[A4] Re: Quattro, Torsen, Haldex and others...

[Brizax at aol.com]

GOOD INFO THERE, SOMEONE HAS BEEN PAYING ATTENTION IN CLASS THE LAST COUPLE  
YEARS HUH? :)

NYT is  wrong!!!

On torsen based quattro system (now version 4, hence quattro  IV), the 
factory torsen is a splitted 2:1, meaning it can transfer up to 2/3  of available 
power to either the front or rear axle.  I've read many  different torque split 
figures from different sources, ranging from 40:60 to  30:70, but I believe 
the 2/3 is the most correct.  There're other  versions of torsen available, for 
example Stasis Engineering offers a 4:1  version, capable of splitting up to 
4/5th (or 80%) to either end.

All  torsen are normally 50:50, whereas the haldex quattro is either 100:0 pr 
95:5  normally, and it's capable of sending all power to either end.  Volvo  
uses this system as well.  So there, NYT got the 2 quattros  confused.

Now here's some distinction of the different versions of  traditional quattro 
systems for longitudinally mounted engines (all B and C  chassis cars, such 
as 4000, 80/90, 5000, 100/200, A4, A6, A8):

quattro  I - original version.  All 3 diffs are open, but the center and rear 
are  manually lockable.  It's very simple and reliable, but it tends to eat  
tires or cause damage to the driveline when people forget to unlock the diffs  
after they unstuck themselves.

quattro II - introduced in '88 I  believe, when torsen replaced the old open 
center diff.  The rear is  still manually lockable, but it's now operated 
pneumatically so that it'll  unlock itself once the car reaches over 25 km/h.  I 
have this in my '90  CoupeQ, it works very well, never needed to lock the rear 
diff, but this  version will not pass the "one wheel with traction only" test.

quattro  III - this was only used in the V8, where both the center and rear 
diffs are  torsen.  Since this car wasn't sold in large quantity I don't know 
how  well it actually works, but it sure sounds promising, almost like the 
HUMVEE's  system!

quattro IV - torsen center diff with open front and rear diffs,  but now it 
has EDL to keep side-to-side traction in check.  This system  combines the 
proactive nature of the mechanical torsen, with reactive aspect  of the EDL.  It 
should in theory able to pass the "one wheel with  traction only" test.

On the TT, its quattro system has no version  because it's not an evolution 
of the original quattro for longitudinally  mounted engines.  As I've said 
before, my brief drive in a TT didn't  impress me much.  I think it's mainly due 
to the completely reactive  nature of the haldex system, ie, it sends power to 
the rear until it detect  slips.  However I hear that Volvo has since modified 
their haldex AWD on  newer cars and they're supposed to be a cut above the 
original haldex  system.

To further continue the AWD topic, BMW's system is not as good  as quattro 
due to its inability of changing torque split between front and  rear, it's 
always at 38/62.  However it's brand new x-drive is supposedly  much better than 
the old AWD system, which started in '88 in the 325ix and was  never 
changed/upgraded until just now.

M-B's system is even  worse.  All 3 diffs are open and it relies on their 
version of EDL to  control wheel spin by braking the spinning wheel(s) hence 
sending power to the  ones that aren't spinning.  It's completely reactive, much 
worse than the  haldex system, and it places too much demand on the brakes.  If 
Audi  decided to deactivate their EDLs above 80 km/h due to potentially 
overworking  the brakes, how well do you think M-B's system copes with this issue?  
 Although I must say this probably won't be a problem for daily driving, it's 
 more of a problem if you really get stuck or actually to  off-roading.


Now to chip in my $0.02 on the center versus rear  applications of the torsen 
diff, they are really quite different.  Hence  if you've only driven quattros 
with torsen center diff, you really have no  idea how it would behave when 
is't slapped onto the rear of a RWD car, of the  front of a FWD car (I believe 
Nissan used to offer torsen on their 240SX and  Maxima as part of their SE 
package).  A friend of mine (240SX owner) has  told me once that having a LSD 
(such as torsen) in the rear is great in the  dry, but it does do some wierd stuff 
when it's wet and slippery, cause the  diff seems to be distributing power 
from side to side as each of the rear  wheels lose and regain traction during a 
turn.

Anyhow, hope this clears  things up a bit, or not... ;-)

Quincy
'01.5 S4 - quattro IV
'90  CoupeQ - quattro II