I am considering putting an FD differential in my S4, and I was wondering if anybody had any advice. Particularly, what parts should I have replaced or upgraded when I do the swap? Also, any advice about finding and purchasing an FD differential would be much appreciated.

 

It won't fit. Period. The FD diff bolts to a powerplant frame that takes up most of the rear of the car, then extends from the diff nose to the rear of the transmission. Unless you do a lot of cutting and welding it has no chance of fitting. Then you'll have to deal with custom half shafts, custom drive shaft, etc. etc.

 

I am referring to the torsen lsd from an fd into an s4. From what I have read in various posts on this forum, I am under the impression that this swap is not particularly difficult. See this post:
https://www.rx7club.com/showthread.p...ifferential+fc

 

Is it cheaper to get an FD one than an LSD made for the S4?

 

Used ones seem to be around $300 plus shipping. They are supposed to last a very long time, and the response is supposed to be more linear. The stock S4 diff apparently wears out after 30-50,000 miles.

 

S5 T2 Differential would be a better choice if longevity is a concern. It doesn't have the clutch packs like an S4 does. Bolt the whole unit in and you're done.

If youre concerned about durability, rebuild the S4. Many FD guys swap the S4 diff in because the FD torsion rear end just can't take a launch / shock like the S4 unit can.