There are several types of AWD such as FWD base AWD(RX350 AWD), RWD base AWD(4Runner AWD), AWD full time(LX470), AWD Selectable(Sequoia 4X4), & AWD quattro(Q7)...
What are their differences? Do they have different performances? Which one is better?

 

Generally, a RWD-based AWD system is preferable to most FWD-based systems because in RWD-based systems, the engine sits underhood longitudinally, with the output shaft pointing straight back ( or, in the case of RWD / AWD Porsches, straight forward ) where it bolts right on to the transmission and AWD differentials without having to have extra bevel gears to re-route the torque around 180 degrees and back another 90 degrees to the AWD hardware. This, of course, makes it not only easier and cheaper to produce, but also to work on and replace components whe they wear out or need repair.
The ubiquitous AWD systems that Subaru uses ( which are considered the best AWD systems under $35,000 ) are unique, and are kind of like the Porsche systems in reverse. Subaru used the same flat-4 engines, with the rear-pointing output shaft, in the old FWD cars that they quit selling in the U.S. market in the mid-90's......a system that made their old FWD designs unique also....and less complex than many competing FWD systems as well.

As far as how the FWD-based vs. RWD-based AWD systems actually perform, there are too many different variables to make a blanket statement or a prediction across the board. Acura's SH-AWD system is superb in many ways.......perhaps the best one in production today; even better then Subaru's ......but it is very complex and expensive to produce.

One blanket statement, though, CAN be made about AWD. All other things equal.......( which, of course, they rarely are in autodom )........ANY AWD system will have slower acceleration and worse gas mileage than its RWD or FWD equivilant, simply from the extra weight and drag of the AWD components.
The new Suzuki SX-4, AWD system, which I have praised very highly, ( see my thread on inexpensive snow cars and my review of the SX-4 ) gets around this to some extent by having a dash-operated switch that allows you to disengage the AWD and run it in FWD only when you don't need the extra traction.....a first for small cars and car-based AWD.

Now.....DON'T confuse AWD ( All-Wheel-Drive ) with 4WD ( Four-Wheel-Drive ), especially Part-Time 4WD.

AWD ( or Full-Time 4WD ) has a center differential or other device to allow all four wheels to turn at different speeds independently of each other and can be used on ANY surface.
Part-Time, or traditional 4WD, confined today mostly to Ford and Dodge pickups and some cheap entry-level SUV's designed for off-roading like the Jeep Wrangler, is a simple, rugged, durable system that locks up all four wheels at the same speed and prevents them from turning independently of each other. This is fine off-road or on slick surfaces but can't be used on a dry surface, therefore greatly limiting the vehicle's flexibility.

Well, you may ask, why would you be concerned with a dry surface if you are using a part-time system in or after a snowstorm? Simple......because even during or after a snowstorm, you are not ALWAYS on a wet or slick surface. A plowed road, especially after the sun hits it, can be slick one moment, wet the next, and dry the next, and of course it will be dry under bridges and underpasses.
A part-time 4WD system is fine, even on dry pavement, as long as you are going in a straight line, but go around a DRY curve and you can screw up the tires and drivetrain because on a dry curve the outside wheels naturally want to roll faster than the inside ones, but the part-time 4WD system doesn't allow this......and the results and stresses involved on the vehicle are obvious.

 

As always good post.

 

Just curious, which are the best AWD systems out there then (ranking)? I've heard a lot of talk from buddies about the Honda SH-AWD system, etc., and in the above post it receives praise. Does the Lexus/Toyota system rank well here? The Audi Quattro...?

 

I think Honda's SH-AWD might be the most sophisticated and capable.

Quattro is certainly completely tested and reliable.

Subaru of course has excellent experience in this area as ALL their vehicles offer AWD.

 

I would say that Subaru has complete tested and reliable systems as well =)

 

In the under-$35,000 category, Subaru clearly does the best AWD.....for reasons which I have explained above. ( Subaru actually has 5 different center-differential systems depending on engine and transmission )

In the over-$35,000 category, Audi used to have the best systems ( Quattro ) but in the last year or so has clearly been superseded by the superb Acura SH-AWD....complex but currently the best and most sophisticated system today on mass-production cars.

 

What about ATTESA? or Mitsu's AWD system on the Evo that can also shift power side to side like SH-AWD?

 

It may or may not be out-of-date since it's from 1998

Part-time 4WD
With a part-time system, you stay in two-wheel drive most of the time. You engage four-wheel drive manually, either by moving a separate gear lever or an electronic switch on the dashboard. That control is connected to a "transfer case," a secondary transmission that transfers power to one or both axles. When you're back on dry road, you have to disengage the system again.

