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About this tool
Convert boost between psi, bar, kPa and inHg, gauge or absolute, with pressure ratio, altitude and charge-air density.
This converter translates turbo or supercharger boost between psi, bar, kPa, inHg and kg/cm² in both gauge and absolute terms, using the relationship absolute = gauge + ambient. It then computes the pressure ratio a compressor map is plotted against, adjusts ambient pressure for altitude with the ISA troposphere model, and applies the adiabatic compression relation for air (γ = 1.4) plus intercooler effectiveness to show the charge-air temperature and density the engine actually receives. Aimed at anyone sizing a turbo, reading a compressor map, or reconciling a boost gauge against a MAP sensor.
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Every unit is shown both ways, so a boost gauge reading and a MAP sensor reading stop contradicting each other.
Ambient pressure drops about 17% by 1,500 m, which raises the pressure ratio for the same gauge boost.
Applies compressor heating and intercooler cooling to show the air mass gain, which is what actually tracks power.
Divide psi by 14.5038 to get bar, or multiply bar by 14.5038 to get psi. So 15 psi of boost is 1.034 bar and 1 bar of boost is 14.50 psi. Both figures here are gauge pressure — add atmospheric, about 14.696 psi or 1.013 bar at sea level, to get absolute.
About 2.02 at sea level. Pressure ratio is absolute manifold pressure divided by absolute ambient pressure, so (15 + 14.696) ÷ 14.696 = 2.021. Compressor maps are always plotted against this absolute ratio, never against gauge boost.
It changes the pressure ratio, not the gauge reading. Ambient pressure falls to about 12.26 psi at 1,500 m, so holding 15 psi on the gauge means a pressure ratio of 2.22 rather than 2.02 — the turbo works harder, spins faster and runs hotter for the same indicated boost, while the engine still receives less absolute pressure than it would at sea level.
Roughly 120 °C at a 2.0 pressure ratio from 25 °C intake air with a 70% efficient compressor, using the adiabatic relation for air. A 75% effective intercooler brings that back to about 49 °C, which lifts the charge density ratio from 1.53 to 1.87 — the intercooler is worth around 22% more air mass at the same boost pressure.
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