New diesel engine: An all-new version of the Duramax 6600 turbo-diesel V-8 engine will debut in the interim 2006 model year as an option on Silverado Heavy Duty models. When mated to an all-new Allison 1000 six-speed automatic transmission, the new powerplant will deliver more horsepower – 360 hp (268 kw) at 3200 rpm – and increased torque, at 650 lb.-ft. (881 Nm). The power increase comes with lower emissions and significantly improved quietness and smoothness. The Duramax 6600’s horsepower and torque increase and emissions reduction are enabled by a strengthened iron cylinder block and a lower compression ratio. The lower compression reduces stress on the engine by reducing the peak cylinder firing pressure. This, in turn, allows more fuel to be burned – more fuel means more power – while the lower compression helps reduce NOx emissions. Lower compression also helps reduce noise and vibration, making the 6600 a quieter and smoother engine. A revised variable-geometry turbocharger also enhances the driving experience of the Duramax 6600. Aerodynamic changes to the turbo’s impellers help tailor controlled application of turbo power for seamless and immediate response at full throttle. The turbo, which spins at up to 120,000 rpm, is high-speed-balanced, reducing vibration and resonance – and contributing to the engine’s overall smoothness and refinement. The revised turbo also helps reduce emissions, while maximum boost remains at 20 psi. Another new feature of the Duramax 6600’s turbo system is the capability of the variable-geometry turbocharger to provide exhaust braking. This function is available on some medium duty truck models and can replace add-on exhaust brake hardware. With the new system, braking is controlled by a signal from the engine controller and can be activated by the driver.