Putting an A8 Transmission in an S8

[mattsimis]

As prompted by:
http://www.tyresmoke.net/forum/audi-...ml#post1347120

I have a 1998 S8 (ie prefacelift) and was wondering if the gearbox from an A8 would work as a drop in replacement, with my current ECU?

I dont mind taller gearing and slightly slower acceleration.

[Conan_the_Librarian]

I believe you need the TCU to match the transmission. Otherwise a straight swap.

[mattsimis]

Sounds reasonable, would the A8 TCU work ok in conjunction with the S8 ECU? The guys on AW are saying "rear differential is not compatible with an A8 transaxle"... I cannot visualise why different ratios on the Rear diff would make its "incompatible" with a different gearbox.. any thoughts?

[Conan_the_Librarian]

With regard to the rear Diff. They are right. Rear diffs are match to the transmission.

TCUs are TCUs as far as the ECU is concerned. You may find that gears shift at od revs as the rev limiter on an s8 is 500 higher than an A.

[mattsimis]

Ah right, they explained it a bit more too and I seen now. Its unlikely changing both the gearbox and rear diff (and possibly TCU) is more cost effective than a rebuild.

[Conan_the_Librarian]

For piece of mind a rebuild is the way ahead. There are a number of specialist that members have used that have got good reputations.

Look in the recommended suppliers list.

[mattsimis]

Originally Posted by Conan_the_Librarian

For piece of mind a rebuild is the way ahead. There are a number of specialist that members have used that have got good reputations.

Look in the recommended suppliers list.

Im in Ireland so none of them would likely suit, however there is a well regarded ZF Specialist in Dublin, apparently been at it nearly 30years.

[PsYcHe]

If it's a proper ZF specialist, then take it there and see what they suggest. At least you know you're guaranteed a good job.

[Mr K]

Originally Posted by mattsimis

I cannot visualise why different ratios on the Rear diff would make its "incompatible" with a different gearbox.. any thoughts?

the front diff is built into the gearbox, change the gearbox and your changing the front diff. If the front diff isnt the same as the rear your front and rear wheels will be going at different speeds.

To be honest, I expect the actual gear ratios within the box are the same, and audi final drive for the A8 and S8 to give the differences.

If your doing this for MPG, I would think about how long its going to take to get the pay back for the new box. Also my s8 (with an S8 gearbox/diff) averaged about 29mpg in both directions on the 600mile round trip to the lake district a few months back, cruising at a steady 70. most of the time I get 20mpg. my mate who had a PF A8 4.2 got about 22-24mpg on average. Bassically dont fit this gearbox if your expecting 30mpg out of your car afterwards, I would expect 1, maybe 2 mpg more?

[Singh]

With the exact same driving style I get 30/1 MPG from an A8 but only 24 on an S8. I think it's the biggest flaw on the D2 S8, there should be a motorway cruise setting that drops the revs by 1K at the flick of a switch.