there is around a 3% difference between front and rear rotational speeds that the diff will cope with for short periods of time before starting to burn out
but it's not like the systems used in the vauxhals etc as it is purely mechanical
imagine that your rwd car has a viscous lsd, once the wheels start to spin, it starts to lock up, sending power to both sides
if you have one tyre bigger than the other it will cope to a degree before it starts to wear out slowly over time
if you compare that with an lsd that is plate type that slowly winds up until it's locked, youwill find that different sized wheels left and right will eventually catapualt you into the scenery when it eventually locks solid
now picture the viscous lsd being fitted to the centre diff in the seirra/escos
it's still the same thing, but it simply controls the rotation of the front and rear props as opposed to the left and right driveshafts
when the rear wheels spin, it starts to channel the torque to the front wheels
so if the rotation of the rear prop is faster or slower than the front, it starts to get working
so you don't have a sudden "it's locked and burning out" sensation as you would with a plate diff
ford state that the 3% speed difference can be held for as long as 30 seconds while you are performing the brake test for an mot before damage will occour (60 seconds and 6% on the dohc cars with the shorter gearing) but don't tell you what scope there is for fitting different sized wheels front and rear