Project Grey Mustang 5.0: Part 5 - Putting the Power Down with Eaton

by Vince Illi

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.

Read more about Project Mustang 5.0 here!

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.


Yes, that would be one of those worm gears I was talking about peeking out of the bottom right corner of the half-masticated differential cover.  Fortunately, this is not my 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.


This exploded view of an Eaton Posi unit shows the basic components of a clutch-type differential.  The clutches are the stacks of flat “rings” on either side.

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.

In this video, you can see that the T-1 Torsen uses worm gears connected to “standard” gears to transmit torque from one axle to the other.

This schematic shows the “Invex gears” used in the T-1 Torsen.  Source: Torsen
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Tuesday, May 20, 2014 2:45 AM
The way torsens work mean they engage very smoothly, which is great for car control. Unfortunately, torque sensitive diffs (TorSen, rings a bell ?) have a problem. Torque sent to the high gripping wheel is always proportional to the grip of the other one.

Which means that the less traction you get, the more it works like an open diff. to the point where it does not lock. that happens mostly when driving on snow or ice, but there is another condition where the less loaded tyre has no traction: when lifting a tire ...

And that is where a clutch based LSD gets superior. Drive on a track, jump a curb ? With a torsen, that results in no traction on the wheel that still is on the road, while an LSD WILL lock, and give you that traction (that has a chance to send you spinning but hey ... you asked for traction)

I am all for using torsens (i do use one and i love it), but i don't recommend it to people who take shortcuts over curbs, or people who need a really strong coupling. for everyone else ... GET ONE.
Tuesday, May 20, 2014 6:16 AM
Crousti, that is true regarding the Torsen differential. In off-roading you can get around that by lightly applying the brakes but that probably isn't too applicable in road racing. They also make a diff called the T2-R which has a clutch pack to apply preload, so that it acts like a combined ATB and LSD, which is interesting.

Dusty, I was under the impression that the Torsen and Eaton True-Trac (which looks to be the same design as a Quaife) were more torque-based than speed based, so they act only under a difference in torque, independent of speed. Probably semantics at this point because the desired result is about the same for road racing.
Tuesday, May 20, 2014 7:00 AM
So, silly question here, my IS300 came with a Torsen T2 differential and I know it isnt clutch type but it seriously feels worn out. I can get the occasional peg leg on it. Is there any chance an article could be done on refurbishing one of those or what can be done to do such a thing?
Dusty Duster
Dusty Dusterlink
Tuesday, May 20, 2014 7:06 AM
Emeyer: Only some of the T-2Rs are preloaded. The one for the Ford 8.8 is not. I'm not entirely sure what they use the clutches for in that application. This was one of the reasons for my decision of the Eaton. It had 90% of the T-2R's bias ratio at half the cost and far better durability. The addition of actual preload clutches to the T-2R might have made me go with that one instead.

Tom: Both Eaton and Torsen say that the only way to refurbish their diffs is to remove them and send them to the manufacturer. I, personally, would love the chance to refurbish one, though. It looks like it would be quite interesting.
Tuesday, May 20, 2014 7:10 AM
I just cant fathom why they dont have some sort of refurbish kit for people who dont want the down time of sending it in. Also it isnt like they are complicated units to disassemble.
Tuesday, May 20, 2014 8:13 AM
@ -Tom-

Being entirely mechanical, the wear is probably just increased gear lash. Nothing really to be refurbished. They probably make you send it in because it requires carefully measuring the lash between every component and replacing the worn out parts. Only other option for someone who does not have this capability would be to just replace everything.
Tuesday, May 20, 2014 9:51 AM
"Peg legging" as in the tire is completely unloaded and so spins freely under power? Because that is the exact downfall of torsen diffs. If you lift a drive wheel with a torsen diff, it might as well be an open diff. Some cars are going to lift drive wheels often on corners. Roll stiffness, droop travel, weight bias, it all plays a role. It's a big part in selecting the correct diff for the platform and the application.

The torsen gears have an axial load associated with them as they are helical gears. The clutch packs that have been added to them help convert that axial load into a locking force in the diff. Without preload, I beleive it makes the lockup more progressive and higher torque bias capable. Without preload though, it's still going to act like an open diff when a wheel goes into the air.

