Over at the autos.ca forums, I came across this very interesting, and very long, post from a forum member (a mechanical engineer), regarding AWD systems:http://www.autos.ca/forum/index.php?topic=91790.0
"...Meh, cheapy system [Mercedes Benz 4Matic], no centre diff and not much different from the Escape what with the single electronically controlled clutch integrated into the rear axle, to which marketers attribute magical properties and much brand chest-beating puffery. Haldex type reactor really - just different details like disconnecting the propshaft most of the time which slows down engagement time. Apparently Toyota and some Hyundai/Kias add a lock button on some models to join front and rear axles so you can try to get out of the ditch you just slid into. Palliatives to overcome the otherwise overt cheapness. Just another part-time AWD system, which I lump together as Tier 3 Smoke and Mirrors AWD. The most common kind.
My rambling comments generally apply to automatic transmission AWD systems. Subaru, Mitsubishi and Audi all made decent manual transmission AWDs for their regular performance cars, and even better versions for their rally machines. All three featured three differential systems - centre, front and rear, which is the basis for the best full-time AWD.
In the beginning, Audis had semi-decent AWD systems with three open diffs and manual centre and rear lockers in the original 1980 quattro , but it was not as good as the AMC Eagle, which pipped them all to the post by a couple of years in 1978. And various Jeeps too.
The Eagle was certainly more advanced than Audi's first AWD system - which I owned in blissful ignorance that Chrysler's New Progress Gear subsidiary had my whip all beaten by the late 1970s before quattro even emerged!
NPG had thought up pretty well any reasonable AWD gear-type technology before anyone else and developed the first mass-produced viscous limited slip devices based on the English Ferguson Formula from the late 1960s. Probably VW/Audi was far too snooty and never thought to look at what they could have bought cheaply right off the shelf from Detroit when they made that first quattro! No, surely German engineers aren't convinced of their prowess that much are they?
Maybe not, but Ferdinand Piech was in charge at Audi then and in his mind he's the Leader of the Universe. VW is trying to save $5 billion in costs annually in 2015 to pay for the MQB boondoggle, but Piech as High Priest of the VW Empire decided that Phaeton Mark II would proceed anyway. The Mark 1s cost VW a $28,000 loss per car according to research published in the Financial Times of London recently, along with speculation that Piech finally went off his rocker at age 80. Is this a man who asks others for advice? Who even imagines that Americans are good engineers? No, so a pretty basic AWD system was featured in the ur quattro and touted as world-beating.
Besides NPG, Subaru designed the first quantity-manufactured "AWD" for auto transmissions back in the mid 1980s and updated it for 1998 from part time to full time. No centre differential, just a Multi-Plate clutch Transfer system or MPT, and it's still going strong today. The company had already designed two more advanced systems earlier in the 1990s, one a somewhat better version of the other - the better one actually stuffed in a manual gearbox for their rally special STI. Again, these were just variations on NPG technology if Subaru had bothered to look them up. I've always been intrigued if all these companies' engineering reps ever got around to chatting to each other at SAE conferences or just wandered around in a cloud of their own private thoughts.
Forgetting Haldex and similar types and there's no particular reason not to because part time AWD isn't AWD by definition and that includes this particular Benz system, the first besides Subaru to ditch the centre diff in any quantity in what they thought of as a "premium" AWD system was the second iteration of BMW X-drive from 2004.
The next was Honda SH-AWD the following year, but it was not a bare bones effort. It had (has) two variable clamping force clutches, one for each rear driveshaft, hence real torque vectoring ability not relying on brake force reacting through a differential, so it dispensed with both a centre and rear diff. In fact the new Focus RS has the same system, probably the GKN Twinster which looks like a pure copy of pre 2014 SH-AWD.
To get the 70% rear torque maximum both Acura and Ford natter on about without a biasing centre differential, the transfer drive from the front has to speed overdrive the rear wheels by a few percent compared to the front. If this were not so, the best would be a 50/50 f/r split. These aren't full time 70% to the rear systems or the tires would get chewed up, or the clutches worn out, whichever comes first. Of course, if you are either a Honda or BMW fanboy and have thoroughly swallowed the company Kool-Aid, their systems are the best, and the rest are rubbish. These folks live in a bubble and it's wise not to disturb them, but it is fairly obvious that SH-AWD is a slight step ahead of BMW X-Drive or Subaru's MPT.
