AWD: The five types of Subaru systems

Continuous All-Wheel Drive:

In Subaru models equipped with a five-speed manual transmission, a viscous-coupling locking center differential built into the transmission case distributes engine power 50:50 front-to-rear. Slippage at the front or rear wheels causes more power to transfer to the opposite set of wheels.

The viscous coupling contains a series of opposing discs attached to the front and rear drive shafts, surrounded in a type of silicone fluid. Slippage at the front or rear wheels causes a rotational difference between the front and rear discs in the viscous unit, which in turn shears the fluid.The shearing action heats the fluid, causing it to thicken.

As the fluid thickens, power transfers from the plates rotating faster (the slipping wheels) to those rotating more slowly (the wheels with the best traction). When the slippage ceases, all the discs turn at the same speed, restoring the 50:50 power split.