Calculator Inputs
Example Data Table
| Setup | Crank | Blower Pulley | Engine RPM | Slip | Drive Ratio | Estimated Blower RPM |
|---|---|---|---|---|---|---|
| Street baseline | 7.00 in | 3.50 in | 6,500 | 3% | 2.000:1 | 12,610 |
| Mild upgrade | 7.00 in | 3.20 in | 6,500 | 3% | 2.188:1 | 13,805 |
| Track pulley | 7.00 in | 2.90 in | 6,800 | 5% | 2.414:1 | 15,595 |
Formula Used
Drive ratio = crank pulley diameter ÷ supercharger pulley diameter.
Effective total ratio = drive ratio × internal step-up ratio × (1 − belt slip ÷ 100).
Blower RPM = engine RPM × effective total ratio.
Estimated boost = current boost × (proposed blower RPM ÷ current blower RPM)2.
Target pulley = current pulley ÷ √(target boost ÷ current boost).
Minimum pulley from RPM limit = engine RPM × crank pulley × step-up × slip efficiency ÷ max blower RPM.
These formulas model pulley speed and an estimated boost change. Real boost also depends on blower map, inlet restriction, intercooler efficiency, cam timing, exhaust flow, air temperature, altitude, and belt traction.
How to Use This Calculator
Enter the crank pulley diameter and your current supercharger pulley diameter. Add the proposed pulley you want to test. Use the same unit for all pulley diameters. Enter the engine RPM where you want to check blower speed. Add the internal step-up ratio when the supercharger has one. Use 1.00 for direct pulley speed. Enter a realistic belt slip value. Then add your current boost, target boost, atmospheric pressure, and the maximum blower RPM. Press calculate. Review the result above the form. Use CSV for spreadsheet logs. Use PDF for a simple printable report.
Supercharger Pulley Planning Guide
Why pulley size matters
A smaller supercharger pulley spins the blower faster. More speed can raise boost. It can also add heat, belt load, and knock risk. The correct pulley is not just the smallest part that fits. It should match the blower range, fuel system, tune, and engine limit.
Use speed before boost
Blower RPM is the first number to check. Every supercharger has a safe speed range. Overspeed can damage bearings, rotors, seals, or gears. This calculator compares current and proposed pulley speed. It also shows the margin to your entered blower limit.
Understand estimated boost
Boost is estimated from the change in blower speed. The estimate assumes the same engine, same airflow path, and similar efficiency. Real boost may differ. A better intake, headers, or cam can lower gauge boost while increasing power. A restriction can raise boost while adding heat.
Account for belt slip
Belt slip reduces effective blower speed. Slip often appears at high RPM. Dust, glazing, weak tension, poor wrap, or worn belts can increase it. Add a slip estimate to make the result more realistic. Watch logs for falling boost near redline.
Plan safe changes
Use the target pulley value as a planning guide. Check fuel injector duty, pump capacity, spark timing, intake temperature, and air-fuel ratio before driving hard. Use conservative steps. Recheck after each hardware change. Final tuning should be done with proper data logging.
FAQs
1. Does a smaller supercharger pulley always make more boost?
Usually it raises blower speed and boost. Results still depend on airflow, belt grip, air temperature, engine displacement, cam timing, and exhaust restriction.
2. What unit should I use for pulley diameter?
Use inches or millimeters. Keep crank, current blower, and proposed blower pulley diameters in the same unit for accurate ratios.
3. What is internal step-up ratio?
Some superchargers use internal gears. The step-up ratio multiplies pulley speed. Use 1.00 when the blower speed is directly based on pulley ratio.
4. Why include belt slip?
Belt slip lowers actual blower speed. It is common at high RPM or high load. Adding slip gives a more conservative speed estimate.
5. Is the boost estimate exact?
No. It is a planning estimate. Real boost depends on blower efficiency, altitude, inlet flow, intercooler performance, engine airflow, and tune quality.
6. What does smallest pulley by RPM limit mean?
It is the smallest pulley that should keep blower speed at your entered limit. A smaller pulley can exceed that limit.
7. Should I use peak RPM or shift RPM?
Use the highest RPM the engine will reach under load. Shift RPM or rev limiter RPM gives the safest blower speed check.
8. Can I tune from this calculator alone?
No. Use it for planning. Confirm with wideband data, knock monitoring, intake temperature, fuel pressure, and professional calibration when needed.