GM Duramax Heavy Duty Trucks
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General Motors isn’t waiting until its next-generation fullsize pickup comes out to offer a significantly improved Duramax 6600 diesel engine. Redesigned and re-engineered Chevy and GMC fullsize pickups aren’t expected to hit the dealerships for at least another year as 2007 models. But GM is giving the current generation truck a majestic going-away party as the heavy-duty models now get the most powerful diesel engine available to pickup consumers.
The new 6.6-liter Duramax V8 is rated at 360 horsepower at 3200 rpm with an impressive peak torque of 650 lb-ft at 1600 rpm. That beats the Ford Power Stroke, which is rated at 325 horsepower and 570 lb-ft, and the Dodge Cummins, which is rated at 325 horsepower and 610 lb-ft. For now, Chevy and GMC own the bragging rights as king of the torque mountain.
But wait, there’s more.
Backing up the increased power is a new Allison 6-speed automatic transmission. With an additional overdrive gear, engine speed is reduced by 250 rpm at 60 mph to help with fuel economy and range. This new tranny also offers such innovative features as tap shifting for easier manual control of the gears and a low-traction mode to help limit tire slip on wet or icy road surfaces.
Making this power combo even more impressive is a long list of refinements that have smoothed out and quieted down vehicle operation. But those perks may get lost in the rush of acceleration and pulling power now available in Silverado and Sierra 2500/3500 pickups.
I spent an afternoon driving a variety of heavy-duty models, some with trailers, up hills and over a closed road course at the GM Proving Grounds near Milford, Michigan. My first impressions are without criticism at this point. Driving an empty 2500 Extended Cab is like piloting a sport truck. Stab the throttle at 60 mph and in a short distance you’re pegging the speed control at 96 mph (limited not by engine power but by the engine-management computer saying that’s too fast for the tires). Towing 10,000 pounds, I had no problem reaching Interstate speed or accelerating from 40 to 70 mph in a passing exercise. I also towed up a 7.2-percent grade with ease. GM officials set up acceleration contests with comparable models from Dodge and Ford. I’ve been to at least 10 vehicle introductions where similar drag races are held, and host manufacturer has never lost.
The GM trucks were easy winners on flat roads and up the hill, but that’s not the main point that needs to be stressed. Maximum GCWR of the Big 3 heavy-duty trucks are similar so that most towing consumers are going to get enough truck to do the job within those ratings. (Ford, 26,000 lbs.; Dodge, 23,000; GM, 23,500 lbs. But remember, ratings vary between vehicle configurations and maximum tow capacities change due to axle ratios, vehicle trim levels, type of hitch and transmission choice.) The bigger selling points that need to be considered are vehicle comfort, drivability, reliability, utility and the capability of outfitting your truck to suit your needs.
The Dodge Ram 2500 Quad Cab with the Cummins diesel engine can accelerate briskly without a load. But the overall experience appeared more pleasant in the GM truck during this comparison because of the engine and transmission refinement. Gear shifts are more precise and harmonious with the engine speed and torque. I had no way of actually measuring noise levels, but the Silverado felt quieter, especially over the Super Duty. And itâ€™s not just dB levels. There are quality levels to noise. Some frequencies are just more irritating than others. Dodge has made significant improvements in its 2006 cab to isolate road noise, and the F-150 is among the quietest trucks on the road. It's going to come down to the details and personal preferences as pickups become more SUV-like in their ride and comfort.
The new Duramax achieved its higher power ratings mainly through an upgrade in the fuel system and a new engine control unit (ECU). The Bosch common-rail fuel system now offers 26,000 psi, up from the previous maximum of 24,000 psi. New 7-hole injectors allow up to five separate pulses or events per combustion cycle, up from three events in the previous Duramax.
“This gives us the ability to tailor the combustion pressure characteristics to reduce the noise and emissions in the cylinder,” explains Charles Freese, executive director of diesel engineering at GM Powertrain.
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