Boost Horsepower Calculator (turbo/supercharger power from boost & charge temperature)

Understanding the boost horsepower calculator — concept and principle

Turbochargers and superchargers push more air into an engine, and more air allows more fuel and more power. This calculator gives an educational estimate of the power after adding boost, based on how much denser the intake air becomes.

More air means more fuel and power, but the warmer charge is less dense, so the estimate is 214 rather than 232.

How it is calculated

Manifold pressure (absolute) = ambient pressure + boost (gauge); 1 psi = 6.894757 kPa.
Pressure ratio = manifold pressure ÷ ambient pressure.
Density ratio = pressure ratio × Tambient ÷ Tcharge (temperatures in kelvin).
Boosted power ≈ naturally aspirated power × density ratio.
Power with no temperature rise = NA power × pressure ratio.

The principle

At the same engine speed and volumetric efficiency, an engine swallows a fixed volume of air per cycle. The mass in that volume is proportional to air density, which by the ideal gas law is proportional to pressure ÷ temperature. At a fixed air–fuel ratio and fuel consumption per unit power, power is proportional to air mass. Compressing air also heats it, which reduces density; that is why intercoolers matter.

Worked example

Default inputs: NA power 150, boost 8 psi, ambient 101.325 kPa and 25 °C, charge air 50 °C.

Boost = 8 × 6.894757 ≈ 55.16 kPa; manifold pressure ≈ 156.5 kPa absolute.
Pressure ratio = 156.5 ÷ 101.325 ≈ 1.544.
Temperature ratio = 298.15 ÷ 323.15 ≈ 0.923; density ratio ≈ 1.425.
Boosted power ≈ 150 × 1.425 ≈ 214, a gain of 42.5%.
Without the temperature rise it would be 150 × 1.544 ≈ 232.

Each point is the calculator re-run with only “Naturally aspirated power (hp or kW)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Naturally aspirated power (hp or kW)	Estimated boosted power
37.5	53
75	107
112	160
150	214
188	267
225	321
300	427

How to read the result

This is an upper estimate. A supercharger takes power from the crankshaft, a turbo adds exhaust back-pressure, ignition timing may need to be retarded and the engine's fuel system and internal parts set limits. Real gains are usually lower. The 18-unit gap between 214 and 232 shows the value of cooling the charge.

Common mistakes

Adding gauge boost to zero instead of to atmospheric pressure.
Using Celsius instead of kelvin in the temperature ratio.
Ignoring the power needed to drive a supercharger.
Assuming the engine can safely handle the extra cylinder pressure.

Keep in mind: Adding boost can damage an engine and may affect legality, emissions compliance and warranty. Engine modifications require qualified professionals.

Frequently asked questions

How much horsepower does 8 psi of boost add?

In this ideal estimate about 54% before heating losses and 42.5% with a 50 °C charge, but real gains are usually lower.

Why does an intercooler add power?

Cooler air is denser, so more air mass enters the cylinders at the same pressure.

What is the difference between gauge and absolute pressure?

Gauge boost is measured above atmospheric pressure; absolute pressure includes the atmosphere.

Sources: Heywood, J. B., Internal Combustion Engine Fundamentals, 2nd ed. McGraw-Hill (2018) (supercharging and turbocharging) · Watson, N. & Janota, M. S., Turbocharging the Internal Combustion Engine. Macmillan

Checked 9 Nov 2026. Garrett Motion, “How to Select a Turbo, Part 2: Understanding Calculations to Turbo Any Engine”: airflow W_a = HP × A/F × BSFC/60 (airflow proportional to power) and required manifold pressure from the ideal gas law (MAP ∝ W_a × T_m). 1 psi = 6.894757 kPa.

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