8.25 8 1/4 Rebuild and modification

Installing A Powertrax Differential Unit - Mopar Muscle Magazine

Installing A Powertrax Differential Unit – Making Trax
Written by Steve Dulcich on March 1, 2002

Getting Your 8 1/4 Rear Hooked Up With The Power Trax No-Slip
Got horsepower? If you are going to do more than go up in smoke, you’re gonna need some traction to back it up. The conventional open differentials used in most passenger cars utilize side gears that are splined to the axleshafts and in constant mesh with the differential pinion (spider) gears. These spider gears turn freely on the pinion shafts, and act as idler gears when the rear wheels are turning at different speeds, such as around a corner. This cheap, simple, and effective way to provide differential action in a drive axle has been around for a century. For high performance applications there’s a drawback we’re all too familiar with-lay on the power, and the conventional differential sends the power to the wheel with the least amount of traction, producing a smoky one-legged burn-out.

Chrysler, like the other manufacturers in the musclecar years, was hip to this requirement, offering their Sure Grip limited-slip differential as an option. With a Sure Grip, the differential was equipped with a clutch mechanism to create locking action to the rear axles which increases as drive torque is applied to the differential. Unfortunately, the vast majority of passenger cars received conventional open differentials. Since the differential case serves as the carrier for the ring gear, installing an OE-style limited slip requires full disassembly of the rear, and the accompanying “setting-up” of the gear clearances and bearings.


Our ’69 Dart, originally a 273 car with a 7 1/4-inch rear, had been updated with a later A-Body 8 1/4-inch rearend. With its 5 on 4 1/2-inch wheel bolt pattern, availability, and greater torque handling capacity, it was a good swap. Unfortunately, this rear, like most, was a peg leg. With a built 318 up front, and a four-speed in the tunnel, dropping the hammer produced more tire smoke than acceleration. Considering the remedy, we weighed the options. The rear could be torn down and a new OE-style limited-slip case installed, but the down time and setup hassle made us put it off. Then we heard about the Powertrax.