Turbo pressure ratio and airflow calculator
The two numbers you need to read a compressor map: how hard the compressor has to squeeze (pressure ratio) and how much air it has to move (lb/min) for your horsepower goal.
Pressure ratio
- Airflow
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- Airflow in cfm (standard air)
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- Air pressure at your altitude
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The two compressor map numbers
Airflow (lb/min) = horsepower × air-fuel ratio × BSFC ÷ 60
Pressure ratio compares the air leaving the compressor to the air entering it. Any restriction in the air filter and inlet lowers the entering pressure, which is why it's subtracted. Airflow comes from how much fuel the engine burns (horsepower × BSFC) multiplied by how much air goes with each pound of fuel.
Find where your pressure ratio and airflow meet on the map. You want that point inside the efficient islands and away from the surge line on the left.
Method follows Garrett's published guidance on reading compressor maps.
Frequently asked questions
Why do I need to know intake restriction for pressure ratio?
Pressure ratio is about how hard the compressor works relative to what it's actually pulling air in from, and any restriction ahead of the compressor (a tight filter, a small inlet pipe) makes the compressor work harder than boost pressure alone would suggest — leaving it out understates the compressor's real workload.
How do I read my result on an actual compressor map?
Plot the pressure ratio on the map's vertical axis and the airflow in lb/min on the horizontal axis — where those two lines cross tells you whether that compressor can support your goal efficiently, or whether it's being pushed outside its efficient island.
Why does air-fuel ratio factor into the airflow calculation?
Airflow needed scales directly with how much fuel the engine burns and the ratio of air to fuel in the mixture, so both the target air-fuel ratio and BSFC are necessary inputs to translate a horsepower goal into the actual mass of air the turbo needs to supply.
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