General Motors' Heavy Duty pickups for 2011 will feature stronger frames, new suspensions, a more powerful, cleaner-burning Duramax diesel and an enhanced Allison automatic transmission.

The Chevrolet Silverado and GMC Sierra will be offered as 2500 HD 3/4-ton and 3500HD 1-ton models and will go into production this summer, executives of the two divisions said at separate unveilings at the Chicago Auto Show and NTEA's Work Truck Show in St. Louis.

GMC also showed off an upscale Denali version of its 2500HD 4-wheel-drive Crewcab pickup. It has leather-covered seats, brushed aluminum trim, Bose stereo system and 12-way power seats inside, and special chrome grille, door handles, badges and 18- or 20-inch polished, forged aluminum wheels. About 10 percent of customers who buy this type of 2500HD pickup with the standard 6.5-foot box will choose the Denali package, executives estimated. Chevrolet and GMC HDs all get restyled chrome-plated grilles and new wheel designs, and all have domed hoods with powertrain badges.

Mechanical advances in the GM HD trucks are the more notable features, executives said. They give the pickups fifth-wheel and ball-hitch towing capacities of 20,000 and 16,000 pounds, respectively. Payload capacity is up to 6,335 pounds. Thanks to a completely redesigned independent front suspension, front-axle capacity is 6,000 pounds and every 4x4 Silverado and Sierra can mount a snowplow.

Frames are fully boxed, and have increased cross sections and use more high-strength steel for greater strength and durability, and improved ride and handling. Bending and beaming stiffness is increased 92 percent and 20 percent, respectively, with the fully boxed sections enhancing torsional stiffness by a factor of five. Larger engine and transmission mounts, coupled with a 125-percent-stiffer front frame structure, and hydraulic body mounts are incorporated under. Extended and Crew cabs provide greater vibration control. Engineers added access holes to the rear frame section to enable easier installation of fifth-wheel/gooseneck-style hitches. And box-tubes for ball hitches are stronger.

The new front suspension retains a short-long arm/torsion bar design, but now has forged steel upper control arms that are stronger and lighter than their predecessors. The new lower control arms are made of precision-machined cast iron to handle the greater loads. The old single-rated torsion bar is replaced with five different torsion bar rates to support five front gross axle weight ratings. Trim height is adjustable via a single bolt, allowing height to be changed to account for the weight of a snow plow or other accessories.

A stronger rear suspension uses asymmetrical leaf springs with unequal front and rear spring half lengths, which minimize axle hop and enhance traction control. 2500HD models feature a two-stage leafs while 3500HD models have three-stage leafs. All leafs are 3 inches wide, or 20 percent wider than before. Rear gross weight ratings are 6,200 pounds for 2500HDs, up by 116 pounds. On 3500HD models the ratings are 7,050 pounds for single-rear-wheels and 9,375 pounds for dual-rear-wheels, the latter representing a nearly 14-percent increase.

Longer wheelbases, ranging from 133.6 to 167.7 inches, work with stiffer frames, wider axle tracks, new jounce bumpers and shock mounts, and hydraulic cab mounts to make ride smoother and more controlled. Shocks are valved to support the higher weight ratings, and steering systems get bigger gears and pumps to handle higher front-axle ratings.

The enhanced 6.6-liter Duramax V-8 diesel is rated at 397 horsepower and 765 pounds-feet of torque. It features quieter operation and cleaner exhaust. The 2011 Duramax uses a high-pressure, 30,000-psi fuel system and electronic injectors, plus slective catalytic reduction with exhaust fluid, to help reduce emissions to 2010-legal levels. Changes to bearings, connecting rods, pistons, and oil lubrication circuits make the engine stronger and longer lasting. An EGR cooler bypass reduces high-mileage soot deposits in the cooler and EGR circuit. The engine is approved for up to B20 biodiesel.

A "smart" exhaust brake is standard with the Duramax. A driver-selectable feature uses the turbine control of the variable geometry turbocharger and the compression of the engine to generate backpressure, slowing the vehicle without applying the brakes. It is a smart system integrated with the cruise control feature and varies the braking to account for the grade and vehicle load.

The Duramax is standard with an enhanced 6-speed Allison 1000 automatic transmission that more efficiently channels power and torque to the axles for greater fuel economy. The Allison 1000 also features driver shift control with tap up/tap down shifting and a patented elevated idle mode cab warm-up feature. A tow/haul mode reduces shift cycling for better control and improved cooling when towing or hauling heavy loads. Transfer cases on 4x4s have larger adapters, are more securely mounted to the transmissions, and have stronger output shafts. As before, the transmission's 5th and 6th gears are overdrive ratios to reduce engine revs at crusing speeds.

Standard engine in Chevy and GMC 2500HD and 3500HD trucks is the Vortec 6-liter gasoline V-8 mated to a 6-speed Hydra-matic 6L90 automatic. The engine makes up to 360 horsepower and 380 pounds-feet in trucks rated at 10,000 pounds or less, and 322 horsepower with the same torque in trucks rated higher than 10,000 pounds.