If you told someone 10 years ago that General Motors would be selling a diesel truck that makes 517 lb-ft of torque at the rear wheels, he'd think you were crazy. The new for '07 Duramax LMM is the most powerful engine to ever be sold in a civilian truck, and it debuts in the newest generation of the General Motors 3/4- and 1-ton trucks (internally code-named GMT900).
What's a GMT900?
It's the name used by the General to distinguish the new body style and revised frame from the original Duramax introduced in 2001 (in the trucks called the GMT800). Although the 1/2-ton trucks gained a coilover front suspension, the new '07 heavy-duty trucks use a strengthened version of the old frame with the trusty torsion-bar-supported independent front suspension and retuned leaf springs over a 14-bolt rear axle. The real reason behind the GMT name change has to do with what rides on top of the frame. It's a brand-new body and interior that reflects the upscale evolution of fullsize Chevy and GMC trucks. Puffy leather seats have been replaced with firm buckets, squeaks have been virtually eliminated, and the dash no longer looks like it was thrown together from a parts catalog.
Duramax LMM Clean Diesel
At $7,195, the Duramax appears to be the most expensive of the diesel engines being sold by the Big Three automakers. When combined with the required Allison six-speed auto/manual transmission, the diesel option price tag increases to $8,395-just $10 more than the Ford diesel/auto package. Dang, those corporate bean counters are good! Not only did they find a price that's just low enough you couldn't justify buying a gasser, but it's within a few bucks of similar equipment being sold by Ford.
What do you get for $8,395? The most powerful clean-diesel engine being sold to the public: the Duramax LMM rated at 365 hp and 660 lb-ft of torque (the Ford is rated at 350 hp and 650 lb-ft, and the Dodge is 325 hp and 650 lb-ft). It's similar to the outgoing LBZ engine but includes a new computer along with lots of new sensors and emissions features-such as an electronic air-intake throttle, a large exhaust-gas-recirculation (EGR) cooler, a big diesel-particulate filter and oxidation catalyst, and a special venturi exhaust tip to suck in cooler air during soot burns. The new emissions-friendlier setup is good for 90 percent fewer particle emissions than last year along with a 50 percent reduction in the output of nitrogen oxides.
 You can spot an LMM engine...  You can spot an LMM engine by looking for the air/EGR throttle equipment in the intake setup (arrow). |  The new bazooka tailpipe extension...  The new bazooka tailpipe extension gives the LMM a unique sound and draws cool air into the exhaust flow. It also does a good job of catching and cooking mud. |  |
Even with the strangling required to meet the EPA standards, engineers at GM were able to surpass the power ratings of the previous Duramax LBZ. Dyno testing at Spectre Performance showed the new LMM manages to put out 285 hp and 517 lb-ft of torque at the rear wheels. The power is very easy to use thanks to the sophisticated programming that controls the fueling and the variable turbo that does a decent job of keeping boost on tap.
Allison 1000 Six-Speed Auto/Manual
Power from the LMM is transferred into the trusty Allison 1000 automatic transmission that now comes standard with six gears and a manual mode. When the truck is shifted into M (which resides between D and L), the driver is given total control of the upshifts. The truck will not go into a higher gear until the driver clicks the "+" on the shift knob. If the engine spins beyond approximately 3,450 rpm, fueling is immediately cut until the engine returns to normal operating speeds or the transmission is upshifted.
The Allison can also be manually downshifted, but in certain driving conditions, the truck won't be allowed to downshift to prevent shuddering or other problems caused by lugging the engine under load. The Allison's Low setting allows drivers to pull the truck's speed down by using transmission braking. At any speed, when the shifter is yanked into L, the Allison will find the lowest gear available that will not over-rev the engine, then does this in every lower gear until First is reached. This is not as effective as using an exhaust brake, but it gives GM owners more control over their transmissions. When the truck is put into Second gear using the manual mode, transmission/transfer case input/output speeds are used to activate torque management, which reduces power to keep the wheels from losing traction
 A test run in soft dirt revealed...  A test run in soft dirt revealed the new GMT900 still suffers from severe rear axle hop when the rear tires lose traction. |  |  Otherwise, it performed well...  Otherwise, it performed well with plenty of torque available at any speed and the predictable action of the Gov-Lock rear differential. |
Performance
Programming a modern common-rail diesel engine can't be easy, but the engineers at General Motors make it seem that way. While other drive-by-wire electronic systems can make you feel disconnected from the engine, the Duramax is very responsive to input-no matter how dumb you're driving. Mash the throttle, slam the brakes, and give half throttle for a split second before squashing the pedal again, and the LMM won't break a sweat. Good luck trying to upset the truck by confusing the engine and transmission computers.
This comprehensive programming allows the Chevy to take full advantage of the horsepower and torque made by the LMM V-8. There's never a noticeable lag between pedal input and fueling. Full power is not available right off the line, but it steadily builds during wide-open-throttle runs. Fueling ramps-up smoothly and peaks about the same time the turbo sounds like it's building full boost (sorry, there's no factory gauge). Light and quick throttle movements by the driver aren't ignored, although fueling will be limited if there's not enough boost available to keep emissions clean. In most scenarios, the torque management is not very intrusive and allows drivers to forget there is a 32- or 64-bit middleman approving all performance requests before the Duramax and Allison are allowed to play.
Interior
The industrial-grade plastics and sloppily stitched seats found in the GMT800 trucks are gone and were replaced with Escalade-quality panels and controls. GM's luxo-truck testbed has passed along some of its interior DNA to the new GMT900 trucks. Chrome trim, brushed aluminum surrounds, and shiny fake wood provided an upscale feel to our LTZ-equipped pickup. Our tester came with the Luxury-Inspired dash designed for the 1/2-ton SUVs that's integrated into a very large center console. You can also order the HD trucks with the Pure-Pickup dash that eliminates much of the center stack and provides foot space for a middle passenger.
Whichever dash layout you order, you'll get acres of dimpled plastic panels covering the doors, dash, kick panels, and anything else that's not a control button. The material is much better than what GM has used in previous-generation trucks, but it still doesn't have the quality feel of European and Japanese truck interiors. Even on washboard roads, the GMT900 cabin feels solid and isolated from the outside world thanks to tight gap tolerances throughout the interior.