Audi has been the industry leader in the field of everyday all wheel drive practicality since the original Audi Quattro launched in 1980. Audi’s famous system is proactive. Quattro is always on, which is a point an Audi sales rep will hammer into your skull upon entering the showroom. The drive system uses a mechanical torque sensing link between the front and rear axles that can split the power to whichever axle has more grip. The advantage of having a proactive torque sensing system in your car is the ability to cut through turns under acceleration while minimizing the effects of understeer and oversteer. Unfortunately, the required longitudinal mount of the engine and gearbox leaves the front axle behind the engine, enough so that all Audi’s have an inherent “front heavy” feeling. While testing an A4 3.2 (07′), our driver commented on the amount of effort needed to drive the car fast through turns. The steering felt unnaturally heavy at the start of a turn and it was hard to control the direction of the car with the throttle. To drive this car fast, just mash the throttle midturn and let that Torsen center differential pull the nose of the car around with confidence. The car tracked well through the twisties and made mincemeat out of the back roads of Greenwich, Ct.
This confidence inspiring setup may be fine for the occasional spirited driving, but unless you are willing to shell out the money for an S or RS model, you are going to feel slightly cheated and insulted by your Audi. The idiot-proof system doesn’t allow for any power oversteer and the inability to correct your latitude with the throttle will leave most car enthusiasts feeling like something is missing. Enter xDrive. While there is something to be said about the safety of a proactive all wheel drive system, the electronic grey matter that is at the heart of your traction and stability control systems will process information and take any action necessary to save your bacon before its even in the frying pan. Most intrusions from a BMW’s computer will feel natural and unobtrusive; the only sign that anything was wrong in the first place would be a flash of the DTC (dynamic traction control) symbol in the dashboard cluster. All available power is sent to the axle with the most grip, like the Audi’s system. The difference is that while the Audi is constantly moving power from axle to axle, the BMW system will only do it if the situation gets hairy. The system will send power to the front axle if it detects oversteer and cuts power if it detects understeer. By turning off the DTC and DSC (dynamic stability control), the BMW will behave like a textbook rear drive sports car and the rear end can be stepped out with a gentle massaging of the throttle.
Although both system work well, it boils down to the amount of intervention you feel comfortable with. While the Audi sales rep will make a big deal about the truly mechanical system controlling torque distribution, the BMW system will let you have your cake and eat it too. The Audi has many strengths over your typical xDrive equipped BMW but that depends on your automotive outlook. It is impossible to say for certain which of the systems is better, as they were developed with different approaches to driving in mind, but each system does an admirable job of keeping your car properly oriented when the roads get a little wet and your right foot gets a little overzealous.
http://en.wikipedia.org/wiki/Quattro_%28all_wheel_drive_system%29
