Positraction Install

Positraction Install

Installing an Eaton Positraction unit in a Ford 8.8 rear end

Ford's 8.8-inch differential is a strong and popular rear end for retrofitting classic or older cars. It can readily handle the power and torque of a modified 302- or 351-cubic inch Ford small block V8. And Positraction, or Posi for short, is easy to obtain because it came stock in the later-model Mustangs and many other late-model Ford cars and trucks. Positraction is Eaton's trade name for the company's limited slip differential. It was first introduced in 1961 and gained notoriety during the musclecar era as a strong, reliable unit.

As most car enthusiasts already know, one of the best ways to get power to the wheels is through a Posi or limited-slip differential. Before we show how to install one, let's clarify the difference between a limited slip differential and a locking differential.

Torque Split

Basically, the difference between them is that a limited slip differential sends power to the non-drive wheel based on input torque, which is power coming to the axle from the engine. In contrast, a locking rear differential, such as a Detroit Locker, locks the drive and non-drive wheels together based on wheel slip at the drive wheel, regardless of engine power input.

So, with a limited slip unit, when high torque is applied to the differential and transferred to the axle, the spring-loaded clutch pack clamps the gears to the differential case. This clamping action is in proportion to the torque delivered, which means that the higher the torque, the higher the clamping load.

The idea is to prevent wheel slip at either wheel. If one wheel starts slipping before the other, the clamping force drops in proportion to the amount of torque delivered to the slipping wheel. So, when one wheel loses traction, the Eaton locker automatically kicks in, locking the rear wheels together and sending power to both. The whole point of the torque sensing limited slip differential is to reduce the likelihood of wheel slip.

Carbon Inside

Eaton Posi Performance Differentials feature a patented carbon friction material originally designed for racing brakes and clutches. Made from high-temperature carbon fiber wrapped with a carbon anti-wear coating, the discs are virtually indestructible, Eaton claims. And despite repeated hard use, there is no loss of performance. Eaton's warranty says that the patented carbon material provides smooth, quiet operation over the life of the vehicle.

To install a Posi, you'll first need to disassemble the stock rear end. Start by putting a drain pan under the differential and draining the oil. Remove the cover plate's bolts and carefully remove the cover plate. Then remove the disc brake calipers and parking brake drum. Be sure to mark the bearing caps with punches. It is very important that these caps go back in on the correct sides and that they are not upside-down.

After removing the pinion shaft retainer bolt, slide out the pinion shaft which holds the spider gears in place. Then remove the axle C-clips and pull the axles out about a foot. When pulling out the old differential, note the carrier bearing shims located on each side. These go between the bearings and the housing. Remove the bolts holding the ring gear to the differential, and with a punch carefully tap it loose. Next, tap the ABS ring off.

Using a bearing puller remove the old bearing and the race. This is a new 8.8 so we will reuse the bearing and the race. From there, you can install the Posi, as illustrated in the accompanying captions and photos.

We were very impressed with the new limited slip. After installing it in a '39 Ford pickup with a custom chassis, we took it out on both dry and wet pavement and the unit worked perfectly. It really made a big difference in the performance with both wheels hooking up when the power is applied.

Dan Sudal of Dan's Gears usually completes this type of installation in just over two hours, but with us "helping," asking questions, and taking notes, it took about twice the time. With a bit of experience, you can probably install a Posi quicker than that.


Dan's Gears, Dan Sudul, P.O. Box 1338, Sherwood, OR 97140, (503) 692-1547

National Drivetrain, Inc., 3631 S. Halsted St., Chicago, IL 60609, (773) 376-9044, www.nationaldrivetrain.com

Positraction Install
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