Here is the link<ul><li><a href="http://www.edmunds.com/edweb/editorial/tech/wheeldrive.html">What Wheel Drive</a></li></ul>

The author only discusses that most AWD are primarily FWD until the front wheels slip. Quattro on the other hand transfers torque 50-50 to front and back. Also, any idea the delay in transfering power in other AWD's as opposed to quattro?

<br>This article doesn't cover quattro at all. The only awd systems it mentions are viscous coupling based or similar. Also, what's that about the center differential locking the left wheels to the right wheels? Someone doesn't know what they're talking about. It sounds like something patched together from other sources without really understanding what's going on - for the awd part. The rest of it is OK, but not exactly in-depth.<p>This does not pertain to quattro. If you're thinking of buying an awd minivan maybe. They make it sound like the horrible fuel economy of SUVs is due to 4wd, instead of sheer mass and aerodynamics. Just like you can't compare a Dodge van to a BMW (both rwd), you can't compare a minivan or SUV with quattro.<p>Isn't this in the archives? It certainly doesn't deserve being posted twice.<p>Todd W

Forgive my ignorance, but how is Quattro more advanced than the AWD system which he does describe? Before you send me to check the archives, I will check the FAQ page to see if there is anything there. <p>Obviously, the weight and fuel consumption penalty is there. The A4 2.8 Quattro weighs 230 lb more than the FrontTrak and gets 1 and 2 mpg less mileage in city and highway, respectively. Now a couple of mpg's may seem insignificant, but it is a strike against the A4 Quattro when you compare it to a RWD BMW. And as the article mentions, the FrontTrak A4 would be at a disadvantage against a BMW when it comes to handling and acceleration.

<br>A fwd car is lighter than a similar rwd car (irs), which is lighter than a similar quattro. However, big solid axles and transfer cases are the really heavy 4wd parts with more drag. The quattro system is just more of the same lighter parts with similar drag (not including TT Haldex system). In fact, Audi says that the reduced rolling resistance of 4 tires under power makes up for the friction losses. I am content to say that it might cost 2mpg, it is more a matter of driving style. You do notice the difference on acceleration because the rotational mass is higher, which probably doubles the rotational inertia(you do the calculations including the wheels, I'll just use the back of an envelope method).<p>Quattro does not have to be more advanced to be more effective, there have been few changes since 1980 (torsen, traction control). The systems they mention are heavy and/or have a lot of drag. Viscous couplings or computer controlled clutches are like having an unlocked torque converter all the time. Porsche tried what was arguably the most advanced awd system ever in the 959 and ended up deciding that it wasn't worth the effort/expense. Torsen diffs were slow to become popular because no one wanted to pay the royalties.<p>And yes, a 3-series will handle better than a fwd A4. It will also beat a stock A4 quattro on the track most of the time. It reall comes down to what you want to do with the car. Read my post below about the difference between quattro & BMW, I think that you'll like it.<p>Todd W

No I wasn't going to send you to the archives. Yes, I agree there is a weight and fuel consumption disadvantage at the tradeoff of having quattro. My understanding of quattro is that you have power going to the front AND to the back ALL THE TIME. Edmund's discussion of the AWD is the type where power is delivered to the front only EXCEPT when there is a loss of traction. Then power is transfered to the rear. What's the time delay until the power is transfererd? I don't know. Since the quattro delivers power to the back ALL THE TIME, it handles much like a RWD. When there is a loss of torque at one of the wheels (ie loss of traction), torque is almost INSTANTLY transfered to the other wheels. I could be wrong about this.<p>IMO, if you live in an area where it rarely rains or snows, you probably don't need/want quattro.<p>Drew

My Quattro has been getting between 31 and 32 mpg on the highway at speeds ranging from 65 to 75 mph. AND if all I cared about was how fast I could go, or how many BMWs I could blow off of the road I would get a real muscle car with 2 Holley's and a blower. The A4 is the best example of both form, finish, and function that I could find. I think I'll keep 'er.

Ali --<p>See Eliot Lim's excellent F.A.Q. article for an "Introduction to All Wheel Drive systems." On a different topic, I read the other article you mentioned at Edmund's, comparing the BMW 540i sport 6-speed to the Lexus GS400 and the Cadillac STS. Here's an excerpt:<p>One driver even had a hard time outrunning the Seville, thanks to the balky transmission and heavy clutch. What does this mean? Well, it means that our test BMW had some kind of mechanical problem, or that we don’t know how to drive, or that a gaggle of enthusiast magazines have been selling us sunshine and fresh air for a couple of years.<p>I would vote unhesitatingly for option #2. Or more probably, an unstated fourth possibility: they never actually drove any of these cars. As I stated in an earlier post (since archived), I used to test drive and write car reviews for print publications, and the 540i sport 6-speed is without question the finest 4-door sedan I've ever driven. The stick was the proverbial butter knife, and its availability (unique for the U.S. in the 5-series price category) is one of the car's main attractions. So far, Edmund's web-based articles have an awfully low batting average. But thanks for putting me on to them anyway; I love getting all riled up over incompetence like that!<p>-- David F.<br>1.8TqMS<br>E30 325is<p><ul><li><a href="http://www.eskimo.com/~eliot/awd.html">Introduction to All Wheel Drive systems</a></li></ul>

What matters is what the system does.<p>As far as I'm concerned, if the system can drive any of the wheels, it's 4WD. The difference is in how the power is distributed. <p>With a typical pickup or Jeep, the front and rear axles are locked into turning at the same speed. This is what causes axle bind on dry roads and what causes these vehicles to handle poorly in snow(though they won't get stuck).<p>Full-time 4WD as found on some SUV's like Jeeps adds an open, lockable differential to allow differing axle rotational speeds. This is essentially what the original Quattro system was except that Quattro had no 2WD mode. Take a look in an old Quattro and you'll see the knob to lock the center diff. So, since the Quattro system has completely changed over the years, there is no Quattro system. In fact, currently Quattro means two different things if you look at the A4 and the TT or A3.<p>Finally, what's typically referred to as AWD, has a device that will distribute torque unevenly between the two axles. In the case of modern Quattro, the device is a torsen differential. In the case of Subaru and others, it's a viscous differential. Still others have a clutch.<p>For high-performance driving, the Subaru system is probably the best(gasp!). The torsen in the quattro system has some inherent disadvantages that can cause problems on the race track(which I'll discuss if somebody wants me to).<p>There are at least some vehicles that have all of these systems combined in one. The Mitsubishi Montero has a 2WD mode, plus an AWD mode(viscous), plus high and low 4WD with a locked center diff.<p>Finally, one of the things that makes Quattro(as it exists in an A4, A6 or A8) so effective is not the torsen but EDL(technology also used by Mercedes).

Viscous frives suffer on the track from heat. Slippage generates heat which changes viscosity and torque split. <p>It is not marketing BS. AWD is 4WD but 4wd isnt AWD, at least all of the time. Get it? And I beg to differ on the Torsen. It is the heart of the system as it has no controls other than its inherent design.