Audi has recently replacing the Quattro AWD system to the Quattro Ultra on their A6/A7 models. They claim the Quattro Ultra has better off-road capability(who want to off-road a A7 anyway) and save more fuel(a whooping 0.9mpg), compared to the Quattro. AFAIK, the quattro 4WD has three torque-sensing LSD.The center LSD has a base 40/60 ratio, which can change from 70/30 to 15/85 in case loss of traction or performance driving. The front and rear LSD have a max 3:1 torque bias ratio. This is a full-time AWD system. Lower-end models has a center LSD and two open diff. Electronic Differential Lock is used to control wheelspin. The so-called Electronic Differential Lock does not lock the differential, instead, they brake the slipping wheel to slow down to the non-slipping wheelspeed, wasting 50% power as heat. In contrast , diff-locks could transfer 100% torque to the non-slipping wheel. The quattro ultra has a center "split shaft" that connected to the front differential solidly, torque is transfered to the rear axle via a electronically controlled multi-plate clutch. Both the front and the rear differentials are open differentials. Two electronically controlled "locking hub" is available in the rear differential, which can disconnect the driveshaft to safe fuel. In case of off-road condition, the multi-plate clutch can from slip to fully lock up, effectively transfer 100% torque to the rear if the front has zero traction. In order to safe fuel when cruising, the multi-plate clutch and both of the locking hub are disconnected, making the car FWD. This is a part-time AWD system.
Of course full-time AWD is always better than automatic, too bad nobody dares to keep it. Muh fuel economy, muh CO2 tax...pussy ass PC world. Want to save money? Buy a fucking Polo. The whole idea of a luxury item is that you spend more money on it. That offroading talk is pure BS. Outside paved roads, automatic AWD is always inferior to full-time. It's not for offroading! Want to know how it works in the mud? The second axle only kicks in when you're properly stuck. Ok, those who never leave tarmac -a banker, a trophy wife, a vanilla gearhead masturbating to track days- won't notice anything. Those who do will, but it's gonna be too late - and it's not like they had much choice anyway. Get a Subaru before it gets pussified as well it's most likely already pussified, get a Land Cruiser. Or a Niva.
The above poster apparently got out of bed on the wrong side this morning. And while overall I'm inclined to agree that id generally rather have a full-time setup, they aren't factually correct. Part time setups, it's all down to implementation detail. Full time setups, it's all down to implementation detail. A viscous coupled limited slip differential actually doesn't respond instantly, or divert all power where it needs to go. It can and absolutely will fail in some off road scenarios. A clutch type limited slip differential also has a response time, though can at least divert power where needed, but these are super rare in production vehicles and need a lot of maintenance. A torsen differential, utterly useless as when unloaded it acts as an open differential. In an electronic setup, you can respond as quickly as software allows, which in mercedes is really bad response times, it lets the wheels spin a full second, but in some other vehicles it's literally microseconds of response time,*faster* than the viscous coupling ever would be, hell, faster than the clutch type would ever be, the only thing faster is the torsen, which acts open when off-road anyway. So no. I'd disagree with the above, it's just someone being elitist because they haven't a clue how the systems work and therefore it's bad. But still I have seen really bad implementations of electronic setups, and for that reason would prefer the full time. Also. Subaru has already gone electronic (though not always part time) --- Post updated --- As an answer to the question, I'd say neither, depends on if they did a good job or not with the newer system
Subarus seem to have different types with different transmissions: https://www.awdwiki.com/en/subaru, although I won't be surprised if they got rid of full-time completely on the Mk7 Legacy/Outback. Anyway, fixed my previous post. I wouldn't say the Torsen system is worthless, it is what old longitudinal VAGs use and they perform very well offroad.
