Blower Horsepower Calculator: CFM & PSI Guide

This blower horsepower calculator estimates the brake horsepower required to drive an ideal supercharger or blower, based on the airflow you need (in CFM) and the pressure rise you want (in PSI). You enter the desired airflow and boost pressure, and the calculator returns the theoretical horsepower demand, plus the air density ratio and the power absorbed by the blower. It uses the standard adiabatic compression formula, so it gives you a realistic baseline before you account for real-world inefficiencies.

Formula

The calculator uses a simplified version of the supercharger power equation:

Blower HP = (CFM × Boost PSI × 0.00436) / (Efficiency / 100)

Where:

• CFM = airflow at the blower inlet, in cubic feet per minute (standard cubic feet per minute, SCFM).
• Boost PSI = gauge pressure above atmospheric, in pounds per square inch.
• Efficiency = adiabatic efficiency of the blower (typically 55–75% for roots-type, 70–85% for screw or centrifugal).
• 0.00436 = a constant that combines unit conversions and the specific heat of air.

The density ratio is simply (14.7 + boost) / 14.7, assuming sea-level atmospheric pressure of 14.7 PSI. The ideal temperature rise is a rough approximation: each PSI of boost adds about 0.15°F of temperature rise under perfect (isentropic) compression.

Worked Example

Let’s use the default values: 600 CFM, 10 PSI boost, and 70% efficiency.

1. First, calculate the ideal power: 600 × 10 × 0.00436 = 26.16 HP.
2. Then divide by efficiency: 26.16 / 0.70 = 37.37 HP.
3. So the blower requires about 37.4 horsepower to produce 600 CFM at 10 PSI, assuming 70% efficiency.
4. Density ratio: (14.7 + 10) / 14.7 = 1.68 (meaning the air is 68% denser than atmospheric).
5. Ideal temperature rise: 10 × 0.15 × 100 = 15°F (but real temperature rise will be higher due to inefficiency).

Why Blower Horsepower Matters

Knowing the power consumed by a blower is critical for engine builders and tuners. The blower doesn’t create power; it consumes power from the crankshaft to compress air. That power is not free – it comes from the engine’s own output. For example, if your engine makes 400 HP at the crank, and the blower eats 40 HP, you have only 360 HP left to drive the vehicle. The benefit is that the denser air allows the engine to burn more fuel, producing more power overall, but you need to know the parasitic loss to calculate net gain.

This calculator gives you the theoretical minimum power required. Real blowers are less efficient, so the actual power draw will be higher. For a roots-type blower, expect 30–40% more power draw than the ideal; for a centrifugal, maybe 15–25% more.

How to Choose the Right Blower Size

To use this calculator, you first need to estimate the airflow your engine requires. A common rule of thumb:

• For a naturally aspirated engine, airflow (CFM) ≈ (engine displacement in cubic inches × max RPM) / 3456.
• For a boosted engine, multiply that by the pressure ratio (14.7 + boost) / 14.7.

For example, a 350 CID engine revving to 6000 RPM flows about 350 × 6000 / 3456 = 608 CFM naturally aspirated. With 10 PSI boost, the pressure ratio is 1.68, so the blower must move about 1021 CFM. Enter that into the calculator with your boost and efficiency to see the power required.

Common Mistakes and Misconceptions

A frequent error is mixing up CFM at the inlet vs. outlet. The CFM in the formula is the volume of air measured at the blower inlet (atmospheric pressure). At the outlet, the same mass of air occupies a smaller volume due to compression. Always use inlet CFM.

Another misconception is that boost pressure alone determines power. In reality, it’s the mass airflow that matters. Two engines with the same boost but different displacements or RPMs will have vastly different airflow and power. This calculator helps you see that relationship.

Also, the 0.00436 constant assumes standard air density (about 0.075 lb/ft³) and a specific heat ratio of 1.4. At high altitudes or extreme temperatures, results will be slightly off.

Real-World Efficiency and Heat

The efficiency you enter is the adiabatic efficiency of the blower. Roots-type superchargers typically have efficiencies around 50–60%, while twin-screw units can reach 70–80%. Centrifugal blowers vary with speed, peaking around 75–80% at their best operating point. Lower efficiency means more power is wasted as heat, which also increases the temperature of the air entering the engine. That hot air is less dense, reducing power and increasing the risk of detonation. Intercoolers are often used to cool the compressed air, which improves density and reduces knock.

To estimate the actual temperature rise (not ideal), you can use: Temperature rise = (Ideal rise) / (Efficiency/100). For the example above, ideal rise is 15°F, so at 70% efficiency the real rise is about 21°F. At 50% efficiency, it would be 30°F. These are approximations; actual values depend on many factors.

Limitations of the Formula

This calculator is a simplified model. It does not account for:

• Volumetric efficiency of the engine (how well it fills its cylinders).
• Blower drive losses (belts, gears, bearings).
• Heat transfer to the blower housing.
• Pressure pulsations and flow losses in the intake system.
• Changes in air density due to temperature and altitude.

For a quick estimate, it’s very useful. For final design, use a detailed engine simulation or dyno testing. Always remember that the result is the power required to drive the blower, not the power gained. The net power gain depends on how much extra fuel the denser air allows, which is a separate calculation.

Frequently Asked Questions

What is the difference between CFM and PSI in this calculator?

CFM (cubic feet per minute) is the volume of air the blower moves, while PSI (pounds per square inch) is the pressure it creates. Both are needed because power is the product of flow and pressure. More CFM moves more air; more PSI compresses it more. The formula multiplies them together, so increasing either one increases the power required.

How accurate is this calculator for a real supercharger?

It gives a theoretical baseline. Real superchargers have mechanical and thermal inefficiencies, so the actual power draw will be 10–30% higher. Use the efficiency input to approximate that. For a roots blower, set efficiency around 55–60%; for a screw type, 70–75%; for a centrifugal, 70–80%.

Why does the calculator show a temperature rise?

Compressing air heats it. The temperature rise is important because hot air is less dense, reducing the oxygen content and increasing knock risk. The calculator shows the ideal (minimum) rise; real rise is higher. Intercoolers are used to reduce this temperature after the blower.

Can I use this for a turbocharger?

No, because a turbocharger is driven by exhaust gases, not by the crankshaft. The power to drive a turbo comes from exhaust energy that would otherwise be wasted, so it doesn’t consume crankshaft power directly. However, the same compression principles apply, and you can use the CFM and pressure ratio to size a turbo.

What if my engine is at high altitude?

At higher altitude, atmospheric pressure is lower. For example, at 5,000 feet, atmospheric pressure is about 12.2 PSI. The pressure ratio should be calculated as (12.2 + boost) / 12.2. The calculator assumes sea level (14.7 PSI), so results will be slightly optimistic at altitude. You can adjust by using a lower effective boost in the calculator.

How do I convert between horsepower and kilowatts?

1 horsepower ≈ 0.7457 kilowatts. So if the calculator gives 37.4 HP, that’s about 27.9 kW. Many metric users prefer kW, so you can multiply the result by 0.7457 to get kW.

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in cfm | Airflow required | CFM | 600 | 0
in psi | Boost pressure | PSI | 10 | 0
in eff | Blower efficiency | % | 70 | 0
out hp | Blower horsepower | hp | (cfm * psi * 0.00436) / (eff / 100) | 1
out ratio | Density ratio | - | ((14.7 + psi) / 14.7) | 2
out heat | Temperature rise (ideal) | °F | (psi * 0.15) * 100 | 1