Project Grey Mustang 5.0: Part 5 – Putting the Power Down with Eaton
As I hinted at in the previous installment of Project Mustang 5.0, the OEM Ford clutch-type differential (called Traction-Lock or “Trac-Lok”) was beginning to wear out. It was locking up less and less, meaning the inside rear tire was spinning more and more on corner exit. This was caused, in part, by heat created by the clutches locking up causing premature wear. Although the “passive differential cooler” (read: aluminum heat sink differential cover) I installed did help to prolong its life somewhat, it was beginning to slow me down.
Several solutions presented themselves. A rebuild kit for the clutches was inexpensive, and the high-carbon clutches used in the supercharged GT500s are available. Although this would have fixed the problem and led to a longer-lasting clutch pack, I would have still been left with a differential that would need to be rebuilt every other year (or sooner). Furthermore, rebuilding the clutch packs requires removal of the rear differential, so if I’m going to have to take the entire thing out, why not just replace it with something more robust?
The Torsen differentials used in the GT track pack, BOSS 302, and FR500S were looking to be a very good option. Since these differentials utilized worm gears, there would be no clutches to wear out. The problem with the Torsen differentials, however, is survivability under hard launches. Let’s face it: this is a Mustang. It’s going to be drag raced from time to time, and occasionally on slicks. The following image shows what happens when you launch too hard with a Torsen differential.
For something that would perform well, last a long time, hold up to hard driving conditions, and be easy on the wallet, I went to Eaton.
GM was one of the first to offer a mass-market limited slip differential under the name “Positraction.” This technology was developed and manufactured by Eaton. The positraction differential was a clutch-type differential, and Eaton still sells them today for a very wide variety of applications as the Eaton Posi Differential.
Although the Eaton Posi is a great unit with a 50-year history, I still didn’t want a clutch-type differential that could wear out. Instead, I decided on Eaton’s TrueTrac, which is a worm gear differential not unlike the more expensive Torsens.
All limited-slip differentials operate on the same basic principle: when one wheel is spinning faster than the other, something (clutches, gears, viscous coupling, electromagnetism, or PFM) is used to attempt to “force” the two wheels to turn at the same speed.
The bias ratio of a limited-slip differential is the maximum amount of torque that can be transferred to the wheel with the most traction. A differential with a bias ratio of 2:1 can transfer twice as much torque to the wheel with the most traction.
In the case of a clutch-type differential, a spring is used to push a stack of clutches onto the two axles. When one begins to slip, the frictional forces of the clutches resist the two axles turning at different speeds. The advantages of a clutch-type unit include ease of manufacture and very low NVH. The main disadvantage is a bias ratio that decreases as the clutches begin to wear and the fact that maintenance is more involved, as the friction modifier necessary for proper clutch activation breaks down faster than the oil itself and needs to be changed rather often.
The Eaton TrueTrac and Torsen differentials are examples of gear-type differentials. In these types of differentials, more torque is sent to the wheel with greater traction by the use of mechanical gears as opposed to clutches. The advantages of gear-type differentials include (generally) higher bias ratios that do not decrease over time, ease of maintenance, and robustness. Disadvantages include higher cost of manufacture (due to the exact tolerances and machining processes needed to make all those intricate gears) and an increase in NVH.
The Torsen is a widely-used and popular gear-type differential. Describing how it operates in words can be rather difficult, so I’ll just put this video here and let you see for yourself how it operates.