You are right, Subaru is already pussified. The 2018 Impreza STI no longer has front and rear LSDs. --- Post updated --- The thing is an Audi is a sports(or sport-ish) car, neither an econobox or an off-roader. The AWD system should be designed to improve performance on paved road. In a VLSD, the higher the speed differential, the higher the slip limitation, therefore turning off ESC may be beneficial if you get stuck. Because of the Preload of the CLSD, the vehicle will tend to understeer with zero throttle - not good for performance driving. Torsen differential, in contrast, works best on pavement where all wheels have traction. But you need a lot of brake tap(or traction control) when off-roading. The wheelspeed differ from the vehicle speed because of tire creep The higher the surface friction and the heavier the weight, the more of the adhesion area and vice versa. When cornering with little throttle, the outer wheels rotate faster than the inner wheels and the front wheels rotate faster than the rear wheel. When cornering with high throttle, the inner wheels rotate faster than the outer wheels and the rear wheels rotate faster than the front wheel, because of the tire creep and slip angle. If open or viscous differentials are used. Clutch-based differential or torsen differentials will lock the inner wheels to the outer wheels at high throttle, and more torque is transferred to the outer wheels - good for performance driving. The "split shaft" + electronically controlled multi plate clutch in an interesting thing. Because the front wheelspeed is the same as the rear wheelspeed in a stright line, the front wheelspeed is faster than the rear wheelspeed in a corner. The TCU(transmission control unit) could increase clamping force in a stright and decrease clamping force in a corner, in order to keep the front/rear bias nearly constant. Subaru did this on lower-end models. The problem is that this very system cannot send more than 50% torque to the rear when cornering with full throttle, where the rear wheels rotate faster than the front wheels. The solution would be that the shaft is always connected to the rear, and the clutch controls the torque to the front. However the fuel consumption is worse. The Quattro Ultra could do FWD at hypermiling or fuel consumption testings, and do AWD at performance driving. In terms of the front and rear differentials, there are two types of electronically controlled. 1. Electronically controlled multi-plate clutch. 2. Brake vectoring. 1. This type of system behave likes an open differential when the outer wheels are faster than the inner wheels, and clamps the clutch if the wheel sensors detect that the inner wheels are(or about to) rotate faster than the outer wheels, and more power is transferred to the outer wheel. The wear of the differentials is lower, compared to the CLSD or TS LSD. 2. This type of system brakes the inner wheel when cornering, bringing down the inner wheelspeed to the same as the outer wheelspeed, and more power is wasted as heat(not transferred to the outer wheels). And could be regarded as a factory tuned ESC&TCS. Since ESC is mandatory, the added cost is zero. If the tuning is very aggressive, the inner wheels could be slowed down to lower than the road speed, leading to an intentional oversteer, improving handling.
The Q and Allroad ranges are certainly all-terrain cars. Remember - in some climates, offroading doesn't always mean "trail expedition", but often "the snowplow didn't come today".
Don't drink and write. They're executive cars, the idiot here is whoever thinks they're for offroading. AWD is only there to help with some slippery roads, snow or stuff like that. Also, thinking about saving money is something a lot of people I see could use.
Does knee-deep snow on an unplowed street qualify as offroading? Maybe not, but does require some offroading capalities. Also if you have bought something from the Q or Allroad line and never leave paved roads, you're a sad person - or an Eastern European semi-criminal who wants everyone to know who he is.
Or you just want higher seating than in the normal Audis. Either way, ordinary Audis also come as Quattros.
viscous LSD do have response time But clutch or torsen response instantaneously, much quicker than electronic system. This is simple law of physics. When speed difference reversed friction torque is also reversed. During heavy cornering with wide open throttle. The inner wheel tends to spin faster than the outer wheel. A mechanical LSD reacts at the same time(0.00ms) as wheel spin occurs, immediately transfer torque to the non-slipping wheel. Electronic systems only reacts after the sensor pick up the wheel spin signal. Also lots of automaker tend to blurr the difference between torque vectoring (Mitsubishi ACD&AYC) and brakes vectoring (Audi EDL)
Torsen responds instantly, clutch type doesnt always, plus clutch type needs maintenance leaving it unsuitable for most road users. Wheel speed can also be measured over 10000 times per second (I have equipment capable of more) though some bad implementations only use 5 times per second measurement, some better ones are 1000-2000 which is adequate enough that with suitable actuators for the braking (also a sour point on cheap implementations) and logic performed on an adequately fast processor (the calculations aren't hard and a netbook from years gone past can keep up with them) you could actually match the clutch type LSD, with support for more flexible setting and adjustment, and with less wear and tear. The downside. Nobody bothers. It is absolutely possible for electronics to achieve better realistic performance than pure mechanical setups. It's just never done. It's even possible to incorporate load cells into the wheel bearings and measure the torque driving the wheel. But it costs engineering time. Brakes vectoring across an open differential with an adequately tight control loop achieves torque vectoring as is though, the distinction isn't that important
That's why I don't "favor" mechanical system but post a discussion instead, because both types have advantages and disadvantages. Edit: I didn't cast my vote and was expecting a 50%:50% vote, looks like I'm wrong. The distinction is that torque vectoring increase power to the wheels at a given engine power while brakes vectoring decrease it.
