The Subaru 4wd system on WRX, Forrester, GT etc is a well designed package, BUT it has its limitations:


CENTRE DIFFERENTIAL

A maximum of 50% drive can go to the rear. The rest obviously going to the front. The viscous Diff is a speed sensing type Diff and operates similar to an auto Trans torque converter, in that it never achieves full lock up. Because of this fact, it is possible under certain circumstances that you can create large speed differences between front and rear, i.e. if the front wheels lose traction and not the rear wheels.

In some circumstances the car can get stuck, on steep angled driveways or extremely slippery conditions. This is true, but it has to be extreme!!!!!

If you sit and spin the a rear wheel off the ground for longer that 30 seconds constantly, you can permanently damage the rear diff !! The life span of the Diff is greatly reduced by heat build up generated under extreme speed differences.

With one wheel off the ground or on a slippery surface the rear diff will try to drive the rear wheel still on the ground or with most grip, but it can only exert a certain amount of force. If the force required to move the car is exceeded , then the diff gives up and lets all the drive go the the free-spinning rear wheel. The Viscous Diff has a maximum operating capacity, it will try and maintain this until such time as heat build up renders it inoperative.

The rear diff is also viscous-based, so won't "lock up and drive" like LSDs on mechanical type units.


MANUAL TRANSMISSION EQUIPPED VEHICLE

The AWD system on the WRX has a mechanical link to all wheels. The centre differential allows variation in speed between front and rear wheels to avoid transmission 'Wind-Up'. The viscous coupling ensures that if the front wheels turn faster than the rear, then more torque is supplied to the rear and vice versa. The more the speed difference, the more torque is supplied. Because there is a mechanical link, All Wheel Drive is maintained at ALL times.


AUTOMATIC TRANSMISSION EQUIPPED VEHICLE.

Again, this system provides for a mechanical link to all wheels, and also maintains All Wheel Drive at all times.

The Japanese Domestic STI with Manually Adjustable Centre Differential will 'Open' the centre differential when the handbrake is applied, allowing the front and rear wheels to turn at different speeds thus allowing for handbrake turns. This system is entirely different to the UK specification vehicles.