How Superchargers Make Boost
A supercharger is an air pump driven mechanically by the engine crankshaft through a belt and pulley system. Unlike a turbocharger, which uses exhaust gas energy to spin its compressor, a supercharger pulls power directly from the engine to compress intake air. The trade-off is straightforward: you spend horsepower to drive the supercharger, but you gain much more horsepower because the engine now breathes denser air and burns more fuel. A typical street supercharger setup nets a 40 to 70 percent power increase on the same engine displacement.
The boost pressure you see in the intake manifold is determined by three things: how much air the supercharger moves at its operating speed, how much air the engine can swallow at the current RPM, and how restrictive the path between them is. Pulley ratio (crank pulley diameter divided by supercharger pulley diameter) controls how fast the supercharger spins for any given engine RPM. A smaller supercharger pulley means the supercharger spins faster, which means more airflow, which means more boost. This calculator does the math from pulley sizes, supercharger displacement, and engine displacement to predict boost level at any RPM.
Roots vs Twin-Screw vs Centrifugal
Roots-type superchargers. Two rotating lobes that trap air and push it from intake side to discharge side. Cheap to manufacture, simple, and produce strong low-RPM boost — peak boost can come on as early as 2,000 RPM. The drawback is they are inefficient at high RPM and heat the intake charge significantly. Roots blowers are the classic top-mount kit you see on a muscle car hood scoop. Examples: Eaton M62, M90, M112 series used on countless GM, Mercedes, and Mini Cooper applications.
Twin-screw superchargers. Two intermeshing rotors with helical lobes that compress air inside the supercharger itself before pushing it out (true internal compression, unlike Roots). More efficient than Roots, run cooler, and deliver strong boost across a wider RPM range. More expensive to manufacture. Examples: Whipple, Kenne Bell, and the supercharger in the Hellcat and Demon engines.
Centrifugal superchargers. Look and function like a turbocharger, but spun by a belt instead of exhaust. Use a centrifugal impeller that spins very fast (40,000 to 100,000 RPM) and develops boost mostly at high engine RPM. The boost curve is similar to a turbocharger — soft at low RPM, hard at high RPM. Examples: ProCharger, Vortech, Paxton. Common on later-model Ford and Chevy performance applications because the kit packages compactly.
Why Pulley Ratio Drives Boost
The supercharger pulley ratio is the single biggest tuning variable on a positive-displacement supercharger system. A typical factory setup might use a 3.0-inch supercharger pulley spinning off a 6.5-inch crank pulley, giving a 2.17:1 overdrive ratio — the supercharger spins about 2.17 times faster than the engine. At 6,000 engine RPM, the supercharger is doing about 13,000 RPM and producing maybe 9 to 10 psi of boost.
Drop to a 2.75-inch supercharger pulley and the ratio jumps to 2.36:1. At the same 6,000 engine RPM, the supercharger now spins at 14,200 RPM, producing roughly 11 to 12 psi of boost. The math is straightforward: smaller pulley equals more boost, but you pay for it in three ways. First, parasitic load on the engine rises (the supercharger is harder to spin). Second, intake air temperature rises because compression heats air. Third, you can exceed the supercharger’s safe maximum RPM, which destroys the unit. Always check the manufacturer’s max RPM spec before picking a pulley.
Boost, Density, and the Power Equation
Boost pressure itself does not make horsepower — air density does. A boosted engine making 10 psi at sea level on a 70 degree F day breathes very dense air. The same engine making 10 psi at 7,000 feet altitude on a 90 degree F day breathes much less dense air, and produces noticeably less power despite the gauge reading the same. This is why intercoolers matter so much on supercharged builds: cooling the post-compression intake charge by 50 degrees F can recover 5 to 7 percent of the power lost to compression heating.
The conversion from boost to airflow is also non-linear with altitude and temperature. The calculator above uses density ratio math to give you a realistic boost-to-power estimate rather than a marketing-grade promise. Real-world results vary based on fuel quality, intercooler efficiency, ignition timing, and a dozen other factors, but the geometric math of pulley ratio, supercharger displacement, and engine displacement is the foundation everything else builds on.
Frequently Asked Questions
How much horsepower will a smaller supercharger pulley add?
