AutoTech’s Quaife Differential

At Last, a Torque-Sensing Limited-Slip Differential for the VW GTI/Scirocco/Jetta

By Greg Raven
VW&Porsche magazine

Back in the days of 70 hp VWs, it wasn’t necessary to worry too much about getting the power to the ground. Nowadays, however, it seems fairly commonplace to come across a VW with 140, 150, or 160 hp, and in a front-wheel drive car this much horsepower can prove to be a real handful, especially in a corner.

What happens is that weight transfer works against good traction. Under acceleration, the weight transfer is to the rear (off the front wheels) and in a corner the weight transfer loads one side greater than the other. This means that one of the front wheels has very little weight on it, making it easier for that wheel to spin when torque is applied.

Previous solutions included cam-and-pawl type locking differentials and friction plate limited slips (we have yet to see a viscous limited-slip, although they must be undergoing testing somewhere). The cam-and-pawl lockers were great for building up pectorals and biceps, and because they work in an on/off manner (they are either locked or spinning) they are very tough to drive. The friction-type limited slips were only marginally easier to muscle around, but because of their construction they tend to wear out in use, losing effectiveness.

A couple years ago, Gleason introduced a new type of limited slip differential that looked for all the world to be the brainchild of someone who had bought several million dollars’ worth of surplus gears at auction because someone dared him to. But as complex as the Gleason design is, it seems to work. The whole idea is that the gears allow the differential to sense differences in torque and apply wasted torque from a spinning wheel to the wheel with traction. Thus the name, Torsen (Torque Sensing).

The Gleason has two drawbacks, however. First, it is a fairly elaborate piece of equipment that requires special knowledge to assemble. Second, it apparently can not be made small enough to fit in a VW transaxle. On our last visit to Autotech SportTuning, we were shown a torque sensing differential that is simplicity itself, one that WILL fit the VW.

The Autotech differential uses gears, but they are helical cut instead of the worm gears that predominate in the Gleason. The use of helical gears is reportedly one of the reasons why it is simpler to assemble the Autotech differential.

Another reason why it is simpler to assemble is that there are relatively few parts, many of which only fit one way. With a simpler design and fewer parts, it becomes almost trivial to understand how the differential works while examining its innards.

Each stub axle connects to its own large helical gear. There is no direct connection between these two large helical gears. Each of the two large helical gears drives five smaller diameter helical gears (for a total of ten). The smaller gears from one side of the differential mesh with their counterparts from the other side of the differential, providing a pathway for transmitting torque between the two large helical gears.

When one axle tries to spin faster than the other (as it might if it lost traction under power), the large helical gear attached to that axle will attempt to turn its five smaller gears. The smaller gears are indirectly connected to the other large gear, however. And because the smaller gear has the mechanical advantage (it is easier for the smaller gear to turn the larger gear than it is for the large gear to turn the small gear) torque applied to the spinning side is transferred over to the side with traction in proportion to the amount required to even out their relative motion.

In practice, the Autotech differential does increase the steering effort slightly in low-speed maneuvers. This stiffness is the result of the differential splitting the torque between the two wheels, NOT because of any loss of differential action. The Autotech differential provides both a limited-slip and a differential action at the same time. And unlike cam-and-pawl, viscous, friction plate, and other limited slips, the Autotech limited slip does not snatch (slip a little and then catch). This not only makes it more pleasant to drive, it is also easier on tires and other drive train components. In fact, a Formula 1 car equipped with a limited slip of the same design was able to run an entire race on the same tires, an impressive testimony to the smoothness of this differential.

The design is the result of five years of development, with test vehicles provided by various police agencies and ambulance firms. Under these conditions, the differentials have gone over 100,000 miles without wearing out. One further plus is that this differential can be used with ABS, which is not true for cam-and-pawl or Gleason differentials.

As of the end of February, the price of the Autotech differential is projected to be between $650 and $700, not counting the labor involved to install it. This would mean that the Autotech differential would appeal mostly to those in the high end of the performance spectrum. However, if you are running a VW with over 150 hp, this might prove to be the icing that lets you enjoy your cake. Autotech can be contacted at 1800V North Glassell, Orange, California, 92665, telephone (714) 974-4600.

Visit AutoTech SportTuning on the web.