MECHTA
You've got that a little backward Mechta. The turbocharger output is controlled by a WASTEGATE, a variable valve in the exhaust pipe ahead of the turbo that directs exhaust gas either through the turbo or bypass to atmosphere. At low altitude, where little boost is needed, the wastegate is mostly open, bypassing the turbo. As altitude increases, the wastegate gradually closes to direct more flow through the turbo & therefore increase its rpm & output up to the rpm limit of the turbocharger. Properly sized & controlled, the turbo can maintain sea level manifold pressure to any altitude.
The mechanical supercharger does loose with altitude & to overcome this, two speed & later two-stage (one blowing into another) supercharging were employed in later model Mustangs, Spitfires, etc to maintain power above thirty thousand feet.
Boosting air pressure, whether by mechanical or turbo, leads to greatly increased intake air temperature, which is not conducive to high power output. To counter this, complex intercooler & even aftercooler heat exchangers were employed in the intake downstream of the supercharger to reduce intake air temp & prevent detonation. Very high octane fuels had to be developed to work in these highly boosted engines at high altitude.
All of this development, done some 70 years ago during the emergency of a world war, is now beginning to show up in autos, where a tiny 2 liter four cylinder turbocharged & intercooled engine can produce the 300 horsepower of yesterdays V8 at a fraction of the fuel consumption.
Last edited by BobM2; 8th Jan 2011 at 01:12.