GM Brings on the Heavy-Duty Silverado and Sierra

Gary S. Vasilash

Those who are looking for dependability, durability and capability in their trucks now have new vehicles from Chevrolet and GMC to consider, heavy-duty trucks that also include style and comfort.

According to Jeff Luke, executive chief engineer, GM Global Full-Size and Mid-Size Trucks, in 2013, approximately 25%, or some 475,000, full-size pickups were of the heavy-duty variety. And then of that number, he continues, about two-thirds are diesel-powered.

So it should come as no surprise that as Luke and his colleagues roll out the 2015 Chevrolet Silverado HD and the GMC Sierra HD, among the more than 150 build combinations for each of them, there is the 6.6-liter, 397-hp, 765 lb-ft of torque Duramax turbodiesel, an engine that has been in production since 2000, but which, Luke explains, has undergone continuous improvement until it has ended up under the power-dome hoods of the new trucks.

And while on the subject of the diesel, know that: The Duramax features a cast-iron cylinder block with induction-hardened cylinder bores. The crankshaft is nitride-hardened, forged steel. The pistons and the cylinder heads are aluminum. The variable-vane geometry turbocharger has an integrated exhaust brake system. After-treatment is via a urea-based Selective Catalyst Reduction system; urea is held in a 5.3-gallon tank and needs replenishment about every 5,000 miles. There is a diesel particulate filter system that is regenerated by injecting diesel fuel into the exhaust stream rather directly into the cylinder, thereby eliminating concerns of the fuel contaminating the engine oil. And it would probably not make a whole lot of sense to not mention at this point that the diesel is mated to an Allison 1000 six-speed transmission, which has a sixth gear ratio of 0.62:1. As the application is for heavy-duty trucks, it offers features like 250 lb-ft of torque for power take off (PTO) applications; has driver shift control that allows the driver to manually select and hold a gear longer (though it automatically protects the engine by inhibiting downshifts when the engine speed is above allowable rpm limits); has a driver-selectable tow/haul mode (which locks the torque converter earlier, which facilitates the aforementioned engine braking, which is highly beneficial when going downhill and having a load on the hitch); and has an elevated idle function that helps heat up the cab more quickly when temperatures are low (and speaking of low temperatures, the Duramax uses microprocessor-controlled glow plugs that allow engine start in less than three seconds at as low as -20°F).

But, of course, there is still that third of customers who don’t opt for a diesel. The standard engine is a 360-hp, 380 lb-ft of torque 6.0-liter V8. This small-block engine has a cast-iron block, aluminum cylinder heads, polymer-coated aluminum pistons, a steel camshaft, steel roller rockers, and other features for durable operation. (How durable? The engines were dyno tested to an equivalent of 200,000 miles, including 600 hours’ worth of operating at the peak horsepower and torque points.) The engine features standard variable valve timing.

While the standard version of the V8 can run on either gas, E85 or a mixture, there is another variant for the engine, which is a compressed natural gas (CNG)-capable engine, which runs on both gasoline and CNG. Not only does this mean that there is a separate fuel tank to accommodate the CNG, but the engine features specially hardened valves and valve seats.

The transmission for the gas-powered heavy-duty trucks is the heaviest duty of all Hydra-Matic six-speed transmissions, the 6L90.

Late last year, Chevrolet and GMC launched their light-duty versions of the Silverado and Sierra. These trucks have new sheet metal all around as well as significant changes on the interiors. The heavy-duty versions carry on with both. But because these are heavy-duties, there are some modifications that Luke points to, like the large grilles with chrome surrounds and prominent cooling intakes; different hoods and headlamps; and a chrome bumper.

In 2011, the GM truck engineers developed an all-new underlying architecture for the pickups, and this, as in the case of the light-duty 2014 models, is used for the heavy-duty trucks. However, they’ve beefed up the fully boxed frames such that the vehicles have notable performance in heavy-duty applications, like handling 7,374 lb. payloads and towing, with the factory-installed hitch, 19,600 lb.

Speaking of the factory, there are two for the heavy-duty trucks, Flint Assembly in Michigan and GM Fort Wayne in Indiana.