For the record, I've never owned any car that had more than two wheels driving it, and most have only been able to spin one wheel at a time. However I've gotten to drive a fair number of different systems.
Originally Posted by ZeroGravity
One thing to consider is AWD vs "full-time" AWD. Many of the above mentioned vehicles are essentially FWD cars with an electronically activated clutch when the front wheels start to slip. AWD purists (Subaru and Audi owners) would argue that anything less than full-time AWD is not really AWD. I personally own own of each type (Audi A4 and Volvo XC90) and have owned several Subarus. I don't get too bent out of shape about the difference, and in fact in a bigger vehicle like the XC90, I like the idea of not driving all 4 wheels when it isn't needed to save a bit on gas mileage. Besides most clutches like the Haldex 2 in the Volvo transfer power in less than 1s and the Haldex 3 in the new Land Rovers is even faster apparently.
full time AWD, 4x4, Four wheel drive, part time 4x4, full time 4x4. Too many terms with too many meanings to too many different people.
Part time 4x4 where you have to engage it is like full time all wheel drive when it's engaged. Most AWD are self-engaging part time systems.
Let's ignore the labels and describe the systems, and I will tell you what I guess is the purist find wrong with the self-engaging type systems.
Full time 4x4 AWD whatever you want to call it has a differential connecting both sets of wheels to the engine at all times. It could be an open differential allowing wheels to turn at different speeds at all times, or a lockable differential that you could lock by flipping a switch so that front and rear wheels are forced to rotate at the same rate. This (open differential connecting all wheels to the engine at all times) is the easiest one to drive fast in tricky conditions, because there are no surprises.
A front wheel drive system that automatically engages rear wheels when the front wheels start to slip. Well you can adapt to it, but it wouldn't be my first choice. The problem with this is that say you are taking a corner as fast as you dare (and you dare a lot), you are right on the edge of traction, not because you are a klutz and you have your foot on the floorboards, but because you are going quite fast for the corner and you are feeling your way around or maybe you just didn't realize how sharp/icy it was or made some other driver error and entered the turn too hot. The system detects a loss of traction at the front and switches power to the rear, enter additional torque to the rear wheels, front end lift and understeer into ditch. Or you detect the system induced under steer and remove your foot from the gas, enter trailing throttle oversteer into ditch. If you lucky, you get to go a few rounds with the system and make it around the corner. However this system works pretty well if you just have way too little throttle control and that gets you in trouble.
Then there is the limited slip differential that everybody who has had a 4x4 stuck in the back 40 with one wheel spinning at the front and the other wheel spinning at the back thinks is a very good idea. It's problem is that the back end will tend to break traction and sqiurm around when climbing very icy hills which can be disconserting when there are cars on either side of you. With out it you might be spinning a wheel, but the other wheel keeps you straight.
Audi used to have a very good driver controlled system. That was eons ago, I have no clue what they have now.