Turbo Size Calculator

Choosing a turbocharger comes down to two numbers: how much air the engine needs at your power target, and how much the compressor has to raise its pressure to deliver it. Together they form a point on a compressor map. If that point sits inside the map's efficient region, the turbo is a candidate. This calculator works out both numbers using the procedure Garrett publishes for matching its turbochargers.

How Turbo Sizing Works

The method has three steps. Each step uses a value you choose, so the result is only as good as those choices.

Horsepower times BSFC gives fuel burned per hour. Multiplying by the air-fuel ratio turns fuel into air, and dividing by 60 converts hours to minutes. Pounds per minute is the unit compressor maps use.

A four-stroke engine fills each cylinder once every two revolutions, so it swallows RPM ÷ 2 × displacement × VE of volume per minute. The ideal gas law then gives the pressure that volume must be at to contain the required mass of air at the manifold temperature. Hotter air is less dense, so a higher manifold temperature means more boost for the same power.

The compressor has to make more pressure than the manifold sees, because the intercooler and pipes lose some on the way. Its inlet also sits slightly below atmospheric pressure because of the air filter. Both make the pressure ratio higher than boost alone suggests.

Garrett's Worked Example, Checked

Garrett's example uses a 5.7 liter two-valve LS engine with 80% volumetric efficiency, a 6,000 RPM maximum, a 130 °F manifold temperature, an 11.5:1 air-fuel ratio and a BSFC of 0.46. Its results are 57.3 lb/min, a required manifold pressure of 26.0 psia (11.3 psi of boost) and, with a 2 psi pressure loss and 1 psi of inlet depression, a pressure ratio of about 2.0.

One detail is worth knowing. Garrett's article lists the engine as 348 cubic inches, but its published manifold pressure of 26.0 psia matches 346 cubic inches, the LS1's actual displacement. Using 346, our formula reproduces Garrett's figure to within 0.1%. Compressor outlet temperatures in the table assume 70 °F ambient air and 72% compressor efficiency.

The altitude row shows why turbos are sized for where they will run. At 5,000 feet the engine needs the same mass of air, but the compressor starts from thinner air, so the pressure ratio climbs from 2.05 to 2.50 and the air leaves the compressor hotter.

Choosing BSFC, AFR and Volumetric Efficiency

These inputs decide the answer, so choose them deliberately. Garrett's example uses a BSFC of 0.46 and an 11.5:1 air-fuel ratio for a gasoline V8. Turbocharged engines often run richer mixtures and higher BSFC values for detonation margin, and each change moves the airflow requirement directly: a 10% higher BSFC means 10% more air for the same power. Volumetric efficiency describes how well the engine fills its cylinders at peak power. Use dyno data or the maker's figure if you have it. Our volumetric efficiency calculator can estimate it from a known naturally aspirated result.

Plotting the Result on a Compressor Map

A compressor map plots pressure ratio up the side against corrected airflow along the bottom. To check a turbo:

• Plot the peak power point from this calculator. It should fall inside the map, ideally in or near the higher-efficiency islands.
• Plot the peak torque point too. Run the calculator again with the horsepower and RPM at peak torque. Garrett's LS example needs 43.4 lb/min at 4,200 RPM. That point must stay to the right of the surge line, where flow becomes unstable.
• Stay away from the choke region on the right of the map, where efficiency collapses and the compressor overspeeds.
• Compare corrected flow. Makers correct their maps to a standard inlet temperature and pressure, and the reference conditions vary by manufacturer, so check the map's notes.

Twin-turbo setups split the airflow. Each turbo handles half the mass flow at the full pressure ratio, so select the calculator's twin option and plot the per-turbo figure.

Compressor Heat and Intercooling

Compressing air heats it. With a perfect compressor the outlet temperature would rise by the factor PR^0.286. A real compressor adds more heat, set by its efficiency. Boost Lab cites 70 to 75% as typical for a well-matched modern turbo. At a pressure ratio of 3.0 and 72% efficiency, 70 °F air leaves the compressor at about 340 °F, which is why high-boost engines need an intercooler. The manifold temperature you enter should be the temperature after the intercooler, because that is the air the engine actually breathes.

Once you know the boost level, estimate the power gain with the NA to boosted calculator and size the fuel system with the fuel pump size calculator and the injector size calculator.