Formula
Airflow is the mass of air the engine has to swallow to make the horsepower at the air-fuel ratio and the brake specific fuel consumption you enter. Garrett’s matching notes write it as horsepower times AFR times BSFC, divided by 60, in pounds per minute. A rule of thumb is about a tenth of a pound per minute per horsepower. AFR 12 and BSFC 0.55 land a little above that, which is why 400 hp in the full match is 44.0 lb/min rather than 40.
Wa = HP × AFR × BSFC / 60
MAP = Wa × 639.6 × (460 + T_manifold) / (VE × RPM / 2 × Vd)
PR = (MAP + intercooler loss) / (ambient − inlet loss)
Wa corrected = Wa × √((T_inlet + 460) / 545) / (P_inlet / 13.95)
Manifold absolute pressure uses the gas constant 639.6 in these units, manifold temperature in degrees Fahrenheit, volumetric efficiency as a fraction, and displacement in cubic inches. RPM is divided by 2 because a four-stroke takes air every other revolution. Boost is that manifold pressure minus ambient. If the manifold pressure comes out below ambient, the page still shows the numbers and says no boost is needed at that point.
The pressure ratio the compressor sees is not quite boost over ambient. Air lost in the filter has to be made up, and pressure dropped in the intercooler has to be made up too. The page uses (manifold pressure + intercooler loss) / (ambient − inlet loss). The simple ratio (boost + ambient) / ambient is shown beside it. Fifteen psi at 14.7 psia is 2.02. The same boost with 2 psi of intercooler loss and 1 psi of inlet loss is 2.31.
Corrected airflow puts the mass flow on the reference printed on many Garrett maps: 545 degrees Rankine and 13.95 psia. Other makers print a different reference. Use the one on the map you are holding. The CFM figure divides pounds per minute by 0.0765 and is only a standard-density sketch. Cubic metres per minute are that CFM times 0.0283168.
Displacement accepts cubic inches, litres, or cubic centimetres, using 16.387064 cm³ per cubic inch, the same identity as the displacement page. Two litres is 122.05 cubic inches. The Garrett example is typed as 122 cubic inches so the worksheet fractions stay exact. A twin switch halves the airflow and the corrected airflow per compressor. It does not change the engine’s manifold pressure or pressure ratio.
The grid is PR against corrected lb/min for up to four points. It is a blank sheet. Surge, choke, and efficiency islands are properties of one compressor, and this page does not invent them. Turbine housing and A/R change how fast the compressor gets to that point and how much exhaust pressure the engine sees. Those are choices on the turbo maker’s turbine maps, not a number here. A dyno and a tune, with knock monitoring, are what confirm the match.
Worked examples
400 hp, AFR 11.5, BSFC 0.55. 42.17 lb/min. At 122 ci, 6,500 rpm, VE 0.90, and 130 °F in the manifold, boost is 29.9 psi and the pressure ratio with the default losses is 3.40.
Garrett-style full match. AFR 12 gives 44.00 lb/min. At 7,000 rpm and 122 ci the manifold pressure is 43.21 psia, boost is 28.5 psi at 14.7 psia, the pressure ratio is 3.30, and corrected airflow at 70 °F inlet is 44.18 lb/min.
15 psi, as a comparison. 2.02 with no losses. 2.31 with 2 psi of intercooler loss and 1 psi of inlet loss.
Twins on that full match. 22.0 lb/min per compressor. The pressure ratio stays 3.30.
2.0 litres. 122.05 cubic inches, which is not the 122 used in the worksheet above.
Reference
R = 639.6. Garrett reference conditions used for corrected flow are 545 °R and 13.95 psia. Ambient in the worked example is 14.7 psia. Sea level on the altitude formula is 14.696 psia.
Questions
What size turbo do I need for a horsepower target?
Start from airflow. Pounds per minute are horsepower times the air-fuel ratio times BSFC, divided by 60. 400 hp at 11.5 to 1 and a BSFC of 0.55 is 42.17 lb/min. Where that point sits on a compressor map, and whether the engine can use the boost it implies, is a comparison you make against the turbo maker’s map. This page does not name a turbo.
How do I read a compressor map?
Pressure ratio is the vertical axis. Corrected airflow is the horizontal axis. Efficiency islands sit in the middle. The surge line is usually on the left and the choke line on the right. This page plots your operating points on a blank grid so you can lay them against the map that came with the turbo. It does not draw those islands or lines.
What is pressure ratio?
Pressure ratio is the absolute pressure the compressor has to make, divided by the absolute pressure at its inlet. A simple figure is (boost + ambient) / ambient. 15 psi of boost at 14.7 psia is 2.02. The matching figure adds intercooler loss on top and subtracts inlet loss underneath. With 2 psi and 1 psi of those losses, the same 15 psi boost is 2.31.
How many pounds per minute per horsepower?
A rough check is about one pound per minute for every ten horsepower. The airflow formula is horsepower times AFR times BSFC, divided by 60. At AFR 11.5 and BSFC 0.55 that is about 0.105 lb/min per horsepower, so 400 hp is 42.17 lb/min, a little above the rule of thumb.
What is surge?
Surge is the left edge of a compressor map, where the compressor cannot keep airflow moving and flow reverses. A point that lands left of the surge line on the maker’s map is not a point you want to run. This page does not draw a surge line, because that line belongs to one compressor.
How does a twin turbo split the airflow?
The engine still needs the full airflow, and the manifold pressure and pressure ratio stay on that total. Each compressor sees half. The full-match example is 44.0 lb/min. With the twin switch on, each compressor is at 22.0 lb/min and the pressure ratio does not change.
Does altitude change the turbo match?
Yes. Lower ambient pressure raises the pressure ratio for the same boost, because the compressor is starting from less air. The page can take ambient from altitude with the same barometric formula as the psychrometric calculator. A typed 14.7 psia stays 14.7. Sea level on that formula is 14.696 psia.
Sources
- Garrett Motion, Turbo Tech 103 (and the “How to select a turbocharger” matching notes). Airflow, the manifold-pressure equation with R = 639.6, pressure ratio with intercooler and inlet loss, and corrected airflow at 545 °R and 13.95 psia are that published method. No Garrett compressor map is drawn here, and no turbo is recommended.
- John B. Heywood, Internal Combustion Engine Fundamentals, for volumetric efficiency and the air a four-stroke engine consumes.
- Standard-density CFM uses 0.0765 lb/ft³ and is labelled approximate. Cubic metres use 0.0283168 m³ per cubic foot.
- Power-to-weight calculator for pounds per horsepower from the same horsepower figure. It does not size a turbo.
- Altitude pressure is p = 101.325 (1 − 2.25577×10⁻⁵ Z)^5.2559 kPa, the same expression as the psychrometric calculator, then converted with 14.696 psia = 101.325 kPa. A typed 14.7 psia is left as 14.7.
- Displacement uses 1 in³ = 16.387064 cm³, shared with the displacement calculator. Fuel for the same horsepower is the fuel injector calculator. Static compression is the compression ratio calculator. RPM at a road speed is the gear ratio calculator.