Yousuf said:
There are in general three different types of AWD systems that Subaru has.
The AWD system on the base automatics is quite different from the one in the
base manuals, which are different again from the high-end automatics and
manuals.
(1) The system in the base manuals is all-mechanical. It is a refinement of
a classical 4WD system that's been around in similar forms since the
earliest days of automobiles. The mechanical system is considered much
smoother and predictable than the base automatic system (#2). It's also
considered to be lower maintenance, more rugged, and a faster-reactive AWD
system than #2. The system attempts to distribute torque evenly between
front and rear wheels at all times, even while reacting to slippages.
Not quite. When slippage occurs, the viscous center LSD will almost
lock - which means that most of the torque goes to the wheels that grip,
by definition, and that's what you want. Also, the MT has a center
diff, the cheap AT (2) does not.
(2) The system in the base automatics is best describe as
electro-mechanical. It uses sensors at the wheels to measure wheel slippage,
and a computer calculates this, and sends orders to an electronically
activated clutch pack to increase or decrease the amount of torque that gets
distributed from the front to the rear. Under normal circumstances it will
attempt to send most of the torque to the front wheels, only giving torque
to the rear wheels when slippage is detected by the computer. That doesn't
sound too different than system #1, and it may even sound technically
superior. But the all-mechanical system reacts to wheel slippages in a
fashion that the electronic system can't match for reaction time nor
smoothness.
It also uses acceleration and decelleration as a measure, and thus will
provide assistance from the rear at start and in some other situations,
pro-actively. But under normal driving, it is 90:10 (almost FWD) -
which means you can get into trouble much more easily than with the MT
AWD (50:50 from the outset). Also, a number of users have complained
about sudden, unpredictable shifts of power between the back and front
on slippery surfaces (slush, snow, and icy snow), which can even lead to
fishtailing in a car that normally understeers quite heavily (e.g.,
Forester AT). The MT starts with 50:50 and will be almost in a
continuously locked state on slippery surfaces - which is what you want.
It also has more neutral handling characteristics on any type of
surface (especially, if you add a stiffer rear sway bar).
(3) The highend system (available only on WRXs and up, and Outback H6's) is
a combination of the first two systems; it's best described as an
all-mechanical system with electromechanical assist. So it's sort of the
best of both worlds. It's available on the higher-end Subarus either with
manual or automatic transmissions.
Not sure about the availability of (3) on MT transmissions, certainly
not for the standard WRX. And the H6 only comes with AT in the US and
perhaps all markets (or with very few exceptions).
As others have alluded to, driveability, convenience, mileage,
durability/maintenance, and other factors also play a role. Don't
believe advertised (EPA) mileage numbers for the AT - they are almost
always much lower in real life. Conversely, MT drivers have reported
long-term average mileage as high as 30-32 mpg on sustained highway
driving. So, depending on where and how you drive, the mpg figure
difference can easily be 4-5mpg or more.
- D