Pulley Calculator
Compare two pulley setups. Drive ratio, blower or impeller speed against the maker's limit at any engine rpm, a chart of neighbouring pulley sizes, belt speed, and a boost estimate that says which model it used.
Blower and Engine
Current and New Pulleys
Pulley Comparison Results
| Measurement | Current | New | Difference |
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Boost Estimate
Blower Speed Against Engine Rpm
| Engine | Current | New | Difference |
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Pulley Chart
| Blower pulley | Ratio | Blower rpm | Of limit | Vs current | Est. boost |
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Pulley for a Target
How the numbers are calculated
A belt drive turns the driven pulley faster than the driver by the ratio of their diameters: pulley ratio = crank pulley / blower pulley. Overdrive is that ratio minus one. A jack shaft adds a second ratio: the shaft turns at crank pulley over the pulley the crank belt drives, and the blower at the shaft's other pulley over the blower pulley, so tooth counts work as well as diameters because only the ratio matters. Centrifugal superchargers then gear the impeller up inside the case, so impeller speed = engine rpm x pulley ratio x step-up. Vortech's own impeller calculator and the PRI setup guide use exactly this form; roots and twin-screw blowers have no internal gearing, so their rotors turn at engine rpm x pulley ratio.
The speed limit is the number the maker publishes for bearing and impeller life. The chart shows where each setup crosses it, because a pulley that is fine at 6,000 rpm can be over the limit at 7,000. The pulley chart steps the blower pulley up and down from your new size so the next choice is a row, not a new calculation. The boost to horsepower calculator turns a boost figure into an engine power estimate, and the injector size and fuel pump calculators size the fuel system for it.
Boost is the part that cannot be exact. For a roots or twin-screw blower the calculator uses a mass balance: the blower moves a fixed volume per turn and the engine swallows a fixed volume per turn, so at one engine speed the absolute manifold pressure scales with blower speed. Against a published Whipple pulley chart that model lands inside the quoted band. For a centrifugal unit the physics is less tidy, so the calculator shows two first-order models as a band: gauge boost rising with the square of impeller speed, and absolute pressure rising in step with it. A common rule of thumb, about one psi per tenth of an inch of blower pulley, is only a rule of thumb: in the Vortech example above the measured gain was nearer 0.6 psi per tenth, and the slope depends on the pulley size and the boost you start from. Belt slip, heat soak and any bypass valve push the real figure toward the low end of the band.