Also, in my experience, clutch pack diffs are A LOT louder than torsen diffs. A clutch pack with heavy preload clunks and bangs like crazy when turning tight at low speeds. Lots of tire noise too. This and maintenance are the main reasons OEMs go with torsen diffs.
Tuesday, May 20, 2014 9:58 AM
When its acting open I am on smooth flat pavement with fairly equal grip between the wheels. I'm well aware that the differential relies on resistance from both wheels in order to "lock".
Dusty Duster
Dusty Dusterlink
Tuesday, May 20, 2014 10:01 AM
A substantial number of OEMs utilize clutch-type diffs, actually. Most pickup trucks, for instance, used clutch-type diffs until recently, when electrically-locking units became popular.
Tuesday, May 20, 2014 11:31 AM
Many older OEMs use clutch based LSDs. Seems like most modern performance oriented cars (if they even have an LSD option) are torsen though. I'm definitely not saying ALL OEMs use torsen though. Just a lot of them and they do it for maintenance and noise issues of the clutch type.

Nobody cares about trucks here. ;)

Just my opinion, but torsens are great in the front of a car, be it FWD or AWD. A clutch type diff in these situations cause massive understeer at lower speeds. Clutch type in the rear and center make the car lively and fun to drive.
Tuesday, May 20, 2014 11:34 AM
Tom, there is nothing that really wears with typical torsen diffs. The gears and case can wear, but I don't think there is anything you can do at that point but replace the diff?

Could be the fluid maybe that is being used causing the poor lockup? I imagine Eaton for example recommend dino oil largely because it's going to have a higher coefficient of friction between the side gears and case which is where the lock up comes from?
Dusty Duster
Dusty Dusterlink
Tuesday, May 20, 2014 11:59 AM
The viscosity of the gear oil has a definite impact on the efficacy of a gear-type differential.
Wednesday, May 21, 2014 2:51 AM
It is hard to say which oil should be used for a torsen. Honda recommends xxw140 oil for the torsen equipped integra. Nissan recommends 75w90 for all its diffs on S13/14/15, wether they are open, torsens or viscous LSDs.

@tom are you sure your car is equipped with a torsen ? Old VLSDs usually provide no lock when they wear their oil (or overheat). I think OEM LSDs are far too inconsistent to be used on track ( unless they are actually designed for race use ), and non serviceable usually. I had one on my s13, it locked well, for a couple of corners. Then it got too hot and acted like an open diff.

the good side of them is combining the smoothness and lack of wear of a torsen, with the locking capabilities of a clutch type. On the other side, basically you can't maintain them yourself (good luck finding that specific oil viscosity and using the exact right oil volume) and their inconsistency is terrible. Unless they are design for race, but then the price goes up.

Wednesday, May 21, 2014 7:12 AM
@Crousti - I am 100% positive. Not only am I a mod on my.is forums, but I have the original sales brochure, I know the equipment coding in the door sill, I have a the monroney sticker for my car, AND I've had the diff cover off. Its a Torsen. I have had the car for 8 years and 120,000+ miles (currently at 155k) of HARD driving. It used to lock very very well, now, not so much. I've always run the 75W90 synthetic that is called for in the owners manual, changed roughly every 60,000mi. It does not call for an LSD friction modifier (as it isnt clutch type) and I've used ones that have it and ones that dont, I noticed no difference. (I had to change it out due to the rear plate suddenly leaking from striking debris after a change)
Wednesday, May 21, 2014 8:34 AM
Tom, lol. "Yes, I'm sure" would have sufficed I'm sure.
Wednesday, May 21, 2014 2:20 PM
I had a torsen in my Spec Miata. No good in that class on a technical course. Too much suspension compliance and not enough droop to keep the inside wheel down.

I feel that a solid axle car would be ideally suited to a torsen LSD with their near ideal camber curves.

Does that sound right?
Thursday, May 22, 2014 7:56 PM

The FR500S Mustang Challenge Racecars initially came with Torsen T2 differentials. I agree with what you are feeling because while I personally don't know exactly what is wearing out or changing, but the torque from the 320hp 4.6L V8 on slicks caused the T2 to greatly fall off in locking capability and over time, excessive 'peg-legging' was the result. A new T2 would eliminate the issue but would need to be changed out frequently.

The T2R was the solution (and what the Boss 302R's currently race in the IMSA CTSCC) for the 500S, 500C, 302R and I believe most if not all 302S'. These don't seem to wear out or have a drop in performance.

I had a TruTrac in my personal street car and yes with one tire in the snow and one on the asphalt, there would be very little torque sent to the tire with grip. However the TruTrac has been an awesome diff and great for track/drifting/street and I have been very happy with it. The T2R just came out with a stronger casing (they tend to crack) so prior to this, the TruTrac was hands down a better choice, now they are much more even.

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