Text by Chirag Asaravala. Photos and Installation by Chris Monahan.

Whether it is on-road or off-road performance the age old question is how to get maximum straight-line traction without sacrificing the ability to turn the vehicle. When a car goes through a turn the outside wheel must turn at a faster rate than the inside wheel because it will travel a greater distance through the turn. The job then of the "differential" is to allow the wheels to rotate at different speeds, otherwise the inside tire would be pulled along creating excessive tire wear and drivetrain stress.

Most automobiles come stock with an "open" differential. An open

Open differential.

differential splits torque evenly between the two wheels and the amount of torque is limited by the wheel which has the least traction. In other words, if one wheel loses traction and starts to spin it is getting little or no torque and thus the other wheel will also get no torque. This is what causes the common "one wheel peel", where one wheel is spinning and the other is standing still.

One solution used by auto manufacturers since the

Limited Slip Diff. clutch pack.

early 1960's is a limited slip or positraction differential. This device utilizes a friction based clutch mechanism and stiff springs to limit the amount of slip or spin any one wheel can incur. These units are popular because they are quiet and offer adequate traction for low-torque or general street use vehicles. However, as with a transmission clutch, the friction material wears over time and loses its effectiveness. Eventually even a limited slip or posi differential will also result in the one wheel peel effect, or inability to differentiate maximum torque.

Drag racers and off-roaders not concerned with turning ability have always had the option of using a "spool". A spool or mini-spool is

Ford 8"/9" mini- spool.

a simple device which replaces the open or traction lock differential and locks both axles together permanently. (A full spool replaces the differential case while a mini fits within the case.) As a result both wheels always turn at the same rate. This can also be achieved by welding the spider and side gears together in an open differential. While a spool is perfect for a dedicated drag racing vehicle, for the majority of enthusiasts with dual purpose vehicles, this option is just not practical.

In principle the ideal solution would be a device which offered a true mechanical axle lock-up, like a spool, yet functioned

Detroit Locker™

like an open differential while turning. Such a solution has actually been in existence for many decades now. Originally developed for big-torque military and construction vehicles, the Detroit Locker™ by Tractech was adapted and marketed to performance enthusiasts. The Locker is simply a set of toothed gears which engage to lock both axles together when both wheels are moving forward. When the vehicle enters a turn the faster moving outside wheel results in disengagement of the locking gear mechanism, allowing for differentiation. However, the industrial design was a bit rough for street use. The gear lash and constant engagement and disengagement resulted in clanking and banging of the driveline and erratic turning. Furthermore, the installation required replacement of the stock differential case. This made the cost of the unit quite expensive.

Eventually Powertrax® (now a brand owned by Richmond) set out to improve upon the long standing Locker design. Their first product, the Powertrax Lockright, was a major improvement from an installation and cost standpoint in that it utilized the stock case and many of the stock components. However, many of the original shortcomings with the ratcheting annoyance were still apparent. We installed the Powertrax Lockright many years ago in our Project '67 vehicle. While the unit is still standing up to 6000 rpm launches on slicks, it definitely is clunky through turns (see our article "Upgrading the Ford 8-inch Differential Powertrax Lockright Installation" in the Rear-End section of the Tech Department.)

Some companies tried to address the locking differential dilemma by going from automated lockup to on-demand lockup via remote electronic switch or even pneumatic activation. While nice in concept, in practicality installing air compressors and tubing or wiring into a differential case just increased cost and complexity. Richmond continued to plug away at engineering a better automatic locking differential. Their design criteria was to make it even more rugged and durable, while being smoother and quieter. Recently, they announced the new Powertrax No-Slip Traction System. While outwardly similar to the Lockright, the new No-Slip features a synchronizing mechanism which prevents the ratcheting and and constant engagement-disengagement of the teeth during turning. The end result is an automatic locking differential which is suitable for street driven cars.

We installed the Powertrax No-Slip into a 1966 Mustang 8" differential. The results were remarkable to say the least. The car previously was equipped with a mini-spool. While straight ahead traction was ideal with the spool turning was a major chore. We anticipated some ratcheting and bucking through turns with the new No-Slip locking differential, however we were astonished when the unit simply clicked twice as we entered the turn, then again when we powered straight ahead. If we didn't know better we'd have thought we installed it incorrectly! Follow along with our installation process.