Mark Houlahan
Brand Manager, Mustang Monthly
December 21, 2010

Finally we come to the spool. A spool is nothing more than a solid piece of steel (for the most part) that the ring gear is bolted to and the axles are engaged to for 100 percent of the engine's power to both rear wheels. There's no differentiation whatsoever for street use. This lack of differentiation can cause driveline stress/breakage, and extreme tire wear when used on the street. A spool is most at home on the dragstrip where you want all your traction in a straight line and a little tire chirp when turning onto the return road is no big deal. I'm sure you've had a pal or heard cruise night stories of some Mustang with insane power and running a spool on the street, but other than for bragging rights, it's not the easiest thing to have under your Mustang for street use.

Limited Slip

Auburn Gear offers several differentials for Ford axles. The Auburn is a high-bias, limited-slip differential that uses a cone-shaped friction surface clutch arrangement that is coupled to the side gears, and uses centrally mounted preload springs. The preload springs, as well as the normal gear separation forces of the incoming torque, seat these two cones into the differential case to maximize torque output to both wheels. During cornering, the torque input decreases, allowing the cone clutch to slip and differentiate the wheel speeds. Auburn's limited-slip lineup includes the original Auburn High Performance limited slip, the Auburn Pro Series (with higher bias), and the Auburn ECTED Max electronic differential (pronounced Ek-Ted). The original Auburn and the ECTED Max are currently only available for the late-model 8.8-inch axle, in both 28- and 31-spline configurations. The Auburn Pro, in both 28- and 31-spline part numbers, is available for the 8-inch and 9-inch rear, as well as the 8.8-inch late-model (including a 33-spline option for optimum strength on the street).

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If you're working on an 8-inch rear, a popular upgrade is the Currie TSD (Torque Sensing Differential) for 28-spline applications. The Currie folks designed their own limited slip by machining a Traction-Lok housing to accept four spider gears instead of the stock two gears, and then they seal it up with a Currie-designed, nodular iron Traction-Lok cover. Being based on the OE Traction-Lok means that the TSD uses friction clutch plates and a bias spring to control power output and allow differentiation while turning. The Currie TSD is also available for the 9-inch axle in both 28- and 31-spline as well.

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The Detroit TruTrac, today an Eaton Corporation brand, is a different kind of limited slip in that it uses a helical-cut gearset to allow quiet open differential movement while turning, yet when traction is needed, it will slow the spinning wheel and distribute power to the wheel with the best traction. This is done without the use of wear items like clutches and springs, too. The side gears have pinion gears that ride in pockets of the differential housing, and when traction is lost, these pinion gears move outward, wedging themselves into these pockets to slow the spinning wheel. As the input torque is increased, so is the separating force that wedges the pinion gears. The TruTrac is available for the 8-inch, 9-inch, and the 8.8-inch axle.

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The Eaton Posi is only available for the 8.8-inch in Ford circles, so we'll discuss it only briefly here. Essentially, the Eaton Posi is a friction-plate-clutch style of limited slip, upgraded with carbon-fiber clutch discs, and preloaded by a central spring assembly. Like other clutch-based limited slips, when torque input increases, the clamp load on the clutches increases, delivering the power to both rear wheels before there's any chance of tire spin. Also, like many limited slip units, the Eaton Posi is rebuildable.

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Another pair of helical-gear, limited slips offerings are the Torsen T-1 and T-2 differentials. The T-1 fits the 9-inch Ford and the T-2 (and T-2R race model) fit the 8.8-inch Ford. The Torsen design differs between the two versions. The T-1 is a crossed-axis helical gear, meaning the pinion gears cross the differential's side gears instead of running parallel. The T-2 uses a parallel gear arrangement, like other helical gear differentials, which offers quieter operation, finer gear mesh, and low backlash. Both differential styles are a full-time torque sensing/biasing design, and torque is biased instantly between the two rear wheels without the use of clutches, preload springs, or special gear oil additives.

As noted earlier, the majority of our readers will be most familiar with Ford's Traction-Lok limited slip, which has been a factory option in the 8-inch, 9-inch, and 8.8-inch axles for decades. The factory Traction-Lok, like most factory parts, is designed for quiet operation and long life, something that the OEMs do for customer satisfaction. Yet, these very reasons also mean the Ford Traction-Lok is not one of the stronger limited slips either. The Ford Traction-Lok uses a series of clutch plates and a central bias spring like many limited slips. Over the years, rebuilders have used carbon-fiber clutch discs, machined the housing for additional discs, tried different preload springs, and more. They will work up to a point, but if you're pushing a lot of power there are better options out there for your Ford axle.

Yukon Gear & Axle has two different limited slips (and a locker we'll discuss shortly) in its huge catalog of driveline parts that include axles, gears, differentials, and more. The Yukon Trac-Loc is available for the 8-inch and 9-inch axles and is similar in design to the OEM Traction-Lok assembly, but stronger in every way. Yukon starts out with a nodular iron differential case, then stuffs it full of forged internal gears, high-friction composite clutches, and a high-bias spring assembly. The differential is then sealed via a billet steel hat. The Trac-Loc is available in several part numbers for 28- and 31-spline axles, and also in an HD version, which has a higher torque bias giving way to a little clutch chatter when cornering. The Yukon Dura Grip (shown here) is only available for the 8.8-inch when it comes to Ford applications, and like the Trac-Loc, is built around a nodular case with 4320 forged steel internals, composite clutches, and four high-bias springs for both 28- and 31-spline late-model axles.