Supercharger RPM Calculator combines crank and blower pulley diameters with any internal step-up ratio to project blower shaft speed and belt speed at a given engine RPM setting.
Projecting Blower Shaft Speed with the Supercharger RPM Calculator
The Supercharger RPM Calculator multiplies pulley ratio by any internal step-up gearing to find how fast the blower shaft actually spins at a given engine speed. It also checks that shaft speed against the RPM range where common Roots and twin-screw units are rated to operate.
Where the Internal Drive Ratio Number Actually Comes From
Crank and blower pulley diameters get measured directly off the pulleys, in inches or millimeters. Internal Drive Ratio should reflect the specific supercharger’s published step-up ratio — 1.00 for direct-drive Roots and twin-screw units where the pulley shaft is the rotor shaft, but a real multiplier for centrifugal units that gear the impeller up separately inside the case.
How the Supercharger RPM Calculator Projects Blower Shaft Speed
Pulley ratio compares the two pulley diameters directly.
$$ PulleyRatio = \frac{CrankPulleyDiameter}{BlowerPulleyDiameter} $$
This is ordinary belt-and-pulley mechanics: belt linear speed stays constant at both pulleys, so a smaller driven pulley has to spin faster to keep up, the same relationship supercharger tuning references use when describing pulley sizing.
$$ TotalRatio = PulleyRatio \times InternalDriveRatio $$
Multiplying pulley ratio by a separate internal step-up ratio is documented supercharger-tuning practice rather than something specific to this tool — ProCharger describes that internal step-up as the gear ratio between the drive pulley and the compressor shaft, and forum worked examples for centrifugal units multiply the two ratios exactly this way before applying engine RPM.
Leaving Internal Drive Ratio at the default 1.00 while modeling a centrifugal unit silently drops that multiplier, which can understate blower shaft speed by a factor of three or more against real published step-up ratios in the 3.00:1 to 3.44:1 range.
$$ BlowerRPM = EngineRPM \times TotalRatio $$
Crank pulley diameter, blower pulley diameter, and engine RPM all have to be positive, and internal drive ratio can’t drop below 1.00 since internal supercharger gearing steps speed up, never down. The belt speed figure exists as a separate practical check: belt width and material have their own speed limits independent of whatever the RPM ratio math says is safe for the blower itself.
Where 14,000, 16,000, and 18,000 RPM Come From
These three checkpoints aren’t arbitrary round numbers. Whipple publishes a maximum sustained speed of 16,000 RPM for its W275AX twin-screw unit and a maximum intermittent speed of 18,000 RPM for street and racing use, and Eaton lists a typical maximum operating speed of 18,000 RPM for its TVS R1740 Roots-style unit.
Newer-generation Eaton TVS units have pushed that ceiling as high as 24,000 RPM, according to supercharger forum discussion of the change, so the 14,000–18,000 RPM band is a common reference rather than a hard ceiling for every unit.
It also applies specifically to Roots and twin-screw designs — centrifugal superchargers spin their impeller through separate internal step-up gearing to far higher RPM, so this comparison band doesn’t apply to them.
Why a Smaller Blower Pulley Spins the Supercharger Faster
Numbers That Are Easy to Get Backwards Here
Swapping which pulley diameter goes in which field inverts the ratio, turning an intended overdrive setup into an underdrive one and producing a blower speed far below what the actual hardware would run.
Measuring one pulley at its outer belt-track diameter and the other at a bore or hub diameter breaks the ratio, since both diameters need to reference the same point on the belt groove.
Checking blower RPM only at redline and skipping mid-range checkpoints is the third. A pulley combination that looks fine at max engine RPM can already be past a manufacturer’s continuous-duty limit well before redline.
What Comes Up When Sizing Supercharger Pulleys
How do I calculate supercharger pulley ratio?
Divide crank pulley diameter by blower pulley diameter. A 7.5 inch crank pulley with a 3.0 inch blower pulley gives a 2.5:1 ratio.
What is a supercharger’s internal step-up ratio?
It’s the gear ratio inside the unit between the drive pulley and the compressor shaft, separate from pulley sizing. Published examples range from around 3.00:1 to 3.44:1 depending on the specific centrifugal supercharger model.
What RPM can a Roots or twin-screw supercharger safely run?
Whipple rates its W275AX at 16,000 RPM sustained and 18,000 RPM intermittent, and Eaton lists 18,000 RPM as typical for its TVS R1740, though some newer TVS units are rated up to 24,000 RPM.
Does a centrifugal supercharger follow the same RPM limits as a Roots or twin-screw unit?
No. Centrifugal units spin their impeller through separate internal step-up gearing to far higher RPM, so the 14,000–18,000 RPM band that applies to Roots and twin-screw units doesn’t apply to them.
Why does the calculator show belt speed as well as RPM?
Belt speed determines whether the belt itself can physically survive the setup, which is a separate question from whether the blower’s internal RPM limit is respected.
Does making the crank pulley bigger do the same thing as making the blower pulley smaller?
Both raise the pulley ratio, but they work in opposite directions on the hardware — a smaller crank pulley underdrives the supercharger, while a smaller blower pulley overdrives it.