Project Budget 400WHP S197 Mustang Track Car: Part 5 – Upgrading to an Eaton Truetrac LSD

A performance Limited Slip Differential is a MUST for making 2005-2014 (S197) Mustangs handle predictably.  We replace the weak factory LSD with Eaton’s TrueTrac helical (gear-driven) LSD which will transform the driving experience and character of the car.

Eaton Truetrac

Personally, I would upgrade any S197 Mustang with a performance LSD like the Eaton Truetrac before modifying the suspension.  This is because of how much more consistent, predictable, and easy to control they make the car at the limit as the rear starts to slide.  It’s seriously that important.

The solid axle combined with a torquey V8 makes a good LSD far more important for a Mustang than most cars with independent-rear-suspensions like BMWs, Miatas, Elises, and 350Zs with open or worn-out LSDs.  The differential affects the Mustang’s handling far more than simply reducing inside wheel spin.

I have driven and raced more S197 Mustangs of varying builds than I can remember.  From stock cars to supercharged cars and even track-focused cars; the vast majority have sub-par, worn-out differentials that desperately need an upgrade, even for street use.  I would credit the stock LSD for a lot of the Mustang’s reputation for bad handling.

Ford Traction-Lok LSD Mustang

The factory Ford “Traction-Lok” Limited Slip Differential (LSD) is sufficient for the stock power level when new, but they don’t hold up well to track use or more power.  They are not very durable and tend to wear out quickly, reducing the amount of lock and making the car handle worse.

The amount of ‘lock’ is determined by a series of clutch plates that are squeezed together and preloaded by a bent metal “Z-spring” in the center of the diff.  Increasing the pre-load and friction with an aftermarket spring and clutch plates will increase the static lock and holding ability of the diff.  This is fine for drag racing but is really bad for handling because it increases understeer by resisting the different wheel speeds required for turning in the middle of a corner.  To make Mustangs turn, you do not want any drag or lock from the LSD mid-corner when off the throttle.

Our project car’s LSD was in reasonable condition but there was a lot to be desired in the way the car drives off of corners, especially with the increased power.


All cars have differentials, which are fundamental for allowing two driven wheels to turn at different speeds in a corner.  For performance, a Limited Slip Differential (LSD) reduces the difference in speeds, which can cause excessive inside wheel spin and a one-wheel-burnout condition.  Not only do LSDs improve acceleration by increasing rear traction and ability to put the power to the ground through two tires, they greatly affect the car’s handling, feel and consistency.  This is especially important when accelerating out of a corner, where an open differential or weak LSD can make the car feel sloppy with inconsistent and unpredictable changes to the behavior of the car as it transitions from oversteer and understeer.

Performance LSDs are essential for the handling performance, feel, and consistency when driving a car at the limit.  This is even more important in cars with higher levels of power and torque.


Under the LSD umbrella are: Fixed value, Torque Sensitive, Speed Sensitive, and Electronically Controlled differentials.

Technically, the subcategory of “Torque-Sensitive” LSDs encompass both clutch type and gear/helical-driven designs.  Most people refer to all gear-driven LSDs as “torque-sensing” or by the brand name “Torsen”.  However, the helical (gear-driven) LSDs include designs from Eaton, Torsen, Quaife, and Wavetrac.


Eaton developed the first domestic gear-driven LSD back in the 1970s and have continuously refined the design to be more efficient and more durable as technology and metallurgy advanced over the past fifty years.  They take reliability and performance of their differentials so seriously that they have their own 650-acre ORM-level proving grounds in Marshall, Michigan.

Eaton Proving GroundsEquipped with their own 1.6-mile oval, gravel rally track, high and low traction skidpads, sand pit, rock crawling, steep grades of varying surfaces, water fording, and various split-mu and hostile environment obstacles, I’m not aware of any differential company that comes close to testing their product to this level.