But at least these systems are full time AWD. The current trend like this 4Matic featured on the CLA is to attempt to shut down parts of the system in the name of fuel economy most of the time, removing the reason for having AWD, i.e. stability, in the first place unless your AWD dreams are fulfilled by the ability to jump over the Dept of Trans snow mound left by their plows at the end of your driveway.
The Chrysler 200 V6 AWD and Cherokee have the latest system of this type. Their PTO at the front can disconnect, and the clutch on each rear axle can be disconnected as well - result, the prop shaft does not turn, neither does the rear crown and pinion, just rear wheels and half-shafts. So the car goes FWD until AWD is needed and then boy oh boy, it has to ramp up prop shaft speed before engaging the rear axle clutches. I bet it's never late to the action. Who knows how good it is? It's hardly ever doing anything!
Other than that it's SH-AWD when it's actually in operation. Talk about part time! Gotta save gas and keep down parasitic losses. Don't know for sure, but might be an American Axle product. Makes this CLA 4Matic look like a high school project though. And explains why the 200 in normal driving is a ponderous FWD beast, because that is what it normally is. As I found out on a 45 minute test drive during which engine noise and that dreadful 9 speed drove me crazy anyway. Rumour has it that when you select S on the gear selector, the AWD is always on, so there's that.
The Land Rover Evoque has the exact same kind of system except made by GKN in England, including the Twinster rear axle and whatever silly name they gave to the glorified disconnecting front PTO. So GKN and AAM have similar systems, whose basis started out really as the SH-AWD concept from Honda a decade ago. Then chucked it all away by going part time.
The original BMW AWD system initially actually had a centre differential, but then in 2004, they changed to a Subaru MPT type (Subaru's inexpensive one still found in all the 4 cylinder autos except the WRX CVT which is VTD) system only backwards. One hogging big electromechanical clutch with a fixed 60/40 split up to 50/50 as the clutch is gradually locked. Difference is the Subaru system always drives the front wheels - no clutch in the way, while the BMW system always drives the rear wheels with no clutch in the way. In both cases nominal 60% to the always driven wheels. Seems to work well and in these days of misleading advertising, they are both actual real full time AWD systems. No centre diff. Of course, Subaru has TWO better AWD systems than this - you know ones that actually incorporate geared centre differentials with limited slip lockup clutches on the front to rear axle drives to boot.
BMW X-Drive, Subaru MPT and SH-AWD are what I think of as mid-grade Tier 2 AWD systems. They far surpass the dross of the reactively engaged rear wheel part time "AWD" systems, but they're hardly cutting edge.
What's left? The Lexus AWD system in IS350 and LS sedans is a proper three differential system with an electromechanical lockup clutch on the planetary centre diff. Thumbs up, Tier 1, same basically as the Subaru VTD system found in their better automatic-equipped models since 1992 with the SVX sold OUTside North America - here we got the cheaper MPT.
Infiniti is similar, proper gear differential AWD for their sedans. Tier 1.
Cadillac car AWD is just another variation on Subaru MPT type, so Tier 2. The SRX, if anyone cares, is an actual Haldex branded system. The Escalade has real AWD Tier 1.
More expensive MB model 4Matics supposedly contain centre differentials too, but reliable details are hard to obtain (MB's web site is highly confusing implying the CLA has a centre diff when it clearly does not) and advising that the descriptions might not reflect their latest technology. Well, no, actually they don't. Deceptive. Hard to rate as I don't believe they're being straight with their customers.
The Chrysler 300 AWD system is a 3 diff system, but with front axle disconnect. Give it a Tier 3 and a half, but it's really part time. If it kicks in and stays in, then it would be right up at Tier 1.
These new fuel economy standards are playing havoc with AWD systems by shutting them down for 95% of the time. It takes no genius to realize that when the sensors kick in and engage the AWD, no matter how much [censored] is expended telling you things like : "We monitor the wheels for every inch of travel for slippage, and engage the AWD as necessary" the fact remains that after the electronic decision is made, at the minimum, actual physical movement of a clutch plate has to occur. That doesn't happen in .01 seconds. More like 250 milliseconds or one-quarter of a second - or more. At 100 kph, that's 8 metres or more. If you've just hit a sheet of ice on the highway, the vehicle's stability is already compromised by the two wheel drive and perhaps soaring engine revs, and then a drag is suddenly applied to the rear wheels as AWD engages, just as you lift off the gas yourself. This is not ideal, and the reason why full time AWD, always on, will not experience this particular instability.
