It takes power to turn a supercharger just like it does a water pump or alternator since it is belt driven. A turbo is exhaust driven so it is pretty much free horsepower. Superchargers on races cars at max boost can take 300hp+ to turn them.
There is no such thing as free horsepower. Simplified, both the turbo and the supercharger need the same amount of shaft power to force air into the engine: Just the turbo takes the same amount of compression energy back from the expansion of exhaust gases (a bit more actually, but not get too complicated) while the supercharger doesnt take any energy back from the expansion in the exhaust: Rudolf Diesel in his patent description of his invention of the Diesel engine, wrote that the compression could also be done in two or more stages: Essentially what he was describing with those words, is forced air induction: or what we know today as turbos and superchargers. To stay in Rudolf Diesels terminology, the turbocharged engine compresses air in two stages (first the turbocompressor, then the piston compression stroke) and also expands the air in two stages: first the expansion stroke(or combustion, or work stroke) then the expansion in the turbos turbine. The supercharged engine however, has two stage compression but only single stage expansion ! So loss of energy.
To get to the other end:With Miller timing a supercharged engine could be made to do away with the drawback of two stage compression and single stage expansion, and equal the efficiency of a turbocharged engine, because Miller and Atkins valve timing is all about using less of the piston stroke to draw in air (and compress it, so less energy is put into compression) by closing the intake valve early, while the full piston stroke is used to extract energy during the ignition stroke: Due to the heat up of combustion gases they expand, so it is beneficial to have a longer expansion stroke than compression stroke. Since the crankpin offset determins stroke for both compression and ignition, you can close the intake valve at, say 4/5 of the intake stroke, so only 80% of the air is drawn in. Then in the expansion stroke, you can extract 25% more expansion energy...
Miller and Atkins try to achieve the same, but Miller took it a bit further and said you needed a turbo because otherwise you are vacuuming the intake air by about 20% during that last part of intake stroke with closed valve, because the other end of the piston is acting against atmosphere pressure in the crankcase, as well as pushing air against atmospheric pressure out the exhaust pipe. Not that interesting if you keep in mind that spark ignition engines at part load condition, are constantly sucking a vacuum against the throttle valve, and exhausting against atmospheric pressure. At full load, the Diesel is already superior in efficiency, but at part load the gas engine has another major drawback, that is that it needs a fixed air/fuel ratio, but has a fixed displacement so it must create a vacuum.
Thats why Ford is opting for smaller displacement turbocharged gas engines: because of the smaller displacement, it loses less energy by sucking the intake vacuum against the throttle valve, because the turbo doesnt change anything when the exhaust gas pressure and volume (thermal expansion due to combustion heat) is more or less the same as its intake gas pressure and volume. Downsized turbo gas engines just do away with the majority of the throttle valve vacuum loss.
===edit: Reading on, i see two more pages with engine myths, and conclude that my long winded story above, covered most of them
