Sierra and Silverado take on heavy-duty truck market - Getting Around

Chevrolet’s Silverado HD and General Motors’ Sierra HD are GM’s latest entries in the heavy-duty truck market.

The front and rear wheel well areas have defined power bulges and the headlights and grill area of the HD models are different than regular versions. One-ton Big Dooley models will have single piece composite rear fenders.

Frame construction will be the new three-piece method pioneered in the 1999 Silverado-Sierra pickups. The front section is tubular steel hydroformed to provide proper mounting points for the front suspension and engine. The mid-section will continue to be boxed steel, but reinforced to take the added power of the engine and carrying capacity required to compete in this market segment. The rear section carries the semi-elliptical rear spring suspension. It too is boxed steel, but of a lighter construction than the middle section.

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The Vortec 8100 is the new gasoline engine. Engineers expect some improvement in fuel economy, but since the truck is a heavy duty model and not rated by Transport Canada, they were reluctant to give specifics. Engineers said their power targets of 340 horsepower and 455 lb. feet of torque have been met. Also, emissions have been reduced and idle quality improved by reworking the firing order and the intake manifold and cylinder heads.

The most important engine in this segment is the new 6600 DuraMax. Engineers had specific targets, the main one being silence. Most diesels are so loud you can’t carry on a normal conversation within two metres of one. I was no more than a third of a metre from this truck and I had to stop talking to hear if it was running.

Increasing power was another goal. The DuraMax 6600 puts out 520 lb. feet of torque and 300 hp. The DuraMax engine is a 90 degree, direct injected, overhead valve, four valves per cylinder, turbocharged, diesel V-8 with high swirl heads and significantly improved cooling characteristics.

Improvements include an integral oil cooler mounted on the side of the engine block rather than in the radiator end tank. Flow from the gear-driven water pump is channeled through the oil cooler before going through the engine. Internal cooling is aided by spraying oil directly onto the backside of each piston bowl.

A nitrided crankshaft and a deep skirted engine block design enhances reliability. Exclusive features include an induction-hardened engine block and hardened valve seat inserts in the aluminum heads. These techniques boost reliability. DuraMax life expectancy is more than 300,000 kilometres.

The direct-injection system developed by Bosch is another first for diesels. This direct-injection, common rail system separates pressure generation from engine speed and the injection event itself. Engineers can now program the system for optimal performance over the full engine speed range. For example, pressure can be varied up to 23,000 psi and the injector can now open or close as directed by the electronic control module and the electronic driver unit. This sophistication allows the Duramax to employ a process called “pilot injection,” which involves injecting a small amount of fuel into the cylinder prior to the main injection event. Combustion starts on a smaller scale, making for reduced engine knock-noise, less vibration and smoother warmups.

To take advantage of this turbocharged engine’s power, GM developed a six-speed manual gear box and a five-speed electronically controlled Allison automatic. The Allison has provisions for two power take-offs, provided you buy a two-wheel drive unit. Four-wheel drive units can have only one pto because the drive shaft to the front runs too close to the tranny for the second pto unit to fit.

I managed to get in half a day of driving with a load on. GM set up a test in the Rocky Mountains with a Duramax and trailer loaded to 19,000 lb. as well as a couple of other HD units with such loads as bales and lumber. Each unit came out to 90 percent of gross combined vehicle weight rating. Limited seat time turned up a truck that was quiet and easy to drive.

There was never a power shortage, even when climbing some of the steep mountain passes that tourists avoid because they are gravel or too steep for comfort.

Price isn’t out of line when compared with competitors. The extended cab full-leather top of the line SLT that I drove was $54,000.

Charles Renny is a member of the Automobile Journalist Association of Canada.