[Adam]

Just thought I'd add to the topic....
SCI
Turbo and Blower Design

A turbo consisits of a centrifugal compressor unit [something like a multi-bladed fan or tiny jet engine] coupled to a separate turbine unit [another ducted fan] sharing the same shaft and driven to speeds of up to 180,000 rpm by hot exhaust gases traveling through at speeds approaching 400 mph. The more boost put into an engine, the more exhaust is generated. In turn, the more boost that can then be generated and the more exhaust you get, and so on in a feedback loop of sorts. Centrifugal compressor airflow increases as the square of compressor rpm.
In order to keep a turbo engine from rapidly self-destructing due to overboost at wide open throttle, a wastegate --controlled, these days, by a computer-- is employed to bypass just enough exhaust around the turbo to maintain the desired level of boost in the engine's intake manifold. At low compressor speeds, centrufigal compressor airflow falls off dramatically, but when coupled with a wastegate, the exponential nature of centrigual compressor boost acutally becomes a great asset for getting low-rpm boost up quickly.
The main fallout of exhaust-driven centrufigal compressors is not a lack of low speed boost but the brief fractin of a second [sometimes infinitesimal] of "turbo lag" that occurs upon application of full throttle at idle or light cruise while the compressor spools up to make boost. In good turbo cars, turbo lag is virtualy non-existent.
Superchargers, on the other hand, are driven directly off the crankshaft by gears or belts. A Top Fuel Roots blower at full howl is consuming 1000 horses of the engine's power just to turn the blower [which is still a pretty good deal]. By contrast, turbos are largely driven by exhaust gases and heat that would othrwise be wasted. Some superchargers--like the Vortech and Paxton-- are essentially turbo compressors that are belt-driven off the crank, but there is a fundamental problem here: When you match the centrifugal blower's gearing to achieve the correct multiple if engine speed for correct boost at maximum power rpm, given the exponential nature of air flow changes with speed, your compressor speed will then typically be too slow to make any boost at low engine speed. [Fixed-displacement-- also called positive-displacement-- blowers like the Roots deliver nearly instantaneous low-end boost.]
Turbos have no such problem. Driven by some or all of the exhaust gases [depending on wastegate position], they are not limited to a particular multiple of engine speed. The wastegate concept enables the turbine and compressor configuuration to be optimized to be optimized to deliver plenty of bost air flow at low rpm and low exhaust flow, while diverting enough exhaust past the turbine to keep boost from getting out of hand at wide open throttle and higher rpm.
The Roots-type Eaton supercharger differs from turbos and centrifugal superchargers in that, internally, there are no fans at all, but internal moving rotors that gulp air, close the "gates" behind them, and then squish or move the air in the general direction of the engine intake. Positive-displacement blowers essentially pump a fixed amount of air per cycle, reguardless of the speed at which they are rotating [this breaks down at very low and high speed due to changes in volumetric efficiency].
The bottom line is, both turbos and blowers can pack extra air in an engine; an important difference between the two is the blowers are a slave to the speed of the enigne while turbos are not. While a supercharger cannot lag behind engine speed, it also can't surge ahead like a turbo to make more power while an engine lugs at low speed. Maximum boost, delivered mechanically on blowers according to the size of the installed blower-belt pulleys, cannot be dynamically increased to compensate for altitude as it can on a turbo, and it cannot be controlled by a computer to compensate for fuel quality or to provide short bursts of extremely high boost for passing. Without computer controls, in general, superchargers on street cars cannot operate in as high a state of tune as cumputer-controlled turbos.
A supercharger is like an engine that has to drive a car with only one gear, whereas the turbo is like an engine coupled to an infinitely-variable transmission. The need for flexible blower speed for a crititcal performance is illustrated by WW II fighter aircraft like the P-51 Mustang, which used a two-speed supercharger [essentially all modern piston-engine aircraft use turbos]. Pre-war European F1 racers--Alfas in particular-- have been known to use multi-stage supercharging.
From a technical point of view, the really interesting thing about turbos is that, within limits, the more power they make, the more power the CAN make. From a marketing pint of view, turbos and blowers both make more power.

--Sports Car International--January 2001-- Page 57