The Mustang Forum for Track & Racing Enthusiasts

Taking your Mustang to an open track/HPDE event for the first time? Do you race competitively? This forum is for you! Log in to remove most ads.

  • Welcome to the Ford Mustang forum built for owners of the Mustang GT350, BOSS 302, GT500, and all other S550, S197, SN95, Fox Body and older Mustangs set up for open track days, road racing, and/or autocross. Join our forum, interact with others, share your build, and help us strengthen this community!

Rear Diff Questions

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

I own a 09 Bullitt. It's been modified and is my daily/occasional track car. Back in February I had a Torsen T2 installed and a new set of 3:73 gears. It has over 100k on the odometer and it was time to give the diff some love. Fast forward a couple months and instead of hooking up well in the corners it started feeling like it was slipping and didn't leave me feeling confident. I had the car into a shop close to me to check on a small coolant leak and to rotate the tires and they noticed that there was too much play in the driveshaft and said something seemed off. I told them about the recent upgrade and they informed me that the lash may be off. I called the other shop and caught a ton of flack for taking it somewhere else which has led me to not go back to the shop that installed it but to have the new shop crack it and take a look. Once they did this was their diagnosis and recommended fix:

""""Notes:
found large amounts of play from pinion movement to when wheels spin. also found rear diff appears to be open. can hold one wheel steady and spin the other freely.
removed cover, noticed gear oil is extreamly dark especially for a fresh rebuilt unit. found 3.73 gears which is the ideal gear for this car. found carrier is an eaton true trac unit which is a worm gear posi, slop is inside carrier and limited slip is not working.
aside from carrier, found preload very loose in total which indicates neither pinion or side bearing proload is within spec, this is likeley why gear oil is very dark. bearings in process of failure. aggressive wear pattern on ring gear teeth, the result of too much backlash. (found .022 backlash, ideally needs to be around .008 - .012)
eaton unit that was installed is known to be problematic, have had or heard of several premature failures. aside from this, ring and pinion was setup poorly.
Rec replace ring and pinion, and carrier. recommending a more conventional opposing plate type posi unit for longevity and performance. (yukon-duragrip posi carrier and yukon gear 3.73 ring + pinion) *?*?*? 31 SPLINE AXLES*?*?*"""


If it was your car what would you do with fixing the issue? Eaton TrueTrac? 31 spline axels? Which manufacturers? Thanks TMO community for your input and feedback in advanced!!
 
179
308
Utah
If you are tracking the car, definitely stay with a "Torsen" type diff. Just for handling sake the posi is not going to be what you want.

When he says it is "open" he is probably using the old trick of spinning one tire, before he opened the diff cover. An open wheel axle will spin the opposite tire the opposite direction. A posi will spin both tires the same direction. A "Torsen" type diff will spin opposite, like an open wheel, because when there isn't any torque on the pinion, it acts like an open wheel.

I would be concerned as well that you thought you had a Torsen T2, and got an Eaton.

It does sound like it was poorly setup. I have done my own axle rebuilds (and done it poorly a few times early on) it doesn't take much to be off.

As recommended by @blacksheep-1 stick with the Ford Performance gears. Make sure your builder knows that the FRPP gears are Face Hobbed and not Face Milled during manufacturing. This makes it different when you setup the gear set in the way you fix the pattern on the gear. I will try to attach a few documents or at least reference to them. Good luck with the fix.
 

Attachments

  • Dana Face Milling vs Face Hobbing.pdf
    151.1 KB · Views: 9
  • Ford Axle Ring and Pinion Assembly How to Guide.pdf
    4.3 MB · Views: 14
  • Ford Racing Ring and Pinion Installation.pdf
    740.6 KB · Views: 10

xr7

TMO Addict?
706
821
Exp. Type
Autocross
Exp. Level
10-20 Years
Minnesota
When I do a integral diff overhaul the diff housing is sitting on jack stands, not in a car. Case spreader and the other tools are nice to do a good job because close isn't good enough when you build a diff. No crush spacers either. I tend to go for the tight side of the specs so as it wears in it stays in spec.
 
1,161
2,116
Exp. Type
Time Attack
Exp. Level
5-10 Years
SoCal
Make sure your builder knows that the FRPP gears are Face Hobbed and not Face Milled during manufacturing. This makes it different when you setup the gear set in the way you fix the pattern on the gear.
👆Truth. And to make it more confusing, the Ford Performance instructions for their gear sets include setup instructions for face-milled.
 
so you paid to have a Torsen installed and you ended up with a Tru Trac?

in any case, go with Ford Performance gears, not the Yukons.
The Torsen either broke with normal driving or the gears weren’t installed correctly so it ruined the Torsen. I’m considering going with the Tru Trac and 31 spline instead of the 28 spline and another Torsen T2
 
179
308
Utah
The Torsen either broke with normal driving or the gears weren’t installed correctly so it ruined the Torsen. I’m considering going with the Tru Trac and 31 spline instead of the 28 spline and another Torsen T2
Your car should have come stock with 31 spine axles. I think that is standard for all S197 GT cars. If it has 28 spline in it, I would have to suspect that they were changed at one point or the entire axle was.

Performance wise I think you are good with either the Torsen or the Eaton.

Frankly I would be surprised if the Torsen that was installed was bad. About the only thing that can hurt it is an impact with a big piece of metal. Spinning one of the wheels while it is in the car would not tell you if it was broken. It will naturally make a gear turning noise when you do that. I would have it pulled, cleaned and visually inspected. The gears inside the Torsen only turn when the car is turning and there is a difference in the rotational speed of one tire to the next. They are not high speed gears. Pretty "Bullit" proof... (sorry, too soon?)

Maybe you should post some pictures of the axle and the differential parts.
 
Last edited:
Your car should have come stock with 31 spine axles. I think that is standard for all S197 GT cars. If it has 28 spline in it, I would have to suspect that they were changed at one point or the entire axle was.

Performance wise I think you are good with either the Torsen or the Eaton.

Frankly I would be surprised if the Torsen that was installed was bad. About the only thing that can hurt it is an impact with a big piece of metal. Spinning one of the wheels while it is in the car would not tell you if it was broken. It will naturally make a gear turning noise when you do that. I would have it pulled, cleaned and visually inspected. The gears inside the Torsen only turn when the car is turning and there is a difference in the rotational speed of one tire to the next. They are not high speed gears. Pretty "Bullit" proof... (sorry, too soon?)

Maybe you should post some pictures of the axle and the differential parts.
I'll do one better than that and give you the link to the report that came with a video.

 
179
308
Utah
I'll do one better than that and give you the link to the report that came with a video.

That is pretty cool. I guess I haven't taken my car in recently. That is a nice service.

Wish I could have seen the diff spin, but that side to side play was really bad! Well, here's hoping you can get it fixed and back on the road. I bet that is a really nice car!
 
That is pretty cool. I guess I haven't taken my car in recently. That is a nice service.

Wish I could have seen the diff spin, but that side to side play was really bad! Well, here's hoping you can get it fixed and back on the road. I bet that is a really nice car!
It is a nice report and I'm glad I found this shop but would it make you think something failed and that the whole rear end needs to be replaced?
 
179
308
Utah
It is a nice report and I'm glad I found this shop but would it make you think something failed and that the whole rear end needs to be replaced?
With the kind of movement in the diff, the housing should be inspected for sure, especially the carrier race mounting surfaces and caps. I have seen good used boss and track pack axles (3.73 and boss 302 torsen) from about $1000 to $1250. Of course an earlier bare housing would be dirt cheap. Any 8.8 GT axle 2005 to 2014 will fit the car.
 

TMO Supporting Vendors

Top