As a rough rule of thumb on a typical Roots or twin-screw setup, every additional 1 psi of boost adds approximately 4 to 6 percent power on a healthy engine with adequate fuel and a properly tuned ignition map. Going from 8 to 12 psi (4 psi gain) often adds 20 to 30 percent power. The gain plateaus at higher boost levels because intake heat rises faster than density does, eating into the gain. Pulley changes must always be paired with a tune, fuel system upgrade, and ignition adjustment.
What is the maximum safe pulley I can run?
Two limits. First, the supercharger has a manufacturer-published maximum RPM (typically 14,000 to 24,000 depending on model). Exceed it and bearings fail, rotor coatings strip, and the unit grenades. Second, the engine has a knock limit determined by compression ratio, fuel octane, and ignition timing. Most pump-gas setups top out around 12 to 14 psi static boost without serious engine work. Above that you need built internals, better intercooling, and high-octane fuel.
Does a supercharger make boost at idle?
Roots and twin-screw superchargers physically move air at idle, but the engine pulls vacuum because it does not need that much air at idle, so the intake manifold reads slight vacuum or near-zero pressure, not boost. Boost only registers once the throttle opens and the engine starts demanding more air than the supercharger can supply at low RPM. Centrifugal superchargers behave more like turbos — essentially no boost below 3,000 RPM.
How much power does the supercharger itself consume?
Parasitic loss on a typical street setup is 25 to 60 horsepower at peak boost, depending on supercharger size and pulley ratio. This loss is built into the net output number. A supercharged engine producing 500 hp at the flywheel is actually making 540 to 560 hp from combustion, with 40 to 60 hp going right back to spinning the blower. Centrifugal superchargers tend to have lower parasitic loss than positive-displacement units at the same boost level.
What is supercharger overdrive?
Overdrive is when the supercharger spins faster than the engine. Ratio above 1.0 is overdriven. Most factory setups run 2.0 to 2.5 overdrive — the supercharger spins 2x to 2.5x as fast as the engine. Aftermarket pulley kits push this to 3.0 or higher for more boost. Underdriven setups (supercharger slower than engine) exist on a few mild factory applications but are uncommon.
How hot does supercharged intake air get?
Compression heats air. As a rough rule, intake air temperature rises about 8 to 15 degrees F per psi of boost on a positive-displacement supercharger without intercooling, depending on supercharger efficiency. 10 psi of boost can mean a 100 to 150 degree F intake charge above ambient — devastating for power and detonation safety. Air-to-water intercoolers on modern superchargers (Hellcat, Mustang GT500, Camaro ZL1) drop intake temperatures dramatically and are the reason those engines can make so much power on pump gas.
Why do some superchargers have a bypass valve?
To reduce parasitic loss at part-throttle cruise. When the throttle is closed and the engine does not need boost, the bypass valve opens and recirculates compressed air back to the supercharger inlet rather than forcing it into the closed throttle. This drops parasitic load significantly, which improves fuel economy at cruise. Centrifugal superchargers have a similar device called a blow-off valve, mounted differently but doing the same job.
Can I supercharge a stock engine?
Yes, within limits. Most factory engines tolerate 5 to 7 psi of supercharger boost on a properly tuned setup with no internal modifications. Above that, head gasket integrity, piston ring lands, and rod bolts become limiting factors. Aftermarket kits designed for stock engines are usually pulley-limited to stay in the safe zone. Aggressive boost levels (10 psi or more) typically require at least head studs, sometimes forged pistons and connecting rods.
What is the difference between blower boost and turbo boost?
The pressure reading at the intake manifold is the same — 10 psi is 10 psi whether it came from a turbo or a supercharger. The difference is the curve. Superchargers make boost as soon as the throttle opens, scaling linearly with RPM. Turbos lag at low RPM (waiting for exhaust flow to build), then ramp on hard once the turbo spools, producing a more abrupt power delivery. Superchargers feel more like “big displacement” engines. Turbos feel like a switch.
Why We Built This
Pulley swaps are one of the most common supercharger upgrades, but tuners and enthusiasts have to do the boost math by hand or trust a vendor’s marketing number. The geometry is not complicated, but it depends on five inputs (crank pulley size, supercharger pulley size, supercharger displacement, engine displacement, target RPM) and a small error in any of them gives wrong boost predictions. This calculator runs the math instantly with all five inputs visible so you can iterate on what-if scenarios before ordering parts. You can be the mechanic.
Help Us Make This Tool Better
Want intercooler efficiency factored in, or a temperature/altitude correction added to the predicted boost output? Send us a note and we will look at every message. Tools improve when the people using them tell us what is missing.