General Motors launched the Duramax Diesel 6.6L V8 engine in the Chevrolet Silverado and GMC Sierra 2500HD and 3500 Series trucks with Allison automatic transmissions in 2001. The application expanded to the Chevrolet Kodiak and GMC Topkick medium-duty trucks for 2003, with 16,000-, 18,000- and 19,500-lb. gross vehicle weight ratings (GVWR).

According to its engineers, the Duramax Diesel 6.6L V8 features two key characteristics: incredible torque to tow or haul loads, and higher-rev horsepower for smooth, flexible over-highway travel.

Turbo Improvements
Refinements in 2005 included an overall reduction in exhaust emissions and an increase in torque output, which was enabled by significant hardware changes in 2004. Those changes included a new variable-geometry turbocharger. Self-adjusting turbine vanes and sophisticated electronic controls automatically adjust boost pressure and exhaust backpressure. The vanes direct exhaust gas at the turbocharger's turbine blades. These vanes can be opened or closed to vary the amount of boost pressure. An electro-hydraulic device operated with engine oil, similar in concept to a camshaft phaser, adjusts the turbine vane angle. The variable-geometry turbo has its own sensors managed by the engine control module (ECM). A solenoid controls oil pressure against a piston that operates a small cam, which works on a unison ring that moves the turbine vanes simultaneously. As the cam turns, it varies the angle of t...