Reliability is one of Eaton’s strengths.  As an OEM-supplier, their differentials meet or exceed OEM standards.  Eaton prides themselves on having the strongest spur gears in the industry and being the only differential company to manufacture their own net forged high strength steel spur gears. This is important because the spur gears are where the majority of failures occur in gear-driven LSDs.


  1. Hey I’ve been there! When I was in college I participated in SAE super mileage, and the competition was held at the Eaton facility. We drove on that oval in something that is best described as a powered soapbox derby car. That oval is not only banked, but it has a pretty serious downhill out of turn 4 before climbing back up the straight.

    I am also envious of the condition of the underside of that car. Here in PA that car could just look at a snowflake and be in worse shape.

    (BTW, there is a typo on pg6, it’s supposed to read bang for your buck, not bang for your bug.)

    1. the early 1960’s Volkswagen Beetle were equipped with gleason differentials in the transaxles …we had a 1963 beetle with a rag top sun roof belonged to my aunt who had the factory service manual break down on the transaxle ..and it was a gleason ..they originally were developed for the 4 wheel drive tow trucks that pull big air planes around on the air strips for traction in ice or snow ..the ones they offer for cars and pickups are only good for about 400 whp ..but it’s good to know they came in the 240-260-280z cars

  2. I have 2 engineering degrees and helical gear LSDs remain 80% magic to me. The exploded view helps: basically the diff casing turns the satellite gears, which turn the axle shafts, but if the axles try to turn the satellite gears they struggle to…or something.

    But my understanding is that because they rely on the angle between the gears to create TBR, they don’t work in reverse?

    Isn’t critical for a track car, but definitely a consideration for a 4×4 application. LSD and thus TruTrac is a pretty attractive option for a front diff…but maybe less so if it doesn’t work in reverse?

    1. Replying to myself with a correction, after a bit of searching: they do work in reverse.

      I was thinking about certain kinds of e-lockers that have a ball-on-a-ramp engagement mechanism; the briefly unlock when going from forward to reverse.

    2. The crossed axis ones always were more comprehensible to me – two worm gear sets, themselves geared together, drive the output gears. Worm gears don’t like back torque and the wedging of them resisting it transfers torque. With the parallel gear ones it’s basically the helical twist of the satellite gears pulling them into one side or another of the groove and the friction there wedging stuff to resist rotation.

      Unless I, with only one engineering degree, am misunderstanding this too. 😉

  3. I was doing a track day at Summit Point when the 1 Lap cars arrived. They were asking us where the best turn-in and braking spots were, and we were happy to help.

    My last track car was a 93 Rx7-TT, with a Torsen in the rear. It was excellent on the track, but a bit too grippy on wet roads. Torsens are all about friction from the gears and the gear end thrust. The outer pairs sit in a cylindrical openings in the carrier with no axle or bearings, just a lot of friction to resist rotation. As the formula for Torsen performance starts with a friction coefficient, I added an extra 1/2 container of the “friction modifier” to make less oversteer. Wonderful LSD like the TruTrac.

    I changed the gear oil a couple times during the season, as all the friction caused high temps in the oil, coming out deep black.

  4. This is what I had in my S197 thanks to the last MotoIQ S197 project car… price is kinda nuts compared to how much more expensive LSDs are for almost every other car out there

  5. Seems like I remember the Gleason being the first gear type. They were popular for the 240-260-280z cars.
    Also, some of the Audi quattros’ had them factory?

  6. I feel like a broken record, but I was super glad to see you walk away from the tire barrier on Saturday at Sebring. Hoping you catch some better luck here soon!

  7. Thank you again for the great article and for including links to make it easy for us to order parts to replicate this build. Most of the links provided in the articles are great in that they take us to specific parts that we can add to a cart and purchase directly (ex. Steeda). This is awesome and thank you. The link in this LSD article takes us to the Eaton general web site. This is a great website and very informative; however, does not tell us specifically which LSD you used in your build or you recommend. If contacting Eaton or one of their retailers is recommended, what kind of information would I need to provide. I’m neither an engineer nor a gear head so any direction may provide would be very greatly appreciated.

