It wasn't long ago that Ford and Dodge diesel truck owners laughed at the 6.2- and 6.5L diesel guys. Historically, GM owners have always been competitive in performance contests despite the best efforts of the other manufacturers. But, no matter how brand-loyal you are, you have to acknowledge that GM's diesel offering was outclassed in 1989, when Dodge began putting the Cummins 5.9L in its 3/4- and 1-ton trucks. And all grounds for bench-racing with Ford guys went down the tubes, when Dearborn released International's new 7.3L Power Stroke in the marketplace in 1994.
It was a dark seven years for GM diesel owners, but all that changed in 2001 when the Duramax 6600 (6.6L V-8) was introduced. This new engine was born out of a partnership between GM Powertrain and Isuzu Motors. Since its debut, more than 650,000 6.6L Duramax diesels have been shoehorned under the hoods of various 3/4- and 1-ton GM trucks. Originally rated at 300 hp and 520 lb-ft of torque, the LB7 regular production option (RPO) code Duramax was a huge leap forward from the 6.2/6.5L it replaced. GM finally had a diesel engine that could compete with the best that Cummins (Dodge) and International (Ford) diesel trucks had to offer, and the turbodiesel torque wars began.
By 2003, Dodge and Ford had retaliated and developed engines that surpassed even the Duramax's power numbers. We all reaped the rewards as the Big Three slugged away at each other like we haven't seen since the musclecar wars of the late-'60s. In 2004, GM countered with its variable vane turbo LLY RPO-code engine that was rated at 310 hp and 565 lb-ft of torque, only to be quickly outdone by Dodge's 600-lb-ft Cummins and Ford's 325hp 6.0L Power Stroke.
A New BeginningGeneral Motors could not afford to be the low-powered option in the diesel segment yet again. To take on Dodge's 610-lb-ft Cummins and Ford's 325hp Power Stroke, GM Powertrain has reworked the 6.6L Duramax for the 2006 model-year. Power output was increased to 360 hp at 3,200 rpm and 650 lb-ft of torque at 1,600 rpm with this new LBZ option code engine that's quickly replacing the LLY engine previously offered in the Chevrolet Silverado HD and GMC Sierra HDs. Why is General Motors' new LBZ-code Duramax so good? 'Cause GM owed it to us!
Secrets Reveled: The Foundation of the LBZThe power increase comes from a new engine calibration that takes advantage of additional fuel-injection pressure that's now quoted to be "more than 26,000 psi." Many aftermarket companies pump up the Duramax using this same technique, but GM engineers have taken this common modification a step further and matched the higher fuel pressures with a set of seven-hole injectors that can fire as much as five times per cycle. Controlling these injectors is a new 32-bit E35 engine control module (ECM) that eliminates the need for the old valve-cover-mounted drive unit that used to fire the injectors. Combined, this new injection system is credited with improving engine power, fuel economy, and emissions. Though no miles per gallon numbers are officially quoted, GM claimed at a press conference last year that the LBZ-code Duramax (along with the new six-speed Allison transmission) will net 3 to 5 percent better fuel economy than Ford's class-leading 6.0L Power Stroke-equipped trucks.
New Six-Speed AllisonIt's no secret that since day one the Duramax engine has been optimized for the Allison 1000 transmission. For the '06 model-year, GM trucks that were previously equipped with the Allison 1000 five-speed (2500HDs, 3500s, and Hummer H1 Alphas) will get a new six-speed version of the transmission. It's basically the same Allison 1000 as before but with an extra tube in valvebody that now activates a different combination of clutches to give the transmission an additional 0.61:1 Overdrive Sixth gear ratio. Basically, the extra gear means the engine speed will drop 250 rpm at 60 mph and around 300 rpm at 75 mph.
In addition to the new Sixth gear, the Allison 1000 transmission has been updated with a stronger 310-mm torque converter, a new solenoid pack (controlled by an A40, 32-bit transmission control module that has double the RAM and ROM of the old controller), and additional C2 clutches were added to increase the transmission's torque capacity from 605 to 650 lb-ft of torque. A new 22-tooth pump is being used along with high helix angle P1 and P2 planetaries (changed from 11 to 17 degrees) to quiet the transmission. The later modification required an additional thrust bearing capacity to be added to maintain the strength of earlier "noisy" Allison 1000s.