Volvo or Audi or BMW

[al-gorithm]

I was just looking for general opinion and realize that this is a Subie site but I wanted to know if I chose correctly for the best AWD between Subaru, Volvo, Audi and BMW.

 

Before I bought the Legacy, I was choosing between the S40 T5 AWD, A4 Quattro and the 330xi, all 2005 except for the BMW.

 

Do you think I made the correct choice in AWD systems? Can anyone who has owned another manufacturer's AWD system please comment with a comparison?

 

I know I got more power on paper and with weight:hp all would be close except for the A4 but that played a lesser role in my final decision than AWD.

[Driver72]

Only the Audi Quattro system is as good (some say better, some say a bit worse)

than Subaru's.

However, Audi just updated their Quattro system for the new cars.

Don't remember exactly what they did, but it's suppose to be even

better.

 

For the money, you made the right choice.

All the other cars would of cost you more and you would of gotten less performance

(marginally less in the BMW) however.

[Deer Killer]

Don't remember exactly what they did, but it's suppose to be even better.

I would think I would have read about it.. perhaps a new marketing term for EDL?

[gt_ltd]

i see a debate coming

[IwannaSportSedan]

Personally, I have never been completely sold on Electronic Brake Distribution in place of an actual limited slip differential.

 

Seems to me that Audi has recently gone from a 3-torsen limited slip drivetrain in the A4 to a limited slip center, open front and rear differentials, with the computer applying any one brake to send power to the opposite side of the axle.

 

The transverse-engine-based AWD that Audi uses is probably just a re-branded version of VW's 4-motion haldex system, which basically ammounts to rear wheel assist. 0-5% up to 50% torque sent to the rear wheels.

 

The subaru has a less complex, yet more direct system. Viscous differential in the center, same in the rear for the GT models, Open front differential, no messing around with the brakes. Technically 3-wheel drive from a standing start (front diff being open...)

 

but the Audi quattro system, IIRC, with the open front and rear diffs are 2 wheel drive, front and rear on the same side, until slip is detected, and power is shifted around by the brakes, all controlled by electronics. With VAG's reliablility track record of recent years, I would be skeptical.

 

Short of a QuadraTrak Jeep, 4-runner, Land Rover vehicle with locking LSDs and multiple gear ranges, I think the Subaru systems are about as good as car-based AWD gets, certainly for the money. Certainly nothing to be unhappy with.

 

I have been reading about things like LSDs, and such, and although I am not an expert, I think if I had the cash to pretty much build to taste, I would set up the subaru's AWD with the Cusco Tarmac Gear, which changes the torque split from the standard 50:50 (manual trans) to 35:65 F/R, leave the same center diff, or use a VTD center in a manual, if it would work. (or full-auto DCCD, no manual settings, which may be the same thing.)

 

I would then add Quaife ATB Helical torque-sensing front and rear limited slip differentials. More info on the Quaife ATB Here. I have a torsen in the Miata (same principle, different brand) and I love it. No viscous or clutch behavior, no wear, just a tight drivetrain that maximizes traction from the tires.

 

Quaife lists applications for the WRX front and rear, here, but their applications page says they don't work for the STi. What does that bode for the Legacy? The center and front differential on an LGT is likely in common with the WRX's 5MT trans, but how about the rear diff? is it in common with the WRX, or the STi, or what else might be the mitigating factor that would make the difference?

 

I've also read the Cusco page about the Tarmac Gear. And am curious about how they really work. It warns that it requires limited slip front and center gears, and the tarmac gear itself is not the limited slip gear. How does the tarmac gear interface with the center limited slip differential, since it appears not to just replace it?

 

Anyway, back on topic, I would certainly think that you made the right choice, considering the price of the BMW Xi, and the ultra-noseheavy nature and questionable long term reliability of the Audi. At least the Subaru's engine is short, and very low, even though it does extend forward of the front axle. The Subaru makes more power for less money, and nice looking, which can't be ignored.

[al-gorithm]

IwannaSportSedan -

 

Thank you for the indepth analysis. I was never sure how other manufacturers systems work. Does BMW and Volvo work similar to Audi's?

 

Anyways, you helped me feel even more comfortable with my decision.

[IwannaSportSedan]

I am not quite sure about BMW's X-drive. I have a feeling that is a viscous center differential, with open front, maybe open or LSD rear differential. Probably integrated with electronic brake distribution. I haven't looked much into the particulars, as the cars are beyond my price range, and don't offer manual transmissions for the most part.

 

Audis (except the TT/A3) and Subaru both have their center and front differentials integrated into the transmission. (transaxle, actually, since it is between two driven wheels.) This puts the front drive axles behind the engine's flywheel, and necessitates almost all of the engine's mass in front of the front wheel's centerline, making the cars front heavy. Subaru less so than Audi, due to the size and center of mass of a flat 4 or 6, compared to Audi's tall I4, V6, or V8.

 

The difference between Audi/Subaru, and BMW's X Drive, but don't quote me on this, is that BMW's with their engine arrangement, and wheelbase, since the car is RWD first, with AWD added later, and the front wheels forward, probably has a separated front differential.

 

BMW's can have the separated differential is probably set in front of, or at least beside the engine block, allowing the engine to be above, or the center of mass behind the front axle. But it means more drive shafts, more space requirements, and some sort of jack-shaft through the oil pan arrangement, if the differential can't be put far enough forward to be in front of the engine block.

Then there can be torque steer issues with unequal length front half-shafts, or a jack-shaft arrangement again, where one side of the differential is extended, and the other isn't, to offset the differential, but still provide equal length half-shafts.

 

Since BMWs (except maybe the X5) are designed as RWD first, they want to push the front wheels as far forward as possible, for weight distribution and handling. That requires the AWD to be made more complex to fit the car, rather than designing the car to fit the AWD layout. (such as Audi and Subaru.) With more driveshafts, and separated differentials, it adds more weight, complexity, more seals to leak, more parts to break... More expensive to build and repair, in the case of most luxury cars and premium SUVs with AWD.

 

Light trucks use a heavy duty system of a similar layout, with simplified, robust components, such as live axles, and straight-engagement transfer cases, rather than a center differential.

BMW and Mercedes cars use a lighter duty, street-oriented layout that was pioneered by 4x4 jeeps and trucks. Engine->Transmission->transfer case or center diff->splitting to separate front and rear differentials, which each split to the front and rear wheels, respectively.

 

As for the Volvos, as well as most Ford/Mazda AWD vehicles by association, and Volkswagen's 4-motion setup, which is adapted to the Golf-based TT and A3 Audis (transverse engine and transaxle, with diverted power to the rear.) run using a "haldex" or similar system.

There is a torque coupler between the front differential and a rear drive shaft, which diverts some power rearward, and through a typical, if sometimes small rear drive layout. Basically it adds rear wheel assist to a front wheel drive car. It helps, but it still inherently behaves more like Front Wheel drive. Fine for point A to point B with more traction than strait FWD, but not particularly robust, nor suited for high performance.

 

Most of the time those either run full Front Wheel Drive, or a high forward bias, like 90%, and only sends more torque to the rear end when the front wheels slip. Most transverse engine AWD vehicles, even some car based smaller SUV's run similar systems.

 

BTW, thanks for the compliment, and I am glad I could help. Hopefully I am not confusing the issue for people. Mostly just trying to give a bit of an over-view, there are members of this board who know more about these systems than I.