Pulley ratio, blower RPM and estimated boost from pulley sizes. This calculator models supercharger pulley ratio, blower RPM at a given engine speed, and estimated boost from crank and blower pulley diameters plus blower max RPM and rated boost. Mustang owners running Whipple, Roush, Kenne Bell, ProCharger, and Magnuson kits on Coyote and Modular engines use it when considering a smaller blower pulley or verifying spin speed before a track event. Enter pulley sizes in inches and engine RPM to see ratio, blower speed, and approximate psi. Compare stock pulley specs against aftermarket crank and upper combos before you order parts for a weekend pulley swap.
Pulley ratio, blower RPM and estimated boost from pulley sizes.
- ESTIMATES ONLY. VERIFY CRITICAL BUILD, TUNING, SAFETY, AND LEGAL DECISIONS WITH A QUALIFIED PROFESSIONAL.
This calculator models supercharger pulley ratio, blower RPM at a given engine speed, and estimated boost from crank and blower pulley diameters plus blower max RPM and rated boost. Mustang owners running Whipple, Roush, Kenne Bell, ProCharger, and Magnuson kits on Coyote and Modular engines use it when considering a smaller blower pulley or verifying spin speed before a track event. Enter pulley sizes in inches and engine RPM to see ratio, blower speed, and approximate psi. Compare stock pulley specs against aftermarket crank and upper combos before you order parts for a weekend pulley swap.
A one-inch pulley change on a positive-displacement blower can add several psi and thousands of blower RPM — enough to overrun the manufacturer's safe speed limit or outpace the factory heat exchanger. Coyote GT owners chasing 700+ HP often swap crank and blower pulleys without calculating blower RPM first, then wonder why inlet temps spike. Pulley math is cheap insurance before you buy a smaller upper on a Whipple or spin a ProCharger harder on a road course Mustang. Exceeding max blower RPM risks catastrophic rotor damage that no warranty covers after an unapproved pulley combination.
Pulley ratio = Crank pulley dia ÷ Blower pulley dia. Blower RPM = Engine RPM × Pulley ratio. Estimated boost = (Blower RPM ÷ Max blower RPM) × Max rated boost. Example: 7.0-inch crank, 3.0-inch blower, 6,500 engine RPM, 18,000 max blower RPM, 14 psi rated: ratio = 2.33, blower RPM = 15,167, estimated boost ≈ (15167 ÷ 18000) × 14 ≈ 11.8 psi. Real boost varies with belt slip, inlet restrictions, and ambient temperature — treat as estimate. Always compare calculated blower RPM against the kit manufacturer's maximum before installing a smaller blower pulley on your Coyote.
Pulley ratio is crank diameter divided by blower pulley diameter, then multiplied by engine RPM to estimate blower RPM.
Measuring pulley diameter at the wrong groove or confusing effective diameter with outer OD throws ratio off — use the belt contact diameter the manufacturer publishes. Another mistake is exceeding max blower RPM because a smaller pulley looked safe on boost alone; Whipple and PD blowers have hard RPM limits separate from the boost number on the gauge.
Usually yes on a positive-displacement blower, but belt slip, inlet temperature, and ECU torque management can cap realized boost. Smaller blower pulleys spin the unit faster, which increases airflow and heat. Coyote Roush and Whipple kits often need supporting mods — heat exchanger pump, colder spark plugs, higher-octane fuel — when pulley changes add more than 2–3 psi over stock. Always log IAT2 before and after a pulley swap on the same ambient day to see whether the extra boost is real power or just hotter, thinner air.
Follow Whipple's published maximum rotor RPM for your specific generation — exceeding it risks rotor clashing and catastrophic failure. Use this calculator before buying an aftermarket crank or blower pulley combo. Many street Coyotes live under the limit with mild pulley swaps; aggressive pulley stacks for drag duty need verified spin speed at redline, not just peak boost on a short pull. Kenne Bell and Magnuson kits have their own RPM ceilings — never copy Whipple pulley math across different blower families without checking that manufacturer's limit.
Larger crank pulleys (or smaller blower pulleys) increase pulley ratio and spin the blower faster per engine revolution. Some kits offer oversized crank pulleys to slow the blower for heat reduction on road course Mustangs. Enter both diameters accurately — a 0.25-inch change on the blower pulley often matters more than expected on high-RPM Coyote engines. Belt wrap and idler alignment affect slip at high blower RPM — if calculated boost exceeds logged boost, slip is a suspect before assuming the formula is wrong.
Centrifugal superchargers build boost exponentially with RPM — this linear RPM-to-boost model is a simplified reference based on your rated max values. ProCharger curves depend on impeller size, step-up ratio, and exhaust backpressure. Use the result for pulley-swap direction (more or less spin) and verify with a datalogged boost trace across the entire RPM range on your tuned Mustang. Low-RPM boost on a centrifugal setup will always lag PD blowers — pulley math alone cannot change that fundamental curve shape.