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

[Quincy Chiang]

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

----- Original Message -----
From: Dan DiBiase <d_dibiase at yahoo.com>
Date: Monday, December 27, 2004 3:20 pm
Subject: Re: [A4] Re: Quattro, Torsen, Haldex and others...

> --- Brian White <brian at bjwhite.net> wrote:
> 
> > This is interesting...
> > http://www.williamtoll.com/images/nytawd.jpg
> > 
> > Check that out.  Notice "can one wheel drive car".
> > The demonstration I saw this was clearly not the case with a 
> Quattro IV
> > equipped A4.  Now, given 
> > that this was a marketting demonstration, it could have been 
> rigged. 
> > But just ask easily, Audi 
> > could be overstating the facts.  I wonder. Hmm...
> 
> That IS interesting, and it shows that VAG seems to have the most 
> flexibleAWD solution out there, being the only one(s) that can go from
> 0-100%/100-0% front/rear wheel torque split, along with driving 
> only 1
> wheel. So what exactly are we debating?! ;-)
> 
> Dan D
> '04 A4 1.8Tq MT-6
> Central NJ 
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