I have a 98 2.8 5spd tip Quattro as my daily. I have a 2000 s4 5spd tip Quattro chassis I bought for a project car. project has been scrapped. My question is can the suspension be swapped? wheel spacing etc...? also can I swap the rear diff's? s4 to a4? they use the same trans gearing but the rear diff on the 2.8 is 3.730:1 and the S4 is 3.511:1. Aside from sharper accel will this cause any long term problems? I wouldn't think so considering the transmissions are geared the same. I cannot find any info on the front diff gearing though.
TIA!! Cheers!

 

Suspension and wheels are the same.

Not sure where you found your ratios as they make zero sense. S4's have 4.11 final drives, and a4 tip have the same. A4 manuals have 3.889 final drives.

The front diff is the same as the rear, and always has to be. Considering the front diff is inside the trans, you wont be changing it. Also, you cannot swap around ratios. Trans ratios don't matter, but the front and rear diffs need to match.

To make a confusing matter simple, you can leave your 2.7 rear diff in the s4 and use you 2.8 trans, and vice versa. If you are trying to swap in an 01a manual, you will have a problem.

 

Well thank you for being slightly courteous and providing partially valid information. It was a simple question of a rear diff swap and possibly providing the front diff specs. The s4 and a4 autos have the same transmissions with the same ratios but for some reason or another they have different rear diff ratios. Audi says you can run from 1% to 4% difference from front to rear but that is for tire size basically changing the ratio.
I got my ratios and specs from the factory but nothing includes front diff info like ratio and if its 50/50 or 60/40 power distribution.

So other than that I really just need to know the front diff ratio and why the these 2 cars run the same trans ratios but different rear ratio's. If somebody educated in mathematics reads this I'm sure they can simply calculate the percentage difference in the ratio's for the rear and cross reference that with audi but unless you can talk to an engineer in Germany i'd clist a grad student.

As far as suspension goes the factory says the s4 is a little different than the a4 and the wheel base on the a4 is actually about an inch or so wider which is backwards to me but whatever they still sold the hell out of them. I haven't looked at them side by side yet because the state I reside in has super awesome weather patterns like clear blue sunshiny day they write songs about to about 2 feet of snow in a few short hours.

So unless the umbrella corp(VW) made them put out the wrong specs 1 of us has an invalid source. "Sadly" I no longer work for the umbrella corp and when I did information was actually really hard to come by.

 

I don't think you understand how quattro works.

The front and rear diffs have to be the same ratios, otherwise the center diff will bind. S4's (manual and auto) have 4.11 rear gears. A4 auto's have 4.11 rear gears. A4's manuals have 3.889 rear gears. That is all the information you need.

 

Well Awd varies from each manufacturer. Sadly I rarely use this site so if I can post links a upload pics I will show you the spec sheets that I have for each year and we can then compare notes. As far as Quattro and awd in general im pretty well aware of how it works real world and conceptually. Differentials can also vary and dictate a power ratio obviously a more advanced setup depending on manufacturer again. We all know the ratios must stay the same but audi has also clearly stated an acceptable margin allowing for different applications. So depending on the pitch would dictate a lot and figuring out who's spec sheet is correct. I see the ratio's you post and they are accurate and stated on my spec sheet as well. But... those are not the ratio's I am referencing. Also the main concern is burning up the center differential so with that added what is the center diff ratio if applicable? So if you can post a pic of you specs or a link and ill check it to mine. Thank you.

 

Yeah...no. You're overthinking this way too much. I don't know where you got your information from but that 3.730:1 ratio for the A4 is bullocks. Your tip A4 has a 4.11 final drive front+rear drive ratio, while your tip S4 uses the 3.51 front+rear diff ratios. In short, they are not compatible

You're right in the aspect that the diffs have a marginal tolerance for different rotating diameters; this only pertains to the WHEELS themselves and NOT the ratio of the diffs. The A4 tip and S4 6-spd use 4.11 front and rear differentials. Transmission ratios do not matter, what matters is what ratios your front and rear differentials are so you do not blow up your center diff and be out a ton of $$$$$.

The center diff is open and is a factory 2:1 torque distribution. You can use shims to gain a larger 4:1 torque split, but just the washers are like $100 anyways.