Boost Horsepower Calculator

Boost Horsepower Calculator for turbo, centrifugal, twin-screw and Roots setups. Enter stock power and gauge boost to see crank power after charge heat, intercooler and drive loss.

Boost Horsepower Calculator for Turbo and Supercharger Builds

Adding boost packs more air into each cylinder, but compressing that air also heats it. The boost horsepower calculator estimates crank power from gauge boost with that heat counted, along with the intercooler, belt drive loss on superchargers, and altitude.

It is built for owners pricing a turbo kit or blower, tuners planning a pulley or wastegate change, and anyone checking a seller’s power claim. The US setting uses hp, psi, °F, feet and cubic inches. The metric setting uses kW, bar, °C, meters and liters, with 1 bar = 14.5038 psi, 1 L = 61.0237 ci and 1 hp = 0.7457 kW.

Why the 14.7 psi Rule Runs High

Most quick estimates scale stock power by the pressure ratio alone. At sea level that means 14.7 psi of boost doubles the air, so it doubles the power. Forum threads repeat this rule constantly, from ClubWRX to drag racing boards.

$$P_{rule} = P_{stock} \times \frac{14.7 + \text{boost}}{14.7}$$

The rule assumes the compressed air reaches the engine as cool as outside air. It never does. On the default inputs, a 200 hp engine at 10 psi gets 336 hp from the rule but about 318 hp once charge heat is counted, even with a good air-to-air intercooler.

Pressure Ratio, Charge Heat and Air Density

Garrett defines pressure ratio as absolute outlet pressure over absolute inlet pressure in its Turbo Tech 103 guide. A boost gauge reads pressure above the local air, so the tool adds local air pressure back before dividing.

$$PR = \frac{P_{atm} + \text{boost}}{P_{atm}}$$

Compressing air raises its temperature. The tool uses the standard compressor relation that Garrett turbo engineer Khiem Dinh works through in his article on compressor efficiency, with k = 1.4 for air.

$$T_2 = T_1 \left[1 + \frac{PR^{0.2857} – 1}{\eta_c}\right]$$

T₁ and T₂ are inlet and outlet temperatures in °R, which is °F + 459.67. The compressor efficiency η_c is the adiabatic efficiency from a compressor map or maker’s data. The intercooler then removes a share of the added heat equal to its effectiveness ε.

$$T_c = T_2 – \varepsilon \, (T_2 – T_1)$$

$$\text{Density ratio} = \frac{P_{atm} + \text{boost}}{14.696} \times \frac{536.67}{T_c}$$

The density ratio compares the charge with a 77°F sea-level day, the reference the stock power is taken at. For a turbo, boosted power is stock power times that ratio. On the default 10 psi turbo, air leaves the compressor near 196°F and reaches the engine near 107°F, for a density ratio of 1.59.

Those temperatures line up with Garrett’s guide, which puts intercooled manifold air at about 100 to 130°F and non-intercooled air at 175 to 300°F. The second result card shows both the heat the compressor adds and the density gain over the stock engine.

Belt Drive Loss on Superchargers

A turbo takes its drive energy from the exhaust, but a supercharger takes it from the crank. The tool charges that loss using the compressor power relation, with airflow in lb/min and 0.24 BTU/lb·°F as the specific heat of air.

$$\text{Drive hp} = \frac{\dot{m} \times 0.24 \times (T_2 – T_1)}{42.41}$$

The same math reproduces Dinh’s example of 50 lb/min at a 2.75 pressure ratio and 72% efficiency, which needs about 71 hp to drive. Efficiency matters here twice, since a poor compressor both heats the air more and takes more power to spin.

That is why Roots blowers sit at the bottom of the default efficiency list at 55%. Vortech estimates a Roots unit at higher boost is 50% efficient or less, with half its input power going into heat. The table below adds 8 psi to a 300 hp engine with each blower type and its typical intercooler.

Airflow and Pressure Ratio for the Compressor Map

The third card gives the two numbers needed to plot an operating point on a compressor map. Garrett’s guide estimates airflow from power, air-fuel ratio and brake specific fuel consumption.

$$\dot{m}\ (\text{lb/min}) = \frac{\text{hp} \times AFR \times BSFC}{60}$$

The tool uses an 11.5:1 air-fuel ratio and 0.55 BSFC, which works out to about 9.5 hp per lb/min. That sits at the conservative end of Garrett’s 9.5 to 10.5 hp per lb/min rule for turbocharged gas engines. The default setup needs 33.6 lb/min at a 1.68:1 ratio.

The ratio shown is the manifold ratio. Garrett notes that an air filter can cost about 1 psi before the compressor. Piping and the intercooler can cost 1 psi or less on a good system and 4 psi or more on a restrictive one, so the compressor itself works at a higher ratio than the card shows.

