The Next FDR Mystery :)

["Buchholz, Steven" <Steven.Buchholz@kla-tencor.com>]

Now that it seems that we've got the 4kQ's figured out I thought I'd bring
up something new ...

Up until last night I was pretty clear about the differential final drive
ratios on the various models of the V8 Quattro.  Well, I spent some time
with the service fiche and the parts fiche last night, and now I'm
completely confused ... :-)

Common wisdom states that the '90 V8's came with a 4.11:1 FDR, and that in
1991 the automatics got a lower ratio with the 5-speeds maintaining the
4.11.  After 1991 with the change to the 4.2 liter engine all cars got the
lower FDR of the '91 automatic.   The parts fiche shows that the automatic
tranny code changed in similar fashion, with ATU used for '90, AYU for '91
and AZG for '92 and beyond.  The manual tranny sold in the US in '91 has a
code AWW.  The strangeness sets in when the codes for the rear diff are
investigated ... the parts fiche states that the rear diff code AFV was used
on automatics in 1990 _and_ 1991, with the manual transmission cars and the
4.2 liter cars getting code AXZ.  The service fiche shows the same codes
(AFV and AXZ) for rear diffs as the parts fiche does ... and goes on to
state that the FDR for both is 4.11:1!  Ready for more?  Remember the tranny
codes ... well, the service fiche gives the FDR for the ATU automatic tranny
(listed above for the '90 V8s as 35:12, or 2.917:1!).  The final piece of
information is a veiled reference that I saw in the parts fiche ... that the
AFV rear diffs come in two different color codes, yellow and green ... there
is no indication of FDR that I could find in the parts breakdown section of
the fiche.  

Sooooo ... as I see it there are a few possible scenarios ...

#1 - All V8 quattros whether they be automatic or manual came with 4.11:1
FDRs.  If this is the case, what is the difference between the AFV and AXZ
rear diffs ... is it beefed up to handle more power from a manual tranny or
a bigger engine?  

#2 - All automatic equipped 1990-91 V8s actually have a FDR of 2.917:1 and
the MT equipped '91s and the cars from '92-on have 4.11:1.  This scenario
assumes that the fiche's statements that the AFV rear diff has a FDR of
4.11:1 is in error and causes one to wonder why they needed two different
automatic tranny codes.  

#3 - Everything is as originally thought, '90 4.11:1, '91 automatics
2.917:1, '91 manual 4.11:1.  This would require that the fiche is mistaken
and that the AYU tranny was the one with the 2.917, with the ATU and AWW
having 4.11:1.  This would also require that the AFV rear diff was available
in two flavors, which could be explained by the color codes, one would be
2.917 and the other 4.11.  That still raises the question about what the
differences between the AFV and AXZ are.  [I don't even want to think about
what the FDR of the '92-on cars is ... ]

... it seems to me I thought of another possibility, but my brain hurts
right now so I'll leave it there.  Notice that there is not a single
scenario that does not involve some error in the documentation ... :-)  I'd
appreciate hearing from anyone who happens to have any other snippets of
information or experience that could be used to help me figure out this
conundrum ...

Thanks for reading this far!  :-)

Steve Buchholz
San Jose, CA (USA)