Double the traction, double...
Double the traction, double the fun. A Powertrax unit had us burning twice the rubber.
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.
The Powertrax is a component...
The Powertrax is a component kit, which replaces the gears in a stock differential case with a locking differential. An inner coupler replaces the side gear and splines onto the axleshaft. The coupler's straight-cut drive teeth on its face side match the teeth on the driver, which is pinned to the case with a heavy shaft. An active spacer between the two works through a syncro system to allow the differential to lock and unlock.
The Powertrax unit is a no-slip retrofit differential designed to fit within the confines of the stock differential case. Make no mistake about it-the Powertrax is a locking differential which results in uncompromised torque splitting when locked, and then automatically unlocks when going around a corner in a normal manner. Simply put, it's either locked or unlocked, and therein lies the caveat. There is a mechanical shift from locked to unlocked and back again. Around a corner, the differential will unlock to disengage the outside wheel under mild acceleration or disengage the inside wheel when coasting or braking. The rear and chassis will react to the change. With the Powertrax unit, extra care is required under certain driving conditions, particularly under hard braking or acceleration in corners and any time the vehicle is driven on wet or slippery roads. Like any locker or limited slip, hammering the throttle in a corner will lock the differential, and the rear will want to come around.
Since it fits in the stock "open" differential case, installing the Powertrax requires only stripping the components out of the differential and installing the Powertrax parts in their place. Because the original case is used, the Powertrax can be installed without fully disassembling the rearend and resetting the gears. It seemed like the ideal upgrade for our street Dart. The installation proved to be as simple and quick as we expected, requiring no special tools, and we were back on the road in a couple of hours. Though we could hear it and feel it while driving, the added straight-line performance made it a worthwhile upgrade.
The Powertrax unit is simple...
The Powertrax unit is simple enough to install at home with basic tools. We set the Dart on jackstands under the framerails to let the rear axle hang. The wheels and tires come off to get to the axleshafts.
Our rear is an 8 1/4-inch...
Our rear is an 8 1/4-inch which like the 9 1/4-inch has a rear cover plate for access to the differential. Getting in to install the Powertrax is just a matter of pulling the cover. For 8 3/4-inch rears, pull the center carrier from the housing.
To take apart the stock open...
To take apart the stock open differential, you need only one wrench to back out the differential pinion shaft lock screw.
Once loosened, the stock pinion...
Once loosened, the stock pinion shaft should slide out easily.
Turn the differential gears,...
Turn the differential gears, and the spider gears rotate out of engagement. Then remove the spider gears and their thrust washer.
The 8 1/4-inch, like the 9...
The 8 1/4-inch, like the 9 1/4-inch, uses C-clips to retain the axleshafts. To remove them, just push in the shafts slightly so they clear the recess in the gears, and slide the clips off. The axleshafts can now be pulled out of engagement with the differential side gears. However, to remove the old gears and install the Powertrax pieces, only the right shaft needs to be pulled out a few inches for clearance.
The side gears and their thrust...
The side gears and their thrust washers are the last things out of the stock differential. With the differential now stripped, the Powertrax can go in its place.
For the syncros to line up,...
For the syncros to line up, the Powertrax components fit together in just one orientation. We marked the couplers and driver halves with a marker in the correct orientation so we could visually confirm that the pieces are correctly installed. The couplers slide in place of the side gears and use no thrust washers in this application. The C-clip on the left-side axle is now installed. The driver half is fitted with springs and the spacer, and is then slipped in to engage the coupler. If the all parts are oriented as specified in the instructions, the teeth of the driver and coupler will seat together.
Next, the four spring assemblies...
Next, the four spring assemblies that fit between the halves of the Powertrax are put into position.
To make sure the clearance...
To make sure the clearance between the halves of the differential is within specs, the Powertrax comes with a simple gauge. This will confirm the assembly is correct; however, the unit is not adjustable.
With everything checking out...
With everything checking out OK, the right axleshaft is pushed back in, and the C-clip is installed through the window in the Powertrax. Pull the axle back outward to make sure the C-clip is properly seated.
The last step is to reinstall...
The last step is to reinstall the pinion shaft, which has to wiggle in to get past the four springs in the pin bore. Use the lock bolt as a handle to work the shaft in, and then lock it down.
Button it back up with the...
Button it back up with the cover, fill it with lube, and it's a done deal. We prefer quality silicone to a gasket here. The Powertrax requires no special lube or additive. Get the tires back on it, and it's huntin' season on Main Street.