snowbird

Hi. I was just wondering what people thought about the new BMW system that is just becoming available. I was thinking of driving both and comparing them. I've been looking for comparisons. Comments? Thanks.

Ron_C

2.5 liter engine Vs a 1.8 liter engine? Didn't we do this kind of thing already today? (S4 v IS300)

CraigB

I think snowbird is asking about the difference between the AWD systems.

jeshuah

Hi snowbird,

I was wrestling with this same decision last week, so I can give you a brief summary of what I was able to dig up on the AWD systems of these two cars. Please take this with a grain of salt because I am certainly not an expert in this area.

First the BMW: from what I can glean from the information on bmwusa.com, the 325xi AWD system is lifted from the X5 and essentially uses the ABS system to redirect power away from slipping wheels. Each of the front and rear half-shafts are connected to open differentials, and these differentials are linked through a fixed central transfer case with a 38%/62% split front/rear. As far as I know, this means that the rear wheels always get 62% of the power and the front wheels always get 38% (and they picked this split based on weight transfer characteristics during acceleration). Then on each axle, the open differential (which would normally allow a slipping wheel to spin freely) can be braked by the ABS system when it detects a spin, transferring power to the other wheel on that axle. The upshot, as I understand it, is that (a) if both rear wheels are slipping you can only get 38% of your normal power applied, and (b) if you're doing any prolonged driving on slick surfaces (imagine a snowy mountain pass) then you can potentially overheat your brakes.

The Audi system, on the other hand, uses torsen differentials. My understanding of this system (and I'm sure many on this forum know better than I) is that it uses a mechanical (torsen) center differential to vary the amount of power delivered fore/aft (and is 50/50 in no-slip conditions). This means that 100% of the torque can be sent to either the front or rear wheels, whichever have traction. I'm not too clear on how the front and rear differentials on the Audi work because the info on audiusa.com is sketchy at best -- either they are open like the BMW and use the ABS traction control to transfer power, or possibly some sort of viscous coupling -- I really don't know. But what I took away from my research is that the Audi Quattro is far more sophisticated than the BMW AWD system, and this ended up being a large factor in my decision to purchase an A4 over a 325xi.

Hope that helps! And if anyone can fill in more info about the quattro please be my guest.

-Jesh

Audiboy

The bimmer uses viscous coupled center differential, hence not fixed, but biased towards the rear, hence up to 30% or so of power can be directed to the fronts. The rear differential is limited slip, which will allow one side to spin a certain angle before the other grips, plus Electronic traction control which uses the brakes as you mentioned.

The Audi Quattro IV (I think it is 4th generation now) on the 90, A and S bodies (not TT) uses a torsen center diff which can transfer as much power as 75% (or 80 I think) front to rear, but maintains a mechanical balance of 50/50. The rear is an electronically lockable center diff (Quattro IV feature) and no, you cannot get upto 100% power to one wheel. Also added are electronic traction control and electronic stability control which uses the brakes, as well as the drive by wire systems control the throttle to minimize wheel spin.

So, both systems are good, the bimmer uses a system to keep its oversteering properties which could be advantageous in performance driving, but I like the mechanical surefootedness of the quattro system for all weather driving

Graves

> The rear is an electronically
> lockable center diff (Quattro IV feature)

Wrong. The rear and front are open diffs. The brakes transfer rotation from side to side. EDL is a misnomer.

Audiboy

right. ooops. Dang, no wonder I can't locate the wire connector

Graves

The xi system is a very simple permanent 4WD, with 3 regular differentials but no Limited Slip Differentials or differential lock-up at all. ALthough this may sound inferior to Quattro it MAY not be. Having 3 open diffs may provide better feel when on the track, etc - i.e. no Torsen bind.

Graves

Chip F.

The center diff on the 325 & 330xi is an open diff not a viscous coupling. The older 325ix (late 80's early 90's) had a viscous center diff.

The thing that made my decision was the the A4 was designed for 4wd, the BMW is a retrofitted rear drive car. Did you know the front axle goes thru the oil pan on the Bimmer? Sounds like trouble to me!

EricB

So without the ABS system, such a car would be STUCK if ONE wheel was on wet grass!