grimmelshanks wrote:i think a good simple way to put it is that superchargers deliver lots and lots of torque and turbos are better for horsepower. diesels aready have enough torque, what they need is more horsepower. also they move so much air that an air driven (exhaust driven, whatever) forced induction system is truly free power
No.
You can size a turbo to give you a huge increase in low end torque but it could run out of breath when the engine gets higher up in the rev band, sacrificing an increase in high end power. You can also size a turbo to not give a lot of boost until you high up in the rev band giving a nice boost in torque at high RPMs (and thus HP) while sacrificing low end torque increases.
A roots or twin screw supercharger feeds the same amount of air into the engine (per engine revolution) at all RPMs so it gives a fairly constant increase in torque across the entire engine speed range. It will have better gains in different parts of the speed range based on the engine's volumetric efficiency at different speeds, the efficiency of the supercharger at different speeds etc. Centrifugal (spelling?) superchargers give very little low end torque increase, but lots up top.
As for why is everything not turbocharged? Cost.
Bob
