How fast is it? When this truck first hit the streets earlier this year, it was still equipped with the original 10-bolt half-ton rear differential. By the time this story was written, a beefier custom rear diff with a locker and disc brakes was in the pipeline. The 10-bolt is only marginally strong enough at stock power levels and would be inadequate once the engine is broken in and horsepower is doubled. However, who could resist doing some preliminary testing? When launching with only a mild powerbrake (to protect the stock rear diff), this truck will peel off 0-to-60 times in seven seconds flat and quarter-mile times in the low 15s at nearly 100 mph, when using factory programming for 300 horsepower and 520 pound-feet of torque. (The only performance mods completed so far include installation of a Magnum Force air-intake system from U.S. Diesel Parts and a Kennedy Diesel four-inch custom exhaust.) To put these performance numbers in perspective, we tested a stock 2001 Duramax-equipped GMC crew cab shortbox 4WD 2500HD weighing 6550 pounds. It produced two-second-slower 0-to-60 and quarter-mile times.
The process of converting the project truck--with its original 5.7-liter gas engine and TH700R4 automatic--to Duramax/Allison power began in spring 2002. Determining the fit of the engine and transmission and fabricating custom motor mounts happened rather quickly. However, the project stalled once we began to ponder the electrical/electronic aspect of the conversion. It wasn't until fall 2003 that a decision was made to strip the truck of the original dash and electrical system and install all-new components to match the new engine and trans. Except for the tail- and headlight harnesses, essentially all of the truck's original electrical system was ripped out and replaced with new factory wiring harnesses, junction blocks, and fuse panels. To complement the new electrical system, a 2001 model-year instrument-panel cluster and dash assembly were installed to update the appearance of the interior and to correctly interface with the Duramax and Allison.
The Allison is much larger than the original TH700R4, but it fits the C/K chassis surprisingly well, requiring only minor adjustments (no cutting) to the transmission tunnel. As a point of reference, the Chevy/GMC TH400 and 4L80-E bell housing and flexplate bolt patterns are the same as those used by the Allison.
Work started in earnest in December 2003 and continued nonstop through May. When added to the time required to fit the engine/trans to the 1989 chassis, the powertrain conversion took approximately six months.
What's next? Once the new rear differential and traction bars are installed, the real performance testing can begin. Plans are to take the engine to at least 600 horsepower, using a combination of currently available aftermarket power products. Diesel-pickup quarter-mile times have steadily fallen as diesel-engine technology improves and the aftermarket makes more strides in horsepower and torque gains. We think this Duramax is up to the challenge, and a full report will appear in a future issue.