Checking the Boost Horsepower Calculator Against Real Dynos

The fourth card is a reality check from Corky Bell’s Maximum Boost. Across the turbo dyno runs he reports, output ranged from 0.052 to 0.077 hp per cubic inch per psi of absolute manifold pressure.

$$P_{Bell} = \text{ci} \times (P_{atm} + \text{boost}) \times (0.052 \text{ to } 0.077)$$

A 183 ci engine at 24.7 psia gives 235 to 348 hp, so the default 318 hp lands in the upper half. Bell’s data came from turbo systems, so a belt-driven blower will usually land lower in the range once drive loss is paid.

The turbo result deserves the same check. Dinh points out that the turbine adds exhaust back pressure, which cuts volumetric efficiency, and the calculator does not charge a turbo for it. A turbo estimate above Bell’s range usually means the stock figure or efficiencies are too optimistic.

Boost Readings Change With Altitude

A gauge that reads 10 psi in Denver is not delivering the same air as 10 psi at sea level. Local air is thinner, so absolute manifold pressure is lower, and the compressor has to spin to a higher ratio to hold that reading.

Garrett’s guide shows 12 psi at Denver jumping from a 1.82 ratio to 2.14 once intake loss is counted. In the calculator, the default 10 psi turbo drops from 318 hp at sea level to about 282 hp at 5,280 ft, with the ratio climbing from 1.68 to 1.83.

Working Back From a Power Goal

The Boost for Target Power mode runs the same model in reverse. It raises boost until the estimate reaches your target, then reports the gauge reading needed.

Taking the default 200 hp engine to 300 hp with a turbo and air-to-air intercooler takes about 8.4 psi. The pressure rule alone would say 7.3 psi. That extra psi is the heat penalty, and it grows with a less efficient blower or no intercooler.

The target must be above stock and no more than three times stock power. If even 60 psi falls short, the tool says so and points to a more efficient compressor or a better intercooler instead of returning an impossible number.

Limits Built Into the Estimate

Every result assumes fuel and ignition are tuned for the added air. Bell puts the practical ceiling for stock engines on pump fuel at about 7 to 12 psi at sea level with good intercooling, and engine internals decide how far past that is safe.

The tool warns when charge air passes 180°F, since hot air under boost invites detonation. It also flags a pressure ratio above 3:1, which is about where Garrett places the widest part of a typical compressor map.

A belt drive taking more than a tenth of the output and an estimate above Bell’s range each get their own note. Power is accepted from 20 to 3,000 hp and boost from just above 0 up to 60 psi. Outside air runs from −20°F to 130°F, altitude from −1,000 to 12,000 ft and displacement from 30 to 1,000 ci.

Stock power means crank power on a naturally aspirated engine. Entering a chassis dyno figure there returns a wheel-based number under a crank label, and entering a boosted car’s current power stacks boost on boost. For a car that is already boosted, the Add Boost to a Boosted Car mode takes the current figure, crank or wheel, and returns the answer in the same terms.

Boost Numbers Tuners Argue About

How much horsepower does one psi of boost add?

There is no fixed number, because each psi adds a share of the engine’s stock power, not a set amount. A TurboBuick thread puts it at roughly 3 to 4% per psi, which is why a stock engine might see 10 hp per psi while a 700 hp build sees 30.

The gain per psi also shrinks as boost climbs, because each step heats the charge more. For the default 200 hp turbo engine, going from 5 to 10 psi adds about 58 hp, while going from 20 to 25 psi adds about 54 hp.

Does 14.7 psi of boost double horsepower?

Not in practice. One racer in a Yellow Bullet thread says an engine making 500 hp naturally aspirated lands around 825 to 840 hp at 15 psi, well short of the 1,010 hp the pressure rule predicts.

The calculator shows why the answer depends on the setup. At 15 psi, that 500 hp engine comes out near 814 hp with a Roots blower and air-to-water cooling, 841 hp with a centrifugal blower, and 772 hp with a turbo and no intercooler. A turbo with an air-to-air cooler comes out near 938 hp, a figure the missing back-pressure penalty makes optimistic.

How do I estimate power when raising boost on a car that already has it?

The usual forum method scales current power by the ratio of absolute manifold pressures. A Turbo Dodge thread uses 200 hp at 10 psi going to 20 psi as its example, which gives 281 hp.

That shortcut ignores the hotter charge that comes with the higher ratio. The Add Boost to a Boosted Car mode backs out the engine’s base output from the current figure first, then adds heat back in. The same example comes to about 271 hp, and the fourth card shows how much of the shortcut’s gain the heat erased.

Does a supercharger make less power per psi than a turbo?

Usually yes, and an LS1Tech thread raises the reason directly, since the supercharger takes crank power to run. At the same 10 psi on the default 200 hp engine, a turbo with an air-to-air cooler estimates about 318 hp while a centrifugal blower with the same cooler estimates about 295 hp.

The turbo is not free either, because its turbine adds exhaust back pressure that the tool does not charge. Real gaps between the two are often smaller than the table shows, and blower type matters as much as the turbo-versus-supercharger choice.