Positraction Install

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.