2011 GMC Sierra Denali HD – Click above for high-res image gallery

Say it with us: 397 horsepower and 765 pound-feet of torque. That's what General Motors has been able to extract from its new B20-capable 6.6-liter Duramax V8 diesel engine in the GMC Sierra HD and Chevrolet Silverado HD, which is mated up to a stout Allison 1000 six-speed automatic transmission. For those who like to keep track of these things, which is to say every single person who's actually interested in purchasing one of these earth-moving behemoths, GM's latest Duramax beats out the 2011 Ford Super Duty in the all-important horsepower and torque wars.

That massively powerful powerplant is also more fuel efficient (by 11 percent, though GM isn't quoting actual numbers just yet) and cleaner than the unit it replaces. Each of these benchmarks has something to do with the exhaust aftertreatment systems employed on the Heavy Duty. GM tells us that the new 2011 truck cycles through its DPF filter cleaning process significantly less often than the unit it replaces, which saves a good amount of fuel.

Interestingly, GM representatives also tell us that they had a bit of a back-and-forth with the Feds regarding how best to handle the required refilling of the diesel exhaust fluid tank, which won't run dry until about 5,000 miles. It seems that when the truck gets dangerously low on the exhaust treatment, its speed will be capped at 55 miles per hour. If you run out, the computer nannies will keep you to just a four mph crawl so that you'll never be stranded completely. Want more? Check out our gallery of high-res images below and click here for the rest of the details.

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