Last month in our discussion on superchargers, we covered the ins and outs of the popular Roots-type, positive-displacement supercharger. This month we'll explore the differences, advantages and disadvantages of centrifugal blowers, the other prominent type of blower found in the sport compact aftermarket.
Difference and designVortec, ProCharger, HKS, Rotrex and Powerdyne all make centrifugal supercharger kits that are capable of producing respectable power. This is primarily due to their compressor design, which is more efficient than Roots-type blowers and is why centrifugal blowers are known to make more peak power than Roots blowers.
A centrifugal supercharger is basically the same as the compressor section of a turbocharger, but is driven off the crank instead of an exhaust-powered turbine.
A centrifugal supercharger, like a turbo, must be spun to a critical speed before any significant boost is produced. The drive system has a gear ratio that speeds up the compressor's rotation because centrifugal compressors spin much faster than the engine's crank will turn-between 30,000 and 60,000 rpm.
Centrifugal compressors are efficient over a rather small speed range. This means the step-up drive ratio must be carefully selected to prevent overspeeding the compressor at high rpm. Because of this, the compressor isn't spinning at its optimal boost-producing speed when the engine is at low rpm. This compromise by design is the centrifugal supercharger's biggest disadvantage. It's also what keeps it from making boost at low rpm like Roots-type blowers.
If the wrong step-up drive ratio is selected it's possible to spin the compressor into surge. Surge results when the engine's flow is less than the compressor's output, causing air to back up in the intake tract. The resulting instability causes air in the compressor housing to oscillate violently back and forth over the compressor wheel.
When the airflow backs up, the pressure after the compressor drops and the airflow resumes, causing a chirping sound. In severe surge, this oscillation can destroy the compressor thrust bearing and even cause mechanical failure of the wheel.
Care must also be taken not to overdrive the compressor, which would cause choke. Choke happens when the inlet vane tip speed of the compressor wheel exceeds the speed of sound, causing flow to drop. Centrifugal blowers build boost slowly because of the low gear ratio required to avoid overspeeding the compressor into choke at high rpm.
This design constraint usually means that centrifugal blowers make maximum boost and power at redline. The faster you spin the motor, the more power you get. This gradually increasing boost curve can make a centrifugal supercharger feel laggy and less responsive at low speeds than a Roots blower.
Not-so-obvious advantagesSome people feel a centrifugal blower is like a super VTEC kicking in. In fact, on a B16A Civic Si, the fat part of the Vortech's boost curve hits just as the high-rpm cam lobe is engaged by the VTEC system, giving an impressive boost in power.
On traction-limited front-wheel-drive vehicles, this sort of gradual power delivery can sometimes be advantageous, making the car easier to drive at the limit, especially on a road course. It can also aid dragstrip launches, an important factor if you run on street tires. A Vortech-equipped Civic Si can run low 14-second quarter-mile times on street tires.
When supercharging a naturally aspirated engine with an aftermarket kit, a centrifugal blower may have some engine reliability advantages over the Roots blower. A naturally aspirated engine creates peak cylinder pressure at its torque peak. As the engine's volumetric efficiency falls off past the torque peak, the cylinder pressure normally drops. A Roots blower pumps lots of pressure into the engine at low rpm, which greatly increases the cylinder pressure at the torque peak. This is good and bad. It's good because more torque is produced. However, if cylinder pressure is too high, detonation will occur.
A properly engineered centrifugal blower will not produce as much boost and cylinder pressure at low rpm. It will reach its maximum flow point near the engine's redline, just when the volumetric efficiency is rapidly falling off. The magic of the centrifugal blower is that it adds cylinder pressure when the engine can tolerate it best.
To combat pumping losses at partial throttle, most centrifugal supercharger kits have a huge bypass valve to vent air around the supercharger to the atmosphere, taking much of the blower's drive load off the engine at neutral-throttle cruise. On cars with a mass airflow meter, this air is recirculated after the airflow meter to avoid a rich condition.
This happens when air is measured by the airflow meter and the ECU uses this value to calculate the right amount of fuel for the air that entered the engine. If air is dumped to the atmosphere through the bypass valve, the ECU's calculation will be wrong, in turn causing a rich condition.
Another advantage that centrifugal superchargers have over Roots blowers is the ease of packaging an intercooler into the system. Since the supercharger is a separate unit from the engine, it is easier to add an intercooler into the plumbing between the compressor outlet and the intake manifold.