The Viscous Coupling is really neat. The design consists of a series of plates with holes cut in them and relies on a dilitant fluid (silicon paste) filling th

The Viscous Coupling is really neat. The design consists of a series of plates with holes cut in them and relies on a dilitant fluid (silicon paste) filling the holes to have a significant resistance to shearing. This allows the outputs to run at different speeds than each other, up to a point where the paste gets hot from shearing and the pressure increase locks the plates to each other. When the output speeds are similar, the diff doesn't lock up but will allow one to overspeed the other. One main benefit to this design over others, is that by changing the number of plates you can change the amount of torque sent to the outputs. When installed with twice as many discs to the rear than the front you end up with twice as much torque to the back wheels, as in the transfer case of a E30 BMW 325iX. Effectively there is no mechanical connection between the engine and the wheels until the transfer case input starts moving which starts to lock up and transfer torque to the rear. The rear diff outputs are not mechanically connected until the shearing in it causes an increases in pressure. But the increase in pressure happens so quickly, lockup being within milliseconds, that you wouldn't know it while driving.