Turbo Boost Pressure Ratio Calculator
Pressure ratio, absolute pressure and air density gain of a turbocharged or supercharged engine from its boost, the atmospheric pressure and the intake temperature.
Results
The dashed line marks the value you entered.
What this tool does
A boost gauge reads pressure above the atmosphere, but a compressor works in absolute terms: 1 bar of boost at sea level is a pressure ratio of about 2, while the same gauge reading high in the mountains is a bigger ratio, because the air outside is thinner. What the engine actually gets is denser air, and compressing air heats it — which is what an intercooler fights, since every degree of heat takes some density back. The ideal power gain assumes power rises with air mass; real engines get less, because of losses and the fuel and ignition limits that come with it.
Formula
PR = (boost + p_atm) ÷ p_atm · density ratio = PR × T_ambient ÷ T_intake (absolute temperatures)
Variables
| Symbol | Meaning | Unit |
|---|---|---|
bo | Boost pressure (gauge) | bar |
at | Atmospheric pressure | bar |
ta | Ambient temperature | °C |
ti | Intake air temperature after the intercooler | °C |
PR | Pressure ratio | — |
AP | Absolute manifold pressure | bar |
PS | Boost in psi | psi |
DR | Air density compared with no boost | × |
PG | Ideal power gain | % |
Worked example
- Boost pressure (gauge)1 bar
- Atmospheric pressure1.013 bar
- Ambient temperature20 °C
- Intake air temperature after the intercooler45 °C
- Pressure ratio1.987
- Absolute manifold pressure2.013 bar
- Boost in psi14.5 psi
- Air density compared with no boost1.831 ×
- Ideal power gain83 %
Limitations
- The formula assumes ideal conditions: no friction losses, no air resistance and no efficiency losses unless you enter them.
- The result is an estimate based only on the values you type. Real situations often include factors this calculator does not know about.