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Custom Performance Modifications - 8.1 Liter Automatic
An avid duck hunter and performance enthusiast found his interests crossing paths one day. His ‘02 GMC, loaded with every option available and a considerable amount of truck accessories, was weighing in as a super-heavyweight vehicle. With a trailer load of four-wheelers and a truck full of hunters, the anemic 496, or 8.1 liter, was struggling and causing the Allison automatic to downshift out of overdrive.
Allison automatics used in these trucks have a 50% overdrive. When they come out of overdrive the RPMS of the engine spike up X2 immediately. Few drivers want to hear or feel that transition repeated. It’s annoying. The 33-inch tires installed on this truck during customization aggravated the situation by watering down the effective gear ratio of the 4.10 differential.
Our job was to make this heavyweight 4X4 light on its feet again... much lighter, indeed, than it had ever been. The entire drive train would need to be massaged at the same time.
A replacement crate motor was purchased to quickly double the horsepower. A larger stroke of 4.5 inches, H beam connecting rods, forged pistons, a custom ground roller cam, custom aluminum heads, and a fabricated intake manifold were delivered assembled, al la crate.
The stock Allison automatic overdrive unit is stout, but a few weak components keep it from handling much more than stock horsepower. Therefore, a crate Allison automatic was prepared with horsepower tolerances, for reliability, in excess of our application. A billet, anti balloon torque converter shell, numerous internal modifications and custom parts, and a transmission-to-transfer case torque strap were all part of our package.
The front and rear differentials would be set up with 4.88 gears, the lowest ratio available to date, to account for the oversize tires. On the exhaust side, custom stainless steel headers; three chamber, oval flow masters; and custom, stainless and chrome, machine gun exhaust tips flow and sound marvelous. On the intake side, a K&N cold air induction kit feeds our heavy breathing heavyweight.
With sports car like acceleration achieved and surplus power to be wielded over any grunt work that may arise, we added a few more accessories. Too much is just right!
