The centrifugal blower's gradual power delivery offers some advantages. On traction-limited FWD vehicles, such a powerband can sometimes be advantageous, making the car easier to drive at its limit-especially on a road course. It can also aid FWD dragstrip launches, an important factor for those using street tires. Another advantage a centrifugal supercharger has over a roots blower is the ease of which intercoolers can be packaged into the system. Since the supercharger remains separate from the engine, integrating an intercooler and plumbing in between the compressor outlet and intake manifold is relatively simple. Centrifugal blowers are also much more compact and easier to package into today's tight engine bays.

So Who Makes Them?
HKS recently introduced its GT series centrifugal supercharger. The HKS supercharger is perhaps the most compact piece on the market, taking up roughly the same space as an A/C compressor. At the moment, HKS offers applications for the Nissan 350Z, Infiniti FX35 and Infiniti G35. Featuring high-quality components and an exceedingly clean installation, the HKS system looks almost as if it was an O.E. application. The GT supercharger is currently limited to an approximate 50whp gain due to reliability concerns since HKS feels any more power could be detrimental to the stock VQ engine's lifespan. However, the GT supercharger is capable of producing several hundred more horsepower when paired with a built bottom end. The GT supercharger's most unique feature is its unique traction drive planetary roller system used to step up compressor wheel drive speeds.

HKS' unique traction drive uses a 9.44:1 gear ratio-a much higher ratio than typical when considering centrifugal superchargers-to spin the compressor wheel up to 70,000 rpm. This gear ratio step-up is accomplished through a roller system, much like the planetary gears found in an automatic transmission, except smooth rollers are used instead of gears. The rollers are preloaded between the circular outer housing of the planetary drive and the compressor wheel's centrally located drive spindle.

Special thixotropic oil in the gear case allows the smooth gears to grip and transfer drive torque between the rollers and drive spindle. The traction oil becomes thicker and more viscous under shear or slippage, providing further transmittal of drive torque. This is where the traction drive's biggest advantage comes into play. At low engine speeds and under light loads, the traction drive allows slip so the blower reaps little in terms of parasitic losses. This translates into decent part-throttle fuel economy. As engine speeds climb and the compressor wheel requires more power, the fluid thickens transferring more power to the compressor to produce more boost.

Traction drive systems are 98 percent efficient in terms of transferring drive power and soak up less power than typical gear or belt drives used by other superchargers, which are, at best, 92 percent efficient. This helps both power and fuel economy, making the GT supercharger perhaps the most fuel efficient supercharger on the market. Anyone with supercharger experience knows that they can be pretty noisy. The traction drive is silent, unlike gear drives and belts, making the HKS system unobtrusive. The HKS kits also have a pending CARB EO for 50-state legality.

Adding It Up
Supercharged engines respond to peripheral modifications just like naturally aspirated ones. Superchargers love headers, exhausts and low-restriction air intakes, usually to a higher degree than their naturally aspirated counterparts. A supercharged engine generally prefers larger header primary tubes, collectors and exhaust systems.

Supercharged engines also respond to high-performance camshafts of nearly the same spec as a naturally aspirated engine. Adjustable camshaft gears will come in handy here since supercharged engines typically like a little less overlap to prevent boost pressure from blowing out the exhaust ports. Such engines also respond well to head porting and other flow-increasing strategies. If high-boost pressures are the plan, especially with low-quality 91-octane fuel, a heavily boosted engine will appreciate lower compression forged pistons in the 8.5:1 compression range and more sophisticated engine management.

A supercharger is usually a more reliable way to make an equivalent amount of power than building a naturally aspirated engine. Increasing the cylinder pressure with forced induction at lower engine speeds puts a compressive stress on the rods and crankshaft. However, to make more power with a naturally aspirated engine requires a boost in engine speeds. Such high speeds puts a tensile stress on the reciprocating components of the engine that is much greater than the compressive load a supercharger adds. This simple display of physics allows superchargers to obtain impressive results with greater reliability and a lower cost in comparison to high-horsepower naturally aspirated engines. Of course, this assumes detonation is avoided and the car has sufficient cooling capacity for its intended use.

To sum things up, roots blowers have the most immediate power, which remains proportional throughout the powerband. Centrifugal blowers make the most peak power, which is achieved by gradual increases throughout the powerband, peaking near redline. And remember, superchargers produce fewer emissions than turbochargers. In short, if you aren't looking for ultimate power but instead are trying to get a large gain of tractable oomph from your engine, or if you're simply looking for a heavily modified car that still needs to pass a smog inspection, then supercharging could just be the best answer.

HKS Thomas Knight