Does anyone know how much speed difference in a viscous coupling input and output starts to give a significant output.

My question is actually relating specifically to a calibra turbo, which uses a VC to send drive to the rear wheels when the front wheels are slipping, but im making the assumption (always dangerous!) that most 4wd VC's will operate at a similar level of slip.

Not after exact figrues, but for example does the slip need to be the front wheels going twice as fast as the rear or similar amounts, or would 5% difference still see a useful amount of torque transfer?