The only thing more frustrating than not having the power to spin the tires is smoking the right rear when you’d rather be accelerating. Single-tire spin occurs with either an open differential or a dead posi-traction unit. Since two legs are better than one in any kind of butt-kicking contest, we’ll take a look at a couple of ways to improve the effectiveness of both the Eaton plate-style and the Auburn cone-design limited-slip units.
The purpose of a posi-traction unit is to tie the two drive axles together for more efficient traction under acceleration. Originally designed to improve traction in mud, snow and icy conditions, such traction improvers were soon adopted to limit tire spin. Under hard acceleration in a rear-wheel-drive car, torque tries to rotate the rearend housing, lifting the right rear tire while planting the left one. While a posi-traction device does nothing to counteract that motion, it does connect the two drive tires with a friction device to prevent the right rear tire from spinning. The effectiveness of the unit can be measured by the amount of torque it takes to break the right rear tire loose from the left rear.
At this point, you might think that the simplest way to permanently tie the two wheels together would be with a solid link between the two axles. Drag racers often employ just such a device, known as a spool. Low-buck hot rodders sometimes weld the spider and side gears together to create an inexpensive spool. Unfortunately, both of those alternatives are dangerous for street use. The reason for the differential in any vehicle is to accommodate the different distances the inside and outside tires must travel in a turn (the outside tire travels a significantly greater distance than the inside tire). If the outside tire is not allowed to travel the greater distance, the axles twist and the outside tire tends to bounce and chirp. That eventually leads to broken axles. When the axle breaks in a GM 10- or 12-bolt rearend, the tire, wheel and axle combination exits the car, tearing up the rear sheetmetal in the process.
A limited-slip unit allows some slippage between the axles in a turn, which means that the device will eventually wear to the point where it cannot prevent torque from spinning the right rear tire under straight-line acceleration or the inside tire when accelerating out of a turn. That’s when you know it’s time to rebuild the posi.
We won’t cover removing the posi from the rearend in this story. If you need more information on that, you can consult a shop manual. Once the differential is removed from the housing, it can be easily disassembled and the clutch plates inspected for wear. To help us detail this rebuild, we enlisted the help of Steve Chrisman at Jack Chrisman Enterprises. In the case of the Eaton-style unit, a Posi-traction rebuild kit with new clutch plates is available through any Chevrolet dealer. If the friction facing on the clutches is worn, you might consider replacing the clutches. The rebuild kit is not expensive.
Usually, the clutch plates are not significantly worn even though the posi slips easily. That can be rectified by adding thicker shims to preload the posi with a higher breakaway torque. The shims are placed either between the clutch pack and the case or between the clutch pack and the large side gear. Chevrolet sells shims ranging in thickness from .007 to .030 inch that will increase the clutch-pack stack thickness and increase the torque required to make the unit slip. That is the most common way to improve the preload of a used posi-traction unit. Too thick of a washer stack will prevent you from assembling the spider gears in the posi unit. Start by using a .010-inch-thicker shim for each clutch pack (for a total of .020 inch). In our unit, that was all we could squeeze into the clutch packs and still assemble the unit. This procedure can be used to increase the preload on an Auburn cone-style posi as well.
Most posi units also employ a spring pack that puts additional load on the clutch packs. Two steel plates locate four springs that can be replaced. Chevrolet offers two different springs for the 12-bolt Eaton posi. The stock replacement springs are rated at 50 pounds per inch, while the high-performance springs are rated at 75 pounds per inch. The heavier springs can be tried, but they also may create a popping sound as they preload and then release the clutch plates, especially in tight, slow turns. Auburn also offers higher rate competition-only posi springs, but the company doesn’t recommend them for the street.
One way to measure the effectiveness of the changes you make to a posi is with a homemade torque adapter. The one we used was built by SCCA autocross racer Dan Livezey. The adapter is simply a flat steel plate drilled with the appropriate wheel bolt pattern and a 1-inch nut welded in the center. To use it we placed the car on jackstands and removed one wheel. We used the right rear since it more closely approximates a tire-spin situation. We blocked the left rear tire and slipped the adapter onto the right rear axle. With the parking brake released, we used a torque wrench to measure the amount of torque required to turn the axle. Our autocross racer sources tell us that a breakaway torque of between 110 and 140 pounds of torque will work well on the street and prevent inboard tire spin even in a tight, slow turn when the power is applied. It is possible to create more breakaway torque in a posi, but excessive torque only creates a bind in the posi that could decrease component life.
Improving your limited-slip probably doesn’t rate style points compared to bolting on a supercharger, but when it comes to putting the power to the ground, two legs are always better than one.
