re: was Quattro vs. Synchro

["Hardwick, Robert C" <Robert.Hardwick@PSS.boeing.com>]

This discussion of the "schedule" of torque transmission to the wheels
of various AWD schemes reminds me that I don't have a very good idea of
how the Quattro concept works or has changed through the years.  

I'm looking at used 90s.  They have a longitudinally mounted engine,
center box and rear differential with a button controlled electric diff.
lock.  OK.  Does it sense different velocities or is the center box
torque sensing?  Are either the front or rear differentials limited
slip?  Aside from automatically locking the rear diff., how does the A4
system differ?

I've been told that the viscous coupled systems don't react very quickly
because it takes a second or two to heat up the viscous fluid.  This lag
makes them ok for getting you unstuck, but not viable for a true all
time-all wheel drive system, i.e. limiting power understeer, etc.  Can
anyone corroborate?

cheers, robert hardwick, seattle usa