Join Date: Jun 2001
Location: Bergerac, France
Interestingly all the turbo cars you mention are 4wd, which is where the difference in performance comes from. Their advantage off the line is down to traction, and your advantage higher up the range is down to losses in the drivetrain. Your typical decent 4wd system (not the audi Haldex) will sap between 75-100 bhp when the engine is making full power. 250bhp at the flywheel > 150-175 bhp at the wheels.
It has nothing to do with the engine being normally of forced induction.
Engines are basically Air-pumps, the more air you pump the more power you make, because the more air you move the more fuel you can burn.
As Nigel pointed out, the advantage of a turbo engine is weight, theoretically speaking, when it is off boost, a 2.0L turbo engine acts as an ineficient 2.0L engine due to its lower compression ration, however once the turbo starts working and makes say 1.35 bar, you are now forcing air into the engine at 2.35 bar absolute, which turns your inefficient 2.0L engine into a superlightweight 4.7L engine.
All turbo installations are different, these days low pressure turbos are all the rage, running very little boost, they allow you to keep the compression ratio of the engine up, but still give you that extra boost. These have the advantage of being small, so they spool up quickly giving you boost when you need it. Older installations tend to run much larger turbos, with much higher boost. Off boost the car is sluggish, but once on boost they tend to be pretty awesome. The advantage of the older style installation is that the larger turbo continues to make lots of boost all the way to the red line, whereas the smaller turbo will not be able to flow as much air and will end up being more of a hinderance than a help at high rpm. All down to the engine being an air pump again.
And as for torque/power and RPM, the 3 are connected by a formula, BHP=(torque * RPM)/5252 , if you don't believe it grab some graphs and work it out for yourself.
If you look at the formula, you can see why the latest Jap engines rev up to @9k rpm and make loads of BHP from relatively low torque figures.
Have I missed anything?