A turbocharger doesn't deliver free horsepower because the back pressure required to drive the
turbo reduces the cylinder feed differential (blower pressure less back pressure to drive turbo)
and subsequently the horsepower output. The mechanical (Roots) blower doesn't suffer from the
back pressure issue but does require a fairly substantial amount of power to compress a very
compressible gas, air. The turbo is capable of much higher rpm and while it's less efficient as
a compressor it more than makes up for it in flow capability.