When it comes to a rearend, one specific part plays the greatest role in how the rear functions and handles power: the differential. This device is designed to deliver power to the driven wheels while allowing them to rotate at different speeds. A good differential can alternate between roles to fulfill several jobs. When efficiency or turn-in is needed, it can disengage, but when it comes time to administer power to the road, it can engage. Clearly, this is a complicated trade-off and involves a lot of compromise.
There are several popular designs of differential which deserve some explanation. We consulted Jeff Anderson at Moser Engineering and learned a little more about the pros and cons of these. We will disregard the cost-effective open differential and its stellar fuel economy and instead focus on those used more in the performance realm of the automotive world.
The common upgrade path for most users takes them to a limited-slip differential—known colloquially as a “posi.” As most users add horsepower and find themselves needing a more reliable differential, these types work well since they operate without harshness or vibration—much like the factory OEM differential. However, the posi will sometimes require a slight bit of wheelspin or an aggressive throttle application to get it to spread torque between the driven wheels.
Most of these posi units are built with a clutch-type design—including Eaton‘s posi. However, this one stands apart from most mid-shelf posi units because it uses carbon friction disks. While this means the Eaton posi is happy handling decent horsepower, it can grab more than some might like, and it does often require a friction modifier to go along with it.
The posi often gets confused with the Trac-Lok since the two are almost used interchangeably. The reason for this is that posi-trac and Trac-lok are both marketing terms typically used for GM and Ford-branded limited-slip clutch-style posi units, respectively.
For those following the OEM+ tuning path, there aren’t many better than the Trac-Lok. As far as NVH standards go, this is one of the best, but it also handles a good deal of power and comes at a reasonable price. This clutch-type differential engages more smoothly than the factory units, is capable of handling moderate torque, and is easily rebuildable.
Another design popular for its price and its strength is the locker. This type will work as an open differential in daily operation and allow for better mileage, but they divide the power equally through both wheels until they disengage—sort of an on-off switch, if you like. A locker works best when driving over loose surfaces and is most commonly used in the off-road segment where inconsistent traction defines the driving experience.
Perhaps the most popular brand of the locker is the Detroit Locker—the unquestioned industry workhorse for more than half a century. It’s proven itself in most veins of motorsport and it still finds a home in many modern muscle cars. Low price, an ability to harness major torque, and the willingness to accept just about any type of oil keep it relevant and widely used today.
A locker has a few downsides, however. “When engaged, a locker behaves more like a spool and can cause the vehicle to either push through a corner and/or scrub the tires pretty harshly. In high-grip situations, we’ve seen stock axle shafts break from this sort of thing,” adds Anderson.
As far as functionality in less competitive situations goes, the Detroit Locker’s unusual ratcheting during cornering may throw the first-timers for a loop. Some think this sound means it’s on the verge of failure, but noisy operation is merely part of this differential’s design. Not the smoothest or most sophisticated, but its reliability and ease of maintenance keep them relevant and popular today.
Another type of differential popular today is helical. The two that stand out in this field are the Autotech Wavetrac and Detroit Truetrac units, both of which are some of the strongest differentials available for a performance street car. These use helical gears which exert friction on the case, which then distributes torque between the driven wheels. Because they require lots of friction to function, owners should not add any friction modifiers which would encourage slipping.
The Wavetrac’s patented wave-styled side gear dramatically improves its road course performance. Because of its unique design, the Wavetrac can actually deliver power to the only tire with traction—even if the other wheel is completely off the ground—making it the real choice for road course guys who like to bounce and cut across the curbs.
The final bonus with the Wavetrac is a lifetime warranty which covers usage in a motorsport environment. This unit does come with a slight price premium, but is definitely cutting edge compared to even a Quaife-style unit.
The price is higher than most, but the Wavetrac is one of the toughest on the market and one of the fastest to engage, and yet it operates more smoothly than the rest. It’s also available with higher spline counts and for a number of popular housings including 9-inch, 12-bolt, and 8.8-inch.
The Detroit Truetrac is another highly regarded diff that comes at a greater cost than most, but offers the user the usability and strength needed for competition. It’s slightly more cost-effective than the Wavetrac, but it doesn’t come with the same sort of warranty.
Like the Wavetrac, the Truetrac uses a helical gear-style design and engages in a similarly smooth, reassuring way. No unpleasant ratcheting like the kind experienced with a Detroit Locker, for instance. It is probably one of the strongest designs on the market before one enters spool territory.
For more information on the right differential for your build, make sure to contact Moser Engineering.