What the inputs mean
- Boost: gauge pressure, the number a boost gauge shows. It is measured above the air pressure around the car, so 0 means no boost.
- Power without boost: what the engine makes naturally aspirated, at the same altitude. Crank or wheel power both work, as long as you read the answer the same way.
- Altitude: sets the outside air pressure from the standard atmosphere, 14.70 psi (1.013 bar) at sea level and less as you climb.
The formula
pressure ratio = (boost + outside pressure) ÷ outside pressureideal power = power without boost × pressure ratioThe engine takes in roughly the same volume of air on each stroke. Pushing that air in at a higher pressure packs more of it into the same volume, and more air can burn more fuel. The pressure ratio says how many times more.
Worked example
An engine that makes 300 hp without boost, running 10 psi at sea level:
Absolute pressure = 10 + 14.70 = 24.70 psi. Pressure ratio = 24.70 ÷ 14.70 = 1.68. Ideal power = 300 × 1.68 = 504 hp.
Going the other way, 450 hp from the same engine needs a pressure ratio of 1.5, which is 0.5 × 14.70 = 7.35 psi at sea level in this ideal model.
Why altitude matters
At 5,000 ft the standard atmosphere is 12.23 psi. The same 10 psi on the gauge is then a pressure ratio of 22.23 ÷ 12.23 = 1.82, higher than at sea level, because the engine starts from thinner air. The power without boost is lower up there too, so enter what the engine makes at that altitude.
Why real gains are lower
This is the ceiling, not a prediction. Compressing air heats it, and hot air is less dense, so an intercooler only wins some of that back. The compressor is never fully efficient, a belt-driven supercharger takes power from the crank, and a turbo adds exhaust back pressure. Treat the ideal figure as an upper bound and expect a dyno to show less.
Questions
How much horsepower does 10 psi of boost add?
In the ideal case, at sea level, 10 psi is a pressure ratio of 1.68, so a 300 hp engine could reach about 504 hp. Heat, compressor efficiency and drive losses make the real figure lower.
How much boost do I need for a target horsepower?
Divide the target by the power without boost, subtract 1, and multiply by the outside air pressure. For 450 hp from 300 hp at sea level that is 0.5 × 14.70 = 7.35 psi, before any losses.
What is the pressure ratio of 1 bar of boost?
At sea level, 1 bar of boost gives an absolute pressure of about 2.01 bar, a pressure ratio of about 1.99.
Does altitude change the pressure ratio?
Yes. The same gauge boost gives a higher ratio in thinner air: 10 psi is a ratio of 1.68 at sea level and 1.82 at 5,000 ft.