Crank diameter ÷ blower diameter
Multiply the pulley ratio by engine rpm to estimate rotor input speed when no internal step-up is used. Always respect the exact unit’s maximum continuous and intermittent speed.
Lesson 5 · Positive displacement
Roots & Twin-Screw Superchargers
Positive-displacement superchargers move a predictable volume per revolution and can deliver strong low-rpm response. Their instant torque is attractive, but drive power, discharge heat, belt load, bypass control, packaging, charge cooling, and engine torque limits must be engineered.
Updated 29 July 2026
Author: LifeStyle Racing Technical Education Team
Worldwide unit guidance
LevelBeginner → Intermediate
Reading time15–22 min
Main decisionChoose by displacement per revolution, efficiency range, drive limits, and the torque curve—not sound.
Roots vs Twin-Screw
Both are crank-driven positive-displacement machines, but they compress air differently and should be compared using manufacturer efficiency data, speed limits, displacement, pressure ratio, inlet restrictions, and the complete manifold/charge-cooler system.
| Type | How it moves/compresses air | Strengths | Trade-offs |
|---|---|---|---|
| Roots / TVS-style | Lobes transfer air from inlet to outlet; most pressure rise occurs as air meets manifold pressure. | Immediate response, compact integrated packages, strong OEM development, effective bypass operation. | Discharge temperature and drive power rise outside the efficient range; packaging often sits above the engine. |
| Twin-screw | Intermeshing male/female rotors reduce trapped volume internally as air moves along the rotors. | Efficient internal compression in its intended pressure-ratio range, strong low/mid torque. | Cost, rotor/gear precision, inlet sensitivity, noise, packaging, and mismatch when internal compression ratio does not suit the operating point. |
Positive-displacement system path
Filter + throttleLow restriction and metering
Supercharger inletFeeds rotors evenly
RotorsMove fixed volume
Charge coolerIntegrated or external
ManifoldDistributes to cylinders
BypassReduces work off boost
Drive Ratio, Belt Load, and Head Speed
Pulley changes affect supercharger speed, airflow, boost, drive power, belt traction, bearing load, and heat. The smallest pulley is not automatically the best performance choice.
Contact area matters
Small pulleys reduce wrap and increase required belt force. Idler position, tensioner range, alignment, bracket stiffness, pulley surface, belt type, and accessory load all affect traction.
Unload at cruise
A bypass valve routes air around the rotors during low-load operation, reducing pumping work and heat. It is not the same job as the boost-control wastegate on a turbo system.
Instant cylinder pressure
Early torque can exceed rods, pistons, head sealing, clutch, gearbox, axles, tyres, and traction limits even when peak horsepower appears modest. Torque management may be essential.
Never assume the boost increase from a pulley percentage. Actual manifold pressure depends on engine airflow, supercharger efficiency, inlet/throttle restriction, charge-cooler pressure drop, cam timing, bypass sealing, belt slip, temperature, and calibration. Log speed/boost/temperature after every drive change.
How to Choose a Positive-Displacement System
The best system fits the engine’s intended rpm band and delivers the requested airflow without living at its speed or temperature limit.
- Use manufacturer airflow, efficiency, speed, and pressure-ratio data for the exact rotor pack or complete kit.
- Check throttle and inlet size, air path, filter location, bonnet/hood clearance, manifold distribution, and charge-cooler core.
- Calculate crank and supercharger speed at the real rev limit, including any internal gear ratio.
- Verify belt length, width, routing, wrap, tensioner travel, bracket support, and service access.
- Plan bypass control, throttle strategy, calibration, fuel, spark plugs, sensors, crankcase ventilation, and knock protection.
- Evaluate intercooler pump, heat exchanger, reservoir/degassing, fluid routing, bleeding, and repeated-use temperature recovery.
Technical references
Educational scope: These lessons help readers form better questions and shortlists. Final part selection, fabrication, calibration, inspection, emissions compliance, insurance, and road-use legality depend on the exact vehicle and country or region.
