Boost and airflow calculator

What these numbers tell you

A turbocharger or supercharger raises the pressure of the air going into the engine, so each cylinder holds more air and burns more fuel. Two figures describe the job: how much air the engine must be fed per minute, and by what factor the compressor must raise the pressure. Together they are the starting point for matching a compressor, which this page does not do.

The two calculations

In the first mode you enter the horsepower you are aiming for, the air/fuel ratio and the BSFC, and the page applies Garrett's formula: airflow = hp × A/F × BSFC ÷ 60. In the second you enter the engine and the boost, and the page works out how much air the engine can pass: the displacement swept per minute (a four-stroke engine fills its cylinders once every two revolutions) times the volumetric efficiency times the density of the manifold air. The density is the ideal-gas value, pressure ÷ (R × temperature), with R = 53.35 ft·lbf per lbm·°R for air. That second formula is our derivation, not a published recipe.

Worked example

A 5.7 L engine (348 cubic inches) at 6,000 RPM with 95 percent volumetric efficiency and 12 psi of boost has a manifold absolute pressure of 26.7 psia. With a 70 percent efficient compressor and a 70 percent effective intercooler (80 °F air), the compressor outlet is 223 °F and the manifold 123 °F. The engine passes 71.0 lb/min, equal to 574 cubic feet per minute at the manifold density of 0.124 lb/ft³. At an A/F of 11.5 and a BSFC of 0.46, that supports about 805 hp. Garrett's rule of thumb gives 674 to 745 hp for the same airflow.

Common mistakes

Using gauge boost as the pressure ratio is the most common: a 15 psi engine at sea level has a ratio near 2, not 15. Second, forgetting the inlet loss: a restrictive filter and pipe lower the pressure at the compressor inlet, so the ratio the compressor must make is higher than the boost suggests. Garrett cites roughly 1 psi or less for a well-designed system and 4 psi or more for a restrictive stock one. Third, forgetting altitude: the ambient pressure at 5,000 feet is much lower than 14.7 psia, so the same boost is a higher ratio. Enter the real ambient pressure. Finally, mixing up the air/fuel ratio and the lambda: Garrett lists gasoline stoichiometric as 14.7:1 and says a lower number is richer.

Limits of this method

This is an educational estimate. The BSFC range Garrett gives for turbocharged gasoline engines, about 0.50 to 0.60 and higher, is wide enough to move the result 20 percent. The volumetric efficiency varies with RPM and boost, and the calculation ignores the pressure drop through the intercooler and pipes, the exhaust back pressure, camshaft timing and the heat soak of the intake. Engine damage from running too much boost or too lean a mixture is real, so do not use the numbers to set a tune. For internal-combustion fundamentals in depth, the standard text is J. B. Heywood, Internal Combustion Engine Fundamentals; we cite it generally and did not check page numbers.

How do I calculate the airflow a turbo engine needs?

Garrett Motion gives airflow in pounds per minute as horsepower × air/fuel ratio × BSFC ÷ 60. Their worked example is 650 hp at an air/fuel ratio of 11.5 and a BSFC of 0.46, which comes to 57.3 lb/min. The 60 converts hours to minutes: hp × BSFC is the fuel in pounds per hour, multiplying by the air/fuel ratio gives the air in pounds per hour, and dividing by 60 gives pounds per minute.

What is pressure ratio and how is it calculated?

The pressure ratio compares the pressure leaving the compressor with the pressure entering it, both in absolute terms. Garrett's definition is (boost + atmospheric pressure) ÷ (atmospheric pressure − inlet loss). At 12 psi of boost, 14.7 psia of atmosphere and 1 psi of loss in the filter and pipe, the ratio is 1.95. At 14.7 psi of boost and no loss it is exactly 2.0: a compressor that doubles the pressure. Gauge pressure alone, 12 psi, would mislead you because the compressor works on absolute pressure.

How much horsepower does one pound per minute of air make?

Garrett's rule of thumb is 9.5 to 10.5 horsepower at the flywheel for each pound per minute of airflow for turbocharged gasoline engines, and it says its newer GTX Gen II and G Series turbos have the potential for 10 to 11. The rule folds in a typical air/fuel ratio and BSFC. This page also computes the horsepower directly from your A/F and BSFC, so the two should agree within the range of the rule.

How hot does air get when it is compressed?

For an ideal (no-loss) compressor, T2 = T1 × PR^0.2857, where temperatures are absolute (Rankine or kelvin) and the exponent is (γ − 1) ÷ γ with γ about 1.4 for air (NASA Glenn). A real compressor wastes energy as heat, so you divide the temperature rise by the compressor's efficiency. At a pressure ratio of 2, with 80 °F intake air and 70 percent efficiency, the air leaves at 249 °F (198 °F if the compressor were perfect). The efficiency is read from the compressor map; 70 percent is an example, not a promise.

What does an intercooler change?

It removes heat from the compressed air before the engine. Its effectiveness is how much of the possible temperature drop it achieves: the drop from the compressor outlet toward the cooling air temperature. With 70 percent effectiveness and 80 °F cooling air, the same pressure-ratio-2 example leaves the intercooler at 131 °F instead of 249 °F. Cooler air is denser, which lets more oxygen into the cylinder at the same boost and lowers the risk of knock. Garrett quotes intake manifold temperatures of roughly 100 to 130 °F with an intercooler and 175 to 300 °F without one.

What is volumetric efficiency?

The ratio of the air that actually enters the cylinders to the air that would fill the displacement at the manifold's density. Garrett puts the peak at about 95 to 99 percent for modern four-valve heads and 80 to 95 percent for two-valve designs, with the peak near the torque peak. It rises and falls with RPM, so use the figure for the RPM you are sizing at.

Can I use this to pick a turbo?

It is a first step. The airflow and the pressure ratio mark a point on a compressor map, and the map shows whether that point sits in a high-efficiency region, away from surge and choke. We do not have maps, and airflow in a map is usually stated as corrected flow, a quantity that includes the inlet temperature and pressure, so this page gives only the starting figures to carry to a manufacturer's chart.

Educational estimate, not tuning advice: wrong boost or fueling can wreck an engine, so have the work checked by a professional. Sources: Garrett Motion, “How to select a turbo, part 2: understanding calculations to turbo any engine” (garrettmotion.com, a turbo maker, so a vendor source): the airflow formula, pressure ratio, the 650 hp example, BSFC, VE and intake temperature figures; NASA Glenn Research Center, “Compressor thermodynamics” (grc.nasa.gov): isentropic relation and γ ≈ 1.4 for air. The engine-airflow formula and the temperature corrections for efficiency and the intercooler are standard thermodynamics, derived by us as shown above. Related: injector size calculator, horsepower calculator, engine displacement calculator. How we check our calculators.