P
Paul Pedersen
I've been spending a lot of time lately studying Subarus
and am starting to see clear through the misleading text
by Subaru and others on the subject of AWD and automatic
transmissions. Personally I drive a manual but I've been
so bothered by the incomprehensible descriptions of the
automatics that I wanted to find out what was really going
on.
With manual transmissions there isn't much confusion.
Most if not all AWD manuals use three conventional
differentials : one to split the power front to back,
then one at each end to split the power side to side.
There is usually something to block the center differential
so if one wheel spins power will get sent to the other
end of the car just the same. Most Subarus use a
viscous coupling in the center differential which locks
up if there's too much of a difference between front and
rear.
My Corolla AWD wagon has an electrically lockable
center differential which is operated at the push of a
button (as does the current Subaru STi). A much better
idea, in my mind, in that the spinning wheel doesn't
get to dig a hole before the differential locks up.
With automatic transmissions the subject should be as
simple, but the literature describes things in such a
was as to make things almost incomprehensible.
The only difference between a manual and an automatic
should be that the clutch is replaced by a torque
converter and the gears and shifted automatically.
An automatic AWD Corolla is like this and the power
out of the automatic transmission goes to three
differentials like the manual. The only difference
is the locking of the center differential is automatic
if speed differences are detected.
Things are not the same with Subaru automatics. First,
there are two kinds : the MPT and VTD. The VTD is on
the higher end models like the WRX and Outback H6.
The VTD is true AWD in that it has a center differential.
But : the MPT, used in most models, does not have a center
differential. As far as I can tell power always goes to
the front unless the system detects a difference between
wheel speeds (using sensors) at which point it will engage
a multi-plate clutch (like in a motorcycle) to send power
to the rear. What Subaru doesn't say is that this will
only happen in the case that one of the front wheels is
spinning.
I've seen lots of complaints about the Honda automatic
CR-V being like this, but I haven't seen anywhere that
the non VTD Subarus are also like this.
In my mind, then, these automatics are front wheel drive
vehicles unless some very particular circumstances are
met.
AWD, in my mind, is much more than something to help
getting unstuck. It's a matter of balance, both in
power application and engine braking, with everything
distributed to all four wheels, each doing 25% of the
work. There is less chance of a wheel spinning during
acceleration, or of a wheel locking up if you engine
break in slippery conditions.
If you read texts by Subaru and Honda on their automatics
you get the idea that the system is constantly adjusting
things front to back depending on conditions. Well, it
may be monitoring things constantly, but it isn't doing
anything most of the time.
So if you're looking at AWD for safety and fun, either
go with a manual, or make sure you get an automatic with
a center differential.
and am starting to see clear through the misleading text
by Subaru and others on the subject of AWD and automatic
transmissions. Personally I drive a manual but I've been
so bothered by the incomprehensible descriptions of the
automatics that I wanted to find out what was really going
on.
With manual transmissions there isn't much confusion.
Most if not all AWD manuals use three conventional
differentials : one to split the power front to back,
then one at each end to split the power side to side.
There is usually something to block the center differential
so if one wheel spins power will get sent to the other
end of the car just the same. Most Subarus use a
viscous coupling in the center differential which locks
up if there's too much of a difference between front and
rear.
My Corolla AWD wagon has an electrically lockable
center differential which is operated at the push of a
button (as does the current Subaru STi). A much better
idea, in my mind, in that the spinning wheel doesn't
get to dig a hole before the differential locks up.
With automatic transmissions the subject should be as
simple, but the literature describes things in such a
was as to make things almost incomprehensible.
The only difference between a manual and an automatic
should be that the clutch is replaced by a torque
converter and the gears and shifted automatically.
An automatic AWD Corolla is like this and the power
out of the automatic transmission goes to three
differentials like the manual. The only difference
is the locking of the center differential is automatic
if speed differences are detected.
Things are not the same with Subaru automatics. First,
there are two kinds : the MPT and VTD. The VTD is on
the higher end models like the WRX and Outback H6.
The VTD is true AWD in that it has a center differential.
But : the MPT, used in most models, does not have a center
differential. As far as I can tell power always goes to
the front unless the system detects a difference between
wheel speeds (using sensors) at which point it will engage
a multi-plate clutch (like in a motorcycle) to send power
to the rear. What Subaru doesn't say is that this will
only happen in the case that one of the front wheels is
spinning.
I've seen lots of complaints about the Honda automatic
CR-V being like this, but I haven't seen anywhere that
the non VTD Subarus are also like this.
In my mind, then, these automatics are front wheel drive
vehicles unless some very particular circumstances are
met.
AWD, in my mind, is much more than something to help
getting unstuck. It's a matter of balance, both in
power application and engine braking, with everything
distributed to all four wheels, each doing 25% of the
work. There is less chance of a wheel spinning during
acceleration, or of a wheel locking up if you engine
break in slippery conditions.
If you read texts by Subaru and Honda on their automatics
you get the idea that the system is constantly adjusting
things front to back depending on conditions. Well, it
may be monitoring things constantly, but it isn't doing
anything most of the time.
So if you're looking at AWD for safety and fun, either
go with a manual, or make sure you get an automatic with
a center differential.