The Garret Aviation VNT-25
The idea of forced air induction by turbine, or turbo, is not new and has it's mass production roots in WWII fighter planes. What is new, however, is its application to passenger automobiles. Unlike a near constant high RPM fighter engine, an automobile requires wide-open throttle (WOT) power availability throughout its entire operating range. Previous automotive turbo applications acted like an on-off power switch with a five second delay, decreasing drivability, rather than providing the smooth linear powerband of a normally aspirated engine. Because the turbine is in a fixed position in the exhaust stream, it was plagued with sometimes uncontrolled production from the compressor at high engine speeds, commonly referred to as boost creep, and a significant decrease fuel economy versus a similar, but naturally aspirated engine. The Garret Aviation produced VNT-25 solved all of these problems with its innovative Variable Nozzle Turbine. Hands down it is the most advanced turbo ever mass-produced and it was the first of its kind on production cars.
One of the most talked about problems with turbo charged engines is the lengthy time it takes for the turbo itself to accelerate to operational speeds
For instance, the size of connection on the intake side of the compressor is two and one quarter inches inside diameter and has a volume of 323 cubic centimeters until it reaches the compressor. In high performance applications it is a turbo that has little to be desired. The ONOFF switch of turbo power is gone and is now replaced by the safer, smoother, and much more linear acceleration comparable to naturally aspirated engines of much larger displacement. However, with disciplined drivers, it does not loose the fuel economy characteristics inherent with small, normally aspirated engines when the turbo is not in use. Previously, turbo lag was limited by decreasing the size of the turbo itself. At high engine speeds, the restriction creates enough pressure in front of the turbine (back pressure) that the wastegate can no longer limit turbine power by bypassing the exhaust around the turbine. The turbine also suffers from a small and compact AR ratio. Chrysler Passenger Cars Factory Service Manual vol. A VNT-25 equipped engine also has the potential to, and usually does, produce much more power than engines twice its size. Bernoulli's principal states that as a gas is compressed the temperature increases as the volume decreases. This resulted in lower rotating mass and more importantly, a smaller cross sectional area, which accelerated exhaust gasses at lower engine speeds. Because the exducer, that is the compressor, is of a compressor type, operational speeds are very high. Fourth Edition, Peoria, IL: MacmillanMcGraw Publishing, 1992. Smaller compressors will tend to heat the compressed air more than would a larger turbo for a given pressure. If it were allowed to back up near the compressor, the compressor would have to work much harder to move the already dense air.