Calculator Inputs
Example Data Table
| Engine | Blower | Crank Pulley | Blower Pulley | RPM | Estimated Result |
|---|---|---|---|---|---|
| 6.2 L | 2.3 L/rev | 7.8 in | 3.4 in | 6500 | Moderate street boost estimate |
| 5.0 L | 2.65 L/rev | 7.5 in | 3.2 in | 7200 | Higher blower speed estimate |
| 376 cid | 140 ci/rev | 198 mm | 82 mm | 6200 | Metric pulley comparison |
Formula Used
Pulley ratio = crank pulley diameter ÷ supercharger pulley diameter.
Total drive ratio = pulley ratio × internal step ratio.
Effective drive ratio = total drive ratio × (1 − belt slip percentage).
Blower RPM = engine RPM × effective drive ratio.
Engine demand = engine displacement × engine RPM ÷ 2 × volumetric efficiency.
Blower supply = blower displacement × blower RPM × blower airflow efficiency.
Pressure ratio is estimated as blower supply ÷ engine demand. Boost is then ambient pressure × (pressure ratio − 1).
Temperature rise uses an ideal air compression estimate with heat efficiency. Intercooler effectiveness then reduces the calculated outlet temperature.
How To Use This Calculator
Enter the engine size and choose the matching unit. Add the supercharger displacement per revolution. Then enter crank and blower pulley diameters.
Use the internal step ratio when the blower has internal gearing. If it does not, keep the value at 1. Enter engine speed at the point where you want the estimate.
Set volumetric efficiency, blower efficiency, belt slip, ambient pressure, inlet temperature, and intercooler effectiveness. Use conservative values when unsure.
Press calculate. Review boost, airflow, pressure ratio, blower speed, heat estimate, and warning notes. Use the export buttons to save the result.
Supercharger Pulley Boost Guide
Why pulley boost matters
A supercharger pulley controls blower speed. A smaller blower pulley spins the unit faster. A larger crank pulley does the same. Both changes can raise airflow and boost. That extra pressure can add power. It also adds heat, load, and tuning risk.
This calculator helps you compare pulley choices before buying parts. It does not replace dyno testing. It gives a planning estimate based on displacement, pulley ratio, slip, airflow efficiency, and ambient pressure. These values describe how much air the blower can move compared with the engine demand.
Airflow is the main idea. A four stroke engine draws its displacement every two crank revolutions. Volumetric efficiency adjusts that demand. The blower moves its own displacement every blower revolution. The drive ratio links both speeds. When blower supply is higher than engine demand, pressure ratio rises.
Belt slip is important. High boost setups can lose speed at the pulley. A small slip number can change the final result. Heat also matters. Compressing air raises temperature. Hotter air is less dense and more likely to cause knock. Intercooler effectiveness shows how much of that heat may be removed.
Use the output as a comparison tool. Try your current pulley first. Then adjust the blower pulley diameter or crank diameter. Watch blower rpm, estimated boost, outlet temperature, and warning notes. A pulley that makes strong boost may overspeed the unit. It may also need larger injectors, better fuel, or a different tune.
Real engines vary. Cam timing, port design, throttle losses, bypass valves, leaks, altitude, and exhaust back pressure change results. Manufacturers also rate blowers differently. Treat every answer as an estimate. Confirm final choices with wideband data, datalogging, and professional calibration. Safe tuning protects pistons, bearings, belts, and the supercharger itself.
Keep records for each test. Note the pulley sizes, weather, fuel, spark timing, and datalog values. Compare one change at a time. This makes problems easier to find. A boost estimate may look safe on paper, yet the engine may show knock or belt dust during testing. Stop when data looks unstable. Good measurement is better than guessing. It helps you build a faster and longer lasting combination. Review logs after every pass or street pull.
FAQs
1. What does a smaller blower pulley do?
It spins the supercharger faster for a given engine speed. That usually raises airflow and boost. It can also raise heat, belt load, and overspeed risk.
2. What does a larger crank pulley do?
A larger crank pulley increases the drive ratio. It can raise blower rpm without changing the blower pulley. The result is similar to using a smaller blower pulley.
3. Why is belt slip included?
Belt slip lowers real blower speed. High boost systems can slip under load. Adding a slip estimate makes the result more realistic than a simple pulley ratio.
4. Is this boost estimate exact?
No. It is an airflow based estimate. Real boost depends on cam timing, leaks, bypass action, blower design, throttle losses, exhaust flow, and tuning conditions.
5. What is internal step ratio?
Some superchargers use internal gearing. The internal step ratio multiplies pulley speed. Use 1 when your supercharger has no extra internal speed increase.
6. Why does temperature rise matter?
Compressed air gets hotter. Hot air is less dense and can increase knock risk. Charge cooling and careful tuning are important when pulley changes raise boost.
7. What is a safe blower rpm?
Safe speed depends on the exact supercharger model. Check the manufacturer limit. Enter that limit so the calculator can flag a possible overspeed condition.
8. Can I use this for final tuning?
Use it for planning only. Final tuning should use datalogs, wideband readings, knock monitoring, fuel checks, and expert calibration on the actual vehicle.