Originally Posted by
SixtyFortyDiff
Well then, I just stumbled across this today, (and also your "Crown Gear Musings" post from December of 2017) and you can consider my interest VERY piqued!!
Walky, you are I are apparently cut from the same cloth (notice my username...). Ever since getting into the VW/Audi scene, and watching some rally/rallycross videos online, I too have been "borderline obsessed" with the nitty gritty details of how various AWD systems work, their designs, and pros/cons of each, regardless of manufacturer. The amount of times you'd find the words/phrases such as "differential" / "asymmetrical torque split" / "active diff" in my search history would likely be mildly disturbing to most.
I drive a 2010 B8 S4 with manual transmission. Note: it has a sport differential (this matters, I think - I'll get to that in a minute). I always assumed I had the Torsen T3 as my center diff, because I thought all manual B8s had the KMR trans, and that the Crown Gear was only in 2013 B8.5 DSG cars. However, upon reading through this post and immediately running full speed to the garage, I discovered that my trans code is "MKY LPE" which based on your research in ETKA, means I *should* have the Crown Gear.
So I crawl under the car to verify using the flange vs. stub-shaft theory, to find a somewhat odd discovery. Mine definitely does NOT have a flanged connection to the rear drive shaft. It DOES have a stub-shaft output, which supports the Crown Gear idea, however, the piece between the shaft and the trans isn't typical of the drawings and pictures in this post. Instead of the silver disc/puck, mine has what looks to be a black plastic piece bolted directly to the trans casing (i.e. - it doesn't rotate). The best way I can describe it is to picture what the asterisk symbol ( * ) looks like, and imagine that six-pointed shape extruded in black plastic approx. 1-2" thick, with the output shaft running through the center. This might be irrelevant, but it's different, so it feels worth mentioning.
Side note: I would post the picture I took, but apparently I can't upload pictures directly to this site?! (Pretty sure this is my only post on AZ, forgive me for being a noob). If anybody has somewhere to host pictures, I can send you the pic to host for me!
Based on my findings, I suppose it looks like I may have the Crown Gear, which... I'm actually not sure is a good or bad thing. I understand the principle by which the Torsen T3 "locks" (biases torque is probably the more appropriate term) as input torque is applied - it's a function of the helix angle of the planet gears. Got it. Makes total sense to me.
BUT, how the Crown Gear "locks" is still somewhat fuzzy to me. I get how the crown gear diameters where they interface with the face gears gives a rear:front nominal torque split of 60/40 (same ratio as the Torsen T3), but the clutch pack actuation is the part I don't fully understand. I actually reached out to Audi multiple times on this for clarification but was unable to get in contact with any technical folks who really understood my question. What I don't get is how an axial force is developed within the diff such that the clutch packs are compressed. From my limited understanding of interaction between spider gear to side gear, ("face gear to crown gear" in this case) I can somewhat understand an axial force being applied when there is a difference in speed - i.e. - when the face gears rotate to allow speed differentiation between the front and rear axles. But if that's the only way lockup force is applied to the clutch plates, then that means you need axle speed difference to occur before the diff can start to limit slip/transfer torque/lock up. Which, seems inferior to how the T3 works, which begins to "lock" as a function of input torque, rather than a shaft speed differential. Any insight would be appreciated!
I made a comment earlier about my car having a sport differential. The RS5 comes standard with the sport diff and the crown gear, which leads me to believe there may be some correlation between the sport diff and the crown gear on models where the sport diff is optional, like the S4. Maybe the sport diff functions more desirably with the crown gear as opposed to with the Torsen T3? But then again, I believe the RS7 has a T3, and it also comes standard with the sport diff, so that would add an exception to my theory. Walky, maybe you could do some digging in ETKA on the RS7 trans (ZF 8 speed) and the sport diff? This might help us find out if there is a correlation between the two!
