÷ 60 (lb/min)

MAP = Wa × 639.6 × (Tm + 460) ÷ (VE × N/2 × Vd) (psia)

Outlet pressure = MAP + losses

Pressure ratio = (MAP + losses) ÷ (ambient − inlet depression)

HP target crank power, A/F air–fuel ratio, BSFC brake-specific fuel consumption (lb/hp·h).
Tm manifold temperature in °F (converted from °C), N engine rpm, Vd displacement in cubic inches (L × 61.02), VE volumetric efficiency.
Boost = MAP − ambient, also shown in bar.

The principle

Power comes from burning fuel, and burning fuel needs air in a fixed proportion. BSFC tells you how much fuel each horsepower burns per hour, and the air-fuel ratio scales that up to air mass. To fit that mass of air into the cylinders, the ideal gas law relates mass to pressure, temperature and volume: hotter air needs more pressure for the same mass, and a four-stroke engine fills its cylinders once every two revolutions. The compressor must also make up the pressure lost in the intercooler and pipes, while its inlet sits slightly below ambient because of the air filter.

Worked example

Default inputs: 350 hp, A/F 11.5, BSFC 0.55, 2.0 L, 6,500 rpm, VE 95 %, manifold 50 °C, ambient 14.7 psia, losses 2 psi, inlet depression 1 psi.

Wa = 350 × 11.5 × 0.55 ÷ 60 = 36.9 lb/min = 0.279 kg/s.
Tm = 50 °C = 122 °F, so Tm + 460 = 582 °R; Vd = 2.0 × 61.02 = 122.0 in³.
MAP = 36.9 × 639.6 × 582 ÷ (0.95 × 3,250 × 122.0) = 36.4 psia.
Boost = 36.4 − 14.7 = 21.7 psi = 1.5 bar.
Outlet pressure = 36.4 + 2 = 38.4 psia; pressure ratio = 38.4 ÷ (14.7 − 1) = 2.81.

Each point is the calculator re-run with only “Target crank power (hp)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Target crank power (hp)	Compressor airflow (lb/min)
87.5	9.2
175	18.4
262	27.7
350	36.9
438	46.1
525	55.3
700	73.8

How to read the result

Plot airflow and pressure ratio on each candidate compressor map. A good match places the point in or near the high-efficiency islands and to the right of the surge line, with room before the choke region. Repeat the calculation at the torque-peak rpm, where airflow is lower but pressure ratio may be similar, to check surge margin. Higher A/F, lower BSFC or a cooler manifold all change the result noticeably.

Common mistakes

Mixing compressor maps in kg/s or corrected flow with lb/min without converting.
Entering wheel horsepower as crank horsepower.

Keep in mind: This is an educational estimate. Real turbo selection also considers the turbine side, engine limits, fuel system and knock, and must be checked on a dyno with proper tuning.

Frequently asked questions

What is a typical BSFC for a turbo petrol engine?

Turbo matching guides often use values around 0.50–0.60 lb/hp·h at full load.

Why use a rich air-fuel ratio like 11.5?

Extra fuel helps cool combustion and reduce knock at high boost in petrol engines.

What is the surge line?

The boundary on a compressor map left of which airflow becomes unstable and can damage the turbo.

Sources: Garrett Motion, "How to Select a Turbo, Part 2: Understanding Calculations to Turbo Any Engine"

Checked 9 Nov 2026. Garrett Motion, “How to Select a Turbo, Part 2: Understanding Calculations to Turbo Any Engine”: W_a = HP × A/F × BSFC/60; MAP_req from R = 639.6, T_m (°F + 460), VE, N/2 and V_d (L × 61.02); PR = P_2c/P_1c with P_2c = MAP + ΔP_loss and P_1c = atmosphere − depression; typical A/F, BSFC, VE, T_m and loss values.

Your numbers