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Turbo pressure ratio
The pressure ratio is the key to reading a compressor map and choosing the right turbo. Enter the boost in bar, and feel free to add the target power for an estimated air mass flow.
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Pressure ratio (P2/P1)
1,99:1
Pressure ratio P2/P1 at sea level (1,013 bar). The airflow follows the rule of thumb of 10 hp per lb/min and does not account for the intercooler or efficiency.
How the calculation works
The pressure ratio is the absolute pressure after the compressor divided by the pressure before. Boost is measured as pressure above ambient, so we add the atmospheric pressure of 1,013 bar both in the numerator and as the reference. A boost of 1,0 bar thus gives a pressure ratio of just under 2,0:1 at sea level.
The airflow is estimated with the rule of thumb that there are around 10 horsepower per pound of air per minute. Together with the pressure ratio it places your engine on the compressor map. It is a rough estimate that does not account for the intercooler or efficiency, but is enough to frame the turbo choice.
Example
1,0 bar of boost gives a pressure ratio of 1,99:1, and 400 hp points to around 40 lb/min (18 kg/min) of air.
Typical pressure ratios
| Application | Pressure ratio |
|---|---|
| Street turbo | 1,5-2,5 |
| Race turbo | 2,5-3,5 |
| Pro Mod and drag | 3,5-4,5 and up |
At high altitude the pressure ratio rises for the same boost, because the atmospheric pressure is lower.
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Common questions about turbo pressure ratio
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