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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 benefcial 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 signifcant changes on the interiors. The heavy-duty versions carry on with both. But because these are heavy-duties, there are some modifcations that Luke points to, like the large grilles with chrome surrounds and prominent cooling intakes; diferent 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. u The new heavy-duty trucks from Chevrolet and GMC are available in more than 150 variants, taking into account things like cabs (regular, double and crew), box size (6 ft, 6 in. and 8 ft), and powertrains (gasoline, diesel, gasoline/ compressed natural gas). AD&P; > April 2014 > FEATURE > GM Brings on the Heavy-Duty Silverado and Sierra > Gary S. Vasilash > gsv@autofeldguide.com 36 0414ADP FEATURE Heavy Duty.indd 36 3/18/2014 12:16:18 PM