Boost Pressure Calculator
Converts boost between psi, bar and kPa, works out the pressure ratio the engine actually sees, and gives a rough power estimate for a given boost level.
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The formula
pressure ratio = (atmospheric + boost) ÷ atmospheric power ≈ NA power × pressure ratio × (0.8 + 0.2 × intercooler efficiency)
Boost on a gauge is pressure above atmospheric, so the pressure ratio — the thing that sets how much more air gets in — needs atmospheric pressure added back. The power estimate scales with air mass, less the density lost to the heat of compression that the intercooler fails to remove.
What to watch out for
What boost really does and what limits it:
- It is the pressure ratio that matters. 10 psi at sea level is a ratio of 1.68; the same 10 psi at 2,000 m is 1.85, because there is less atmosphere to start from.
- Compression heats air. A turbo at a ratio of 2 raises the charge by around 90 °C; the intercooler takes most of it back. Hot charge is less dense and more prone to knock.
- The engine sets the ceiling. Fuel octane, compression ratio, ignition timing and fuelling decide how much boost is safe — the number here is what the air would support, not what the pistons will.
Frequently Asked Questions
How much power does 10 psi of boost add?
About 55–65% over the naturally aspirated figure with a decent intercooler at sea level: 10 psi is a pressure ratio of 1.68, and after charge heating losses the usable gain is roughly 1.6×. 150 hp becomes around 240.
What is 1 bar of boost in psi?
14.5 psi — a pressure ratio of about 2 at sea level, which roughly doubles the air the engine ingests.
Does altitude affect boost?
Yes. A wastegate holds gauge pressure, so 10 psi of boost at altitude is a higher pressure ratio than 10 psi at sea level — the turbo works harder and the charge runs hotter for the same reading.