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. This is commonly referred to as turbo lag or turbo spool up time. Under WOT, turbo lag results in a seemingly underpowered engine that suddenly comes to life as a delayed tire melting rush of acceleration. Previously, turbo lag was limited by decreasing the size of the turbo itself. This resulted in lower rotating mass and more importantly, a smaller cross sectional area, which accelerated exhaust gasses at lower engine speeds. Although the turbo is able to spool quicker due to its size, for the same reason its ability to move and compress large amounts of air efficiently is significantly reduced. Inherently a smaller turbo will produce less maximum horsepower than if it were replaced by larger turbo on the same engine.
             Previous...