Part-time systems essentially yoke together the front and rear axles. That provides excellent traction but isn't suitable for use on dry pavement: With the front and rear wheels unable to turn at different speeds, one or more tires will drag if you try to corner. That doesn't matter so much on dirt or snow, but on dry pavement, it can damage the tires and the driveline.

Low Range
All part-time and some full-time systems have a "low range" position on the transfer-case selector. Low range provides very low speed gearing and high torque for the most dire conditions, such as deep mud, steep inclines, or when you're actually stuck.

Full-time 4WD
Full-time four-wheel-drive systems can be used on any surface at any speed. They have either a third, center differential, or some equivalent mechanism, that allows the front and rear wheels to turn independently when you're cornering.

"Selectable" full time systems are like part-time systems, but more versatile. You can select between two-wheel drive and part-time and full-time four-wheel-drive modes. And you have low range available for off-road use. Full-time mode ets you travel at highway speeds on dry roads. Locking the center differential can give better traction than full-time mode in especialy difficult circumstances. A few do this automatically. Most make you press a button or select "4-High" on the transfer case.

We'd leave such a system in full-time four for all normal driving. It's safer, and our tests show no fuel-use penalty from doing so.

Permanant four-wheel drive may be the best of both worlds. The vehicle stays in four-wheel model at all times. In especially tough conditions the center differential mechanism locks and unlocks automatically as needed. And you can still select low range for off-roading or if you get stuck

All-Wheel Drive is used in sedans and minivans and some sports cars and SUVs. The system is active whenever you're in motion. You don't have to select anything.

With car-based all-wheel-drive systems such as those used in Subarus and the Toyota RAV4, most of the engine's power normally goes to the front wheels. Progressively more goes to the rear if the front starts losing grip. These systems are not intended for rugged off-roading. There's no low-range available, and besides, there may be too little ground clearance for off-road use.

 

i would figure the haldex awd system in the bugatti veyron would be the best since it has to deal with 1000 hp.

 

best as in what? Unfortunatly, due to old technology used in Subaru AWD systems, without any electronic assistance, it is one of the worst performing systems in ice/snow/mud/rain, when it comes to handling, stopping, braking and accelerating.

A bit unknown in US, but for instance IADS part-time AWD system used in new Rav4 is currently most technologically advanced system in its class and got perfect reviews in Europe due to VDIM-like technology integrated together with AWD system. This was Toyota's first true active AWD system, as previously they were mostly fixed 50-50% with LSD distributing the power. It is much better than first generation systems because of advanced sensors that engage it so fast that you dont notice it (while on older systems it was quite annoying when you get, delayed, thug at the back).

In recent german Autobild tests at snow/ice/mud/wet roads, Subaru AWD performance was rated lower than 3x cheaper cars because of no electronic aids that simply left the car uncontrollable in extreme situations.

Audi is probably the most known manufacturer in the world when it comes to AWD because they have been marketing in for a while and many of their cars are sold as AWD. Subaru is well known in Japan and USA for its AWD systems.

And of course, Toyota makes more AWD/4WD vehicles than Audi produces cars total :-).

BMW is also improving their systems a lot, and , again, in recent Autobild's tests their xDrive was rated better than Audi's system - although I am not sure if they compared 5 to A6 or 3 to A4, I forgot.

So at the end, good rule of thumb is that newer AWD systems are better than older ones due to more technology used which effectivly makes it work better than before. Outside of race track, where you might want to control your car yourself, ESP/VSC assisted AWD is consdirably better.

 

not sure about that..

Most reviews and personal testomony of subaru say they are some of the best car at any price in ice/snow conditions

Here is a admiditly biased video showing various Subarus doing very well especially on the 'hill climb demonstration':

-------->>>>>http://video.google.com/videoplay?do...42340359932162

 

Here is an article on Quattro, one of the best AWD systems on the market: http://en.wikipedia.org/wiki/Quattro...re:_Haldex_AWD.

 

this statement does not make any sense, you say '3x cheaper cars' while the most expensive Subaru is only in the $30s so a car that is 3x cheaper means a car that costs ~$12K (?) hows that now?

 

What you are quoting here has to do more with tires than the different types of AWD. I've already covered that in other posts.

Read my earlier post. I said that up to now, in the over-$35,000 category, Audi had what was probably the best AWD system ( Quattro ) until the complex but superb Acura SH-AWD came along for 2005-2006.