Supercharger Pulley & Boost Calculator
Estimate how much boost a pulley change will add, using your current boost as the starting point, and check how fast the blower will spin at redline.
A pulley swap is the simplest way to get more boost from a supercharger. A smaller pulley on the blower, or a larger one on the crank, spins the supercharger faster at every engine speed. How much extra boost that buys depends on the type of supercharger, and the most reliable estimate starts from the boost you already measure.
Pulley Ratio and Blower Speed
Speed change from a swap = new ratio ÷ old ratio
The drive ratio tells you how many times the supercharger input turns for each turn of the crank. A 6.0 in crank pulley driving a 3.0 in blower pulley is a 2.00 ratio. Swapping to a 2.75 in blower pulley makes it 2.18, so the blower turns 9.1% faster at every RPM. Most centrifugal superchargers have an internal gearbox that multiplies the input speed again before it reaches the impeller. The maker publishes this step-up ratio, and you need it to work out true impeller speed.
Measure pulleys carefully. VT Superchargers points out that the effective diameter of a ribbed pulley, where the belt actually runs, can differ from its outside diameter by 0.2 to 0.4 inches, enough to throw off a small pulley calculation. Drives with a jackshaft or cog stage, used on some centrifugal installations, add another ratio: multiply each stage together to get the overall drive ratio.
Roots and Twin-Screw Superchargers
Roots and twin-screw blowers are positive-displacement machines. Each turn of the rotors moves a fixed volume of air, so their airflow rises in direct proportion to their speed. The engine, meanwhile, swallows a roughly fixed volume per revolution. The pressure ratio in the intake therefore scales with the blower's speed relative to the engine.
New boost = (new PR − 1) × atmospheric
The relationship holds up against real results. One MINI Cooper S owner reported on Reddit that the stock supercharger made 10.5 psi at redline and a 15% reduction pulley raised it to 15 psi. A 15% smaller pulley spins the blower 1 ÷ 0.85 = 1.176 times faster. Starting from a pressure ratio of 1.71, the formula predicts 14.95 psi, which matches the owner's measured result almost exactly.
Expect the real gain to fall slightly short on bigger changes. A faster blower heats the air more, rotor leakage changes with speed, and the engine's breathing changes with boost. Starting from measured boost captures your specific combination, which is why this method is more dependable than estimating boost from scratch.
Centrifugal Superchargers Behave Differently
A centrifugal supercharger is a compressor wheel, like the cold side of a turbocharger. It does not move a fixed volume per turn. The pressure it creates depends on how fast the impeller tip is moving, and the basic turbomachinery relationship (the Euler equation) makes pressure rise grow roughly with the square of tip speed. That is why centrifugal boost climbs steeply with engine RPM, and why a pulley change has a bigger effect than on a Roots blower.
The square law is an approximation. It assumes the compressor stays in the efficient part of its map. Push the impeller past its rated speed, or toward the edge of its map, and efficiency falls away, so the boost gain is smaller and the extra work turns into heat. That is a common explanation when an owner fits a smaller pulley to a centrifugal unit and sees less boost than expected. In one Reddit thread, a Miata owner with a Rotrex unit found boost stayed at 7 to 8 psi after a pulley change when they had expected around 12 psi. Belt slip and the compressor's limits are the usual suspects in that situation.
Estimating Boost From Displacement
If you have no measured boost yet, a widely used first estimate for positive-displacement blowers compares the air the blower moves with the air the engine uses. A four-stroke engine fills half its displacement each revolution. So the ideal pressure ratio is the blower displacement per turn × drive ratio, divided by half the engine displacement. A 1.9 L blower on a 6.2 L engine at a 2.2 drive ratio gives 1.9 × 2.2 ÷ 3.1 = 1.35, or about 5.1 psi at sea level if both the blower and engine were 100% volumetrically efficient. Real numbers differ because neither is, so use this only as a starting point and then work from measured boost.
Worked Examples
| Setup | Pulley change | Speed change | Boost before → after |
|---|---|---|---|
| Roots, 6.0 in crank | 3.0 → 2.75 in | +9.1% | 6.0 → 7.9 psi |
| Roots, 6.0 in crank | 3.0 → 2.5 in | +20.0% | 6.0 → 10.1 psi |
| Roots, crank 6.0 → 6.5 in | 3.0 in unchanged | +8.3% | 6.0 → 7.7 psi |
| Centrifugal, 7.0 in crank | 3.33 → 3.0 in | +11.0% | 8.0 → 9.9 psi |
| Centrifugal, 7.0 in crank | 3.33 → 2.75 in | +21.1% | 8.0 → 11.7 psi |
Heat, Belt Slip and Blower Speed Limits
Every supercharger has a maximum rated speed in its maker's specifications. Check the calculated blower or impeller RPM at redline against it before buying a pulley. Speeds vary enormously between designs: Rotrex lists maximum speeds above 200,000 RPM for some of its traction-drive units, while Roots and twin-screw rotors run far slower. Overspeeding risks bearing and rotor damage, and on a centrifugal unit it pushes the compressor off its efficient range.
Smaller blower pulleys also reduce belt wrap, the arc of belt in contact with the pulley. Less wrap means less grip, and a slipping belt makes less boost than the math says while it heats and wears quickly. Larger crank pulleys, idler repositioning or a cogged drive are common fixes at higher boost.
More boost means hotter intake air, more fuel and more cylinder pressure. The engine needs a tune, enough fuel and enough octane for the new boost. Check the extra power with the NA to boosted horsepower calculator, then size the fuel system with the fuel pump size calculator and the injector size calculator. If you are working at altitude, set atmospheric pressure to your local value, because the same pressure ratio produces less gauge boost in thinner air.
Frequently Asked Questions
On a Roots or twin-screw blower, the pressure ratio rises in proportion to the speed increase. A pulley 10% smaller spins the blower 11% faster, so 6 psi becomes about 8.3 psi. On a centrifugal supercharger boost rises roughly with the square of the speed increase, so the same change takes 8 psi to about 9.9 psi.
Divide the crank pulley diameter by the supercharger pulley diameter. A 6.0 in crank pulley and a 3.0 in blower pulley give a 2.00 ratio, so the blower turns twice for every crank revolution.
For boost, yes. Both raise the drive ratio. A larger crank pulley also increases belt wrap on the blower pulley, which helps grip, so it is often used together with a smaller blower pulley at higher boost.
Common causes are belt slip, an impeller running near or past its rated speed, and a compressor operating outside its efficient range, where extra speed adds heat rather than pressure. Check belt tension, wrap and the maker's speed limit.
Every unit has a maximum speed set by its maker. Calculate blower or impeller RPM at redline as engine RPM × drive ratio × internal step-up and compare it with that published limit.
Yes. A supercharger produces a pressure ratio, so in thinner air the same ratio shows lower gauge boost. Enter your local atmospheric pressure to account for it.