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AWD vs 2WD

>>a local mechanic recently opined to me that the quattro system operates (in
>> normal mode) in front-wheel drive; only when activated (i.e.,
>> differential activated) does the four wheel drive mode kick in.

Your local mechanic is very mistaken.  Quattro is always in AWD mode,
with power being applied to _all 4 wheels_.  You have 3 differentials:
(1) splits power 50/50 between front and rear (and adjusts that split when 
slippage occurs)
(1) splits the front power between left and righgt
(1) splits rear power between L&R

Under ideal circumstances, each of your 4 wheels receives 25% of the torque.
It can vary allover depending on conditions.  Various generations of quattro
oeprate a little differently, also.

For most AWD systems, your mechanci is right.  For audi, he is wrong.
There is a theoretical dry-road benefit to the audi method.  It spreads the
torque evenly *before* a slide occurs.  This has two benefits:

(1) it may prevent a slide from ever occuring in teh first place
(2) it provides more neutral handling on dry surfaces

Audi's system was originally developed for sporting use on
dry (or wet) roads, not as an off-road savior.  This is why the Q
system operates all the time - helping traction and handling balance - 
while most other systems just get you going once you strat to slip.