Boost Horsepower Calculator
Estimate what a given boost pressure adds to a naturally aspirated engine, allowing for intercooling and real world efficiency.
Engine and boost
- An estimate. Real results depend on the tune, the camshaft, the exhaust and how well the charge is cooled.
- Adding boost to a stock engine without addressing fuel, spark and knock control is how engines are destroyed.
- Compression ratio and boost trade against each other. High static compression limits usable boost sharply.
Boost is a pressure ratio, not a psi number
An engine's output scales roughly with the mass of air it ingests, which scales with absolute pressure. At sea level ambient is 14.7 psi, so 14.7 psi of boost is a pressure ratio of two and, in theory, double the air.
That is why the same psi means less at altitude. In Denver ambient is closer to 12.1 psi, so 10 psi of boost is a bigger ratio than the same 10 psi at sea level, but the engine started from a lower baseline.
Theory always overpromises
Compressing air heats it, and hot air is less dense, so you never get the full theoretical gain. An intercooler recovers most of it. Without one, charge temperatures climb far enough that you lose a large share of the benefit and invite detonation.
Superchargers cost more still, because the belt drive takes crank power to turn. A roots blower can consume over 10% of what the engine makes. A turbo uses exhaust energy instead, which is not free either but does not show up as a direct belt load.
What actually limits boost
Rarely the compressor. It is usually detonation, which is set by compression ratio, fuel octane, charge temperature and ignition timing. A high compression naturally aspirated engine runs out of detonation margin at very low boost, which is why boosted builds drop static compression.
Frequently asked questions
How much horsepower does 10 psi add?
At sea level, 10 psi is a pressure ratio of about 1.68. With a good intercooler and a proper tune, expect roughly 55 to 60% more power than the naturally aspirated figure. A 300 hp engine lands somewhere near 475.
Why do I make less power at altitude?
Ambient pressure is lower, so the engine draws in less air mass at any given boost. A turbo can compensate by working harder, since it is pressure ratio rather than gauge pressure that matters, but a naturally aspirated engine simply loses power with elevation.
Is a turbo or supercharger better?
Different trade-offs. A supercharger delivers boost immediately and predictably but eats crank power to do it. A turbo is more efficient because it uses exhaust energy, but it has to spool. Neither is universally better.
Can I add boost to a stock engine?
Modest boost on a stock engine works when the fuel system, tune and knock control are addressed properly. Around 6 to 8 psi is a common conservative starting point. What kills engines is boost without the supporting fuel and ignition work.
Everything on this page is an estimate worked out from the numbers you entered and typical figures for the conditions you picked. Your vehicle, your parts and your conditions will differ. Check anything you intend to act on against the manufacturer's specification, the service manual, or a qualified technician. Verifying the result before relying on it is up to you.