Two "limited-slip" Ferguson-type viscous couplings are fitted on the 325iX. These are filled with silicone and are not computer controlled. The one behind the transmission splits torque front to rear (nominally 37/63%). A series of plates with holes and slots turn in the silicone fluid. Some plates are attached to the front axle driveshaft and some are attached to the rear axle driveshaft. Normally the plates turn at the same rate without relative motion. The silicone becomes very viscous as soon as it is heated by friction and shear caused by differences between the motion of the plates. This tends to lock the driveshafts. If the rear wheels and driveshaft are slipping and turning faster than the front, friction between the plates increases, slippage is reduced, the rear wheel spin is reduced and the power from the input shaft is transferred to the front. Side View -The drive system add only 145lb. to the vehicle and is very reliable. The viscous couplings are sealed permanently and require no maintenace. The only additonal maintenance items required are changes of 1) the front differential oil (GL-5 90 weight, same as the rear) and 2) the transfer case fluid which is ATF. All drivetrain fluids should be changed at "Inspeciton II", nominally at 30K miles. The front wheels always have some torque transmitted to them. BMW choose a ratio of 37/63% front to rear because this corresponds to the weight distribution on each axle under full acceleration. This avoids the problem typical of front wheel drive vehicles in which the front wheels spin under acceleration due to weight transfer to the rear. Road and Track (April '88) states that "The net effect of the center differential is to act as a power-management system, transferring engine torque away from the end that is slipping and to the end with greater grip; as much as 90 percent of the torque may be shifted to the front or rear as required. This is done actively, quickly and without the occupants of the car ever being aware of it." In it's stock form, the iX understeers slightly more than most BMWs. It is 0.4 seconds slower than an ordinary E30 from 0 to 60, and 0.3 seconds slower in the quarter mile. The rear axle ratio is changed slightly (3.91 vs.3.73:1) to minimize the effect of the added weight on acceleration. Car and Driver specifically notes that on dry surfaces, the iX stops 13 ft. shorter than an 325is, probably due to wider stock tires (205s vs. 195s). -The basic technique used to drive the iX quickly is to "point and shoot". :blink: