Originally Posted by douger
Of course power is also a factor, in that it is much harder to spin an underpowered car. That being said, I have driven the 328i (RWD) and it also slides around during aggressive driving. The fact that the 335xi is faster than a 335i (according to BMW) highlights this point. Unless you have installed a LSD, the BMW RWD cars are really "one wheel drive" -- as opposed to an xi where you have power going to all 4 wheels.
No, you don't have power going to all 4 wheels unless you are driving perfectly straight with all four wheels having identical traction. The sames goes for RWD cars, they have power going to both rear wheels in equal amounts, except when there is an imbalance in traction.
Or, when one wheel is spinning faster than the other. This happens all the time, to both RWD and AWD cars, no matter how sophisticated the system. Every time you make a turn, the outside wheels will spin faster than the inside ones. This is the reason for the differential, and where it gets its name from. If the wheels couldn't spin at different rates, you'd wear down your rear tires in a hurry. If the front wheels couldn't spin at different rates, you would have a very hard time turning.
But that is getting a little technical.......
The xi system does not have LSDs in the front or rear differentials, just like the i series don't have rear LSDs. It uses electronics to control wheel spin, just like the i series does. But if you defeat DSC/DTC fully and place the car on a surface where one front and one rear wheel have zero traction, the car won't move.
So while the i may be called "one wheel drive", the xi should be called "two wheel drive." The only true "four wheel drive" would be a vehicle with front and rear locking differentials and a locking transfer case. You will find that kind of system on a rockcrawler Jeep, but not in a passenger car.