(LAC) Forgive me father, for I am about to sin

[Roy Wendell]

On Feb 26, 2007, at 4:48 PM, LL - NY wrote:

> Do the regular WRX have the same degree (as in # of units failed) as
>  STi's, or is this power exacerbated problem? I regularly Autocross the
>  Saabaru (I agree with your assessment of the power on understeer
>  issue with WRX's on low friction surfaces, kinda boring) and I'm 
> curious
>  if I'm setting myself up for a center diff failure. At least the mode 
> of
>  failure isn't as frightening as Taka has described for DSM's.
>
>  LL - NY
>

The WRX as well as all manual transmission AWD Subarus are the ones 
with the viscous  locking center differential. The STI has the DCCD 
which is an magnetorheological (say that three times real fast) device 
meaning it contains a fluid that changes viscosity in the presence of a 
magnetic field. It's an entirely different unit.

The viscous locking unit is a viscous coupling that's connected to the 
center diff case on one side and the rear output shaft on the other. It 
contains a bunch of slotted plates that are alternately keyed to either 
shaft and surrounded by silicone fluid. When the speed differential 
between the front and rear axles (and therefore between the diff case 
and rear output shaft), the shearing action of the fluid causes it to 
thicken and radically increase in viscosity. It then starts to transfer 
power in the attempt to equalize the front and rear axle speeds 
although it is incapable of doing so completely. So when you combine a 
viscous locking unit with a normal open differential you end up with a 
speed sensing limited slip differential.

The failures that I've encountered so far have both been on the Legacy 
so the longer wheel base might have something to do with it but that's 
only a theory of mine based upon the idea that longer wheel base equals 
more speed differential when cornering. But I'm not sure that I'm even 
correct in that assumption. Unit one was on an '03 Legacy wagon, the 
other was an '02 sedan both with over 100k miles.

I'm sure you know to not even think about doing ebrake turns with the 
Saabaru. Very very hard on the viscous unit. To the best of my 
knowledge the failures in the above cars were not caused by such 
stupidity.

Roy