Originally Posted by DGthe3
Its not a simple percentage, but that is a much more accurate rule of thumb than a static figure is.
You are probably thinking of the power that it takes to accelerate a wheel or drive shaft, and its true that these things take only a fixed amount of power to be accelerated (until you swap them out for something different of course). But the instant that they are up to speed, they no longer rob anything of any power (and they also 'release' power as you try and decelerate). This does not hold true for the gears in the transmission & differential, or the half-shafts going out to the wheels, or the bearings or tire flex any of the other sources of friction. Those losses can be approximated as a percentage of input power, though in reality its a somewhat complex curve that depends on a number of factors. Point being, if you run your car stock on a chassis dyno and calculate that it lost 55 hp ... don't assume that after you've supercharged it its still only losing 55 hp.
I'm not going to ruin this already awesome thread any more but there is no percentage, period.