Supercharged - Garage.Tech

Centrifugal compressors work best over a rather small speed range. This means that the step-up drive ratio must be carefully selected to prevent low-rpm compressor surge. Too high of a drive ratio can easily spin the compressor into surge. This happens when the engine's flow is less than the compressor's output, causing air to back up in the intake tract. When airflow backs up, the pressure after the compressor drops and airflow resumes causing a sort of chirping sound. Severe surge can become a violent oscillation that destroys compressor thrust bearings and can even cause wheel damage.

Care must also be taken not to overdrive the compressor wheel into choke. Choke occurs when the compressor wheel's inlet vane tip's speed exceeds that of sound, which causes a marked drop in flow. Centrifugal superchargers build boost slowly at lower speeds, usually reaching maximum power and boost at redline, due to the lower gear ratio built into the supercharger's gearbox necessary to avoid overspeeding the compressor into choke or surge at high engine speeds. The faster the engine's spun, the more power you get. This gradually increasing boost curve makes centrifugal superchargers feel somewhat laggy and less responsive at low speeds when compared to roots blowers. This compromise by design is perhaps the centrifugal supercharger's biggest disadvantage.

Perhaps the most important, yet not entirely obvious, advantage of the centrifugal supercharger is the possibility of greater durability when matched with a stock engine. Most engines are not designed to receive blower treatments, especially considering some engine's higher compression ratios and pump gasoline. This increases the chances of detonation. Most late model factory pistons feature top rings located as close to the piston's crown as possible to trap fewer hydrocarbons for reduced emissions. This area is especially prone to detonation damage. A naturally aspirated engine sees its highest cylinder pressure at its torque peak. As an engine's volumetric efficiency tapers off past its torque peak, cylinder pressures also drop. Roots blowers pump a good amount of pressure into the engine at low engine speeds, which greatly increases cylinder pressures at torque peaks. This is both good and bad. It's good because more low-end torque is made. However, if cylinder pressure gets too high, engine destroying detonation will occur, especially at the torque peak.

A properly engineered centrifugal blower won't produce as much boost and cylinder pressure at low engine speeds; it won't reach peak boost at its torque peak either. It will reach its maximum flow point near the engine's redline, however, when the engine's volumetric efficiency is rapidly falling off, giving the engine a boost in airflow as cylinder pressure drops. Cylinder pressure remains lower at the engine's torque peak as well, reducing the chances of detonation. The magic of the centrifugal blower is that it adds cylinder pressure when the engine can tolerate it best, at higher engine speeds. And to partially combat parasitic losses at partial throttle, most centrifugal supercharger kits feature huge bypass valves for venting excess air from the supercharger to the atmosphere, taking much of the blower's drive load off the engine when cruising without the recirculated heat exhibited by Eaton blowers.