The newish Audi CROWN gear centre diff is a hoot from my mechanical engineer's perspective. They have succeeded in making a more expensive and complicated version of what acts like a simple planetary diff for no apparent reason other than obfuscation and to say it can be done. Makes me shake my head. Perhaps it's easier to package the included lockup clutch in a small space, but if you've seen a Subaru VTD (WRX CVT, Legacy/Outback 3.6 whether CVT or old 5 speed auto), you'd no doubt wonder if that's really the case. The Audi cannot be as robust as it is not symmetric - planetary diffs torque capability is directly related to the number of pinions. Just add more if you need to. Nevertheless, if you avoid the A3 Haldex system, Audis are Tier 1.
Of course, the Mitsubishi system as fitted to their Evos and high end CUVs is very good as well, but due to small sales, not significant. Plus nobody has ever seemed to be capable of fully explaining just exactly what's going on. Shades of their TV ads for lame cars and SUVs and the sealed room containing their AWD secrets. Probably Tier 1.
Otherwise the AWD landscape is basically littered with differential-less part time systems using slip and grab clutches of one kind or another along with brake torque-vectoring, so as to save $25 on gears in a centre diff. Not wonderful, but easy to control electronically and apparently so long as all 4 wheels turn, they'll start a vehicle on pond ice which is level, and on which kids seem to have no trouble starting either. Make that a Halifax hill three days ago after 30 mm of rain on top of snow, frozen solid in a mere two hours and then see how they work. Yes I'm cynical. A rear wheel drive 1960s Volvo will start on pond ice and be fun to drive - ask me how I know! So, this kind of test is apropos of almost exactly nothing except to the terminally credulous.
Now, put in the same brake torque-vectoring as these cheaper "AWD" systems rely on, add the usual ABS sensors for road speed, sensors for longitudinal and lateral g, steering wheel angle sensors, throttle sensor, actual gear in use sensor, temperature and humidity sensors, whether the handbrake is being yanked or the car is upside down, plus goodness knows what else, add a planetary gear centre diff with a sudden overspeed mechanical clutch locker on the front and rear axle outputs, PLUS an electronically controlled limited slip clutch squeezer on the front and rear axle outputs based on those sensor inputs for more gradual terrain or corner changes to vary front to rear torque split, put a Torsen limited slip diff in the back axle, and a Quaife rip-off limited slip differential on the front axle, and Voila! Subaru STI. That's AWD. No messing about, no qualifications, no ifs ands or buts. Including EVO and GT-R, this is the cream of AWD.
Hamburger or prime rib, they both fill you up. But cheap beef is often disguised with cheese, bacon, mustard, ketchup, a pickle, tomato slice, a bit of lettuce and a sugary bun with barely cooked dough and maybe even some special sauce. Marketing at work letting YOU know YOU bought the BEST In THE WORLD. So that you completely miss the point. That's mass market "AWD". Satisfying for average tastes.
Prime rib? Well, some folks like it with a dash of horseradish, but otherwise the flavour's just fine with no hype required. There's not much prime rib around in the "AWD" world.
Most of the AWD systems are part time cheap single clutch set-ups for the cheaper cars. You get what you pay for and included free is the most vividly imagined benefits courtesy of the company marketing department that apply unearned praise to the limited capability hardware. The only standout is of course Subaru with a full time Tier 2 system in even the cheapest Impreza.
Chrysler runs between Tier 3 and 2 by using great hardware and then disconnecting it!
The other Tier 2 systems are Caddy ATS, BMW and Acura, with Acura a bit better 'cuz it has two, count 'em TWO clutches and torque vectoring.
Tier 1 is Audi, Lexus, Infiniti, Mitsubishi, and of course Subaru VTD.
If you read this distillation of AWD all the way to here, congrats. I've owned only real AWD cars since 1988 - Audi, Mitsubishi and Subaru. Being a (now retired) mechanical engineer I have also insisted on knowing how the systems work. AWD is just another ticked box on the order sheet these days. People dully think, oh! AWD, and never give a moment's thought to it - it's sold like "Want mayo on that?".
Remember, these opinions are my own. I'll certainly stand to be corrected if you know better and can explain exactly where I'm wrong. In fact I'd like to know more. The Internet sites trying to explain AWD are in the main not very good, and I've spent a fair amount of time finding out what truth I can over the years, to strip away mystery, because I have zero loyalty to any car company…"