    1. The make is “Eaton”. The model is “Trutrac”.

      Just search for a “Truetrac” for a Ford 8.8″ and you can get them from s number of places. It’s hard to go wrong.

  8. I have something of a bizarre, somewhat-related question concerning…driveshafts in these cars.

    During my research phase of selecting an S197, so many S197 owners getting rid of the stock 2-piece setup in favor of a 1-piece…and I’m then seeing a ridiculous amount of vibration issues after the switch to a 1-piece.
    The one issue I’ve run into time and time again with Ford RWD products is their perpetual cheaping out on driveshaft tech (hardly any RWD car built by Ford enjoys rear gear upgrades beyond 3.27 gearing).

    I’m looking at buying a 2005-2009 S197, I seem to recall that there were issues with the 2010-2014 2-piece units, but what do you recommend…if anything…to run for a DS?

    1. Apologies, I may have answered my own question, but from what research I’ve been able to do, the stock 2-piece driveshaft…while in itself is a brilliant idea, as horribly-engineered long driveshafts are one of Ford’s weak points throughout their RWD history…its execution left a lot to be desired, especially considering Ford apparently doesn’t service the carrier bearing in the middle of the driveshaft, and it wasn’t exactly the highest quality piece to begin with.
      The unit I’m seeing the least amount of 1-piece vibration grief with is the DSS (Drive Shaft Shop) piece (available in both aluminum and carbon fiber), as the rear joint is a proper CV unit, although the carbon fiber unit is a weeeeee bit spendy…north of $1300.

      1. I’m probably a little late with this reply. But I just did a Ford performance 1 piece DS in my 2011 GT track pack. I wouldn’t have given it a second thought, but the carrier bearing on the stock 2-piece was clunking during shifts, so I needed to fix it. The ford performance unit is high quality, and a decent amount cheaper than the DSS one, fyi. It’s also roughly the same price as buying a factory replacement 2 piece since ford decided to make the bearing non-serviceable. I don’t notice any vibration issues with the one piece, NVH has remained the same as my factory 2 piece DS. It should be noted that I have other NVH issues from the poly bushing/third link on the rear end, so that may be masking any change in vibration from switching driveshafts. Some like me, report no issues going 1 piece, and others seems to have problems.

  9. Thank you for this excellent write up! If you are changing this out would it make sense to change the ring and pinion ratio? What is best gear ratio for a street/track car? I a 2011 GT with an automatic transmission.

    1. Thank you. It depends on your goals and uses. Although changing the final drive to a 3.55 or 3.73 will be a nice change for an automatic in most cases.

      1. What do you recommend for the best gear ratio for the 2012 Mustang GT 6 spd for street/track with this setup? Thanks

  10. There is TWO versions of the eaton tru-trac. The HD version uses FOUR pinion gears on each side, instead of 3. The casing all the gears are contained in, is thicker and stronger. BUT it’s made for 33 spline axles, not 31 spline axles. ( oem is 31 spline). AFAIK, the only companies making 33 spline axles are …’strange engineering’….and also “Moser engineering’. The HD version is typ used by drag
    racers, when their 60′ time is down to 1.4 secs.

    Moot point. The normal version is more than ample for street / auto X/ road racing… even with high rear hp..and forced induction.

    Point here is…. buy the standard normal version. I installed the normal version of the eaton tru-trac AND the white line Watts link at the same time. The difference was apples and oranges.

  11. Hey Billy, I am going to be ordering a Detroit TrueTrac for my FoxBody Mustang. I still have the 28 spline axles. Will the bias push too much power to one side possibly breaking an axle? Should I upgrade to 31 splines or just get the 28 spline diff and save some money? The engine is only 300 hp. You drove the car at the Cruise for a Cause autocross in the Dells last year.

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