viscous coupling

In the auto, it is not a VC (viscous coupling). It is a computer controlled wet clutch.

That's probably a better way to describe it, yes.

Yousuf Khan

So, on my auto Subie Outback, if I insert fuse (for running the small spare) does that simply tell the computer to disengage the wet clutch?

Yes, that's exactly what it means.

Yousuf Khan

That is incorrect. The Imprezas (and Forester) do not have a center diff. The front axle is directly driven by the engine. The manual transmisson has a viscous coupling that connects the drive shaft to the rear axle. When there is slippage between the two, the fluid thickens and some torque will be transmitted to the back. If the front wheels have very little traction but the back wheels have traction, almost all of the torque will be to the back (because a slipping tire does not transfer torque).

Only more expensive Subarus have a center differential.

No, he is right. You are wrong. There is no center differential in Imprezas (or Foresters). Just go to the Subaru web site, it is all laid out there for the different models.

All manual transmission Subarus have a center differential. Only the cheap Impreza, Legacy and Forester automatics don't have one. These are front wheel drive until the system decides to transfer some torque to the rear via the clutch pack.

The more expensive automatics with the VTD transmission also have a center differential.

Hey---it's that foofnicky guy...the one that got fired from the Subaru dealership!!!

Wow..foonick uses the same terminology under two different posting nicks...how courageous of him.

Got a link? Better yet, cut'n paste some text.

Wrong.

My 1985 Subaru GL 2WD wagon has a 5-speed manual transmission and no center differential. In fact, I rather suspect that

*no* 2WD Subaru has a center differential...

My 1985 Subaru GL 4WD wagon has a 10-speed manual transmission (hi/lo range: 10 forward and 2 reverse) and no center differential.

Took me 20 years, but they are just starting to get broken in...

I expect that all AWD Subarus do have a centre differential, can't see how they would work otherwise. (My MY02 Impreza certainly does...) No, a viscous coupling doesn't take over that function, it only comes into action when there is a difference between the front and rear wheel speeds.

For those who like specific quotes, just look at the service manual of any AWD manual transmission Subaru, it will match the above (tho' likely better stated!)

Larry Van Wormer

I know for a fact my Forester has a center diff. Where are you getting this info?

Hunh? Were you looking at just the automatic transmission models? If you looked at a manual transmission model, you would've seen it.

Yousuf Khan

You know, maybe it is time for an Subaru AWD FAQ? There are so many different systems that Subaru uses that this question comes up over and over again.

I can tell you from experience how they work. My 1985 Subaru GL 4WD wagon has a 10-speed manual transmission (hi/lo range:

10 forward and 2 reverse) with differentials at the front and rear but no center differential.

Unlike modern AWD Subarus that are designed to drive all four wheels on dry pavement, the '85 system was designed for off-road and rain-slick conditions only. If you put it in 4WD on dry pavement and try to make a tight low-speed turn, the front and rear wheels try to take different paths and thus roll at different rates, but the 4WD forces them to rotate at the same rate, which means that the rear wheels are being dragged forward relative to what they would normally do and the front wheels are being dragged backwards relative to what they would normally do. The little 1.8 liter engine isn't powerful enough to make them skid, so this does an excellent imitation of having the brakes on. At that point you can't get out of 4WD; the lever will not move. You have to back up a bit and then go into 2WD. And that, my friends, is why all modern AWD Subarus have some way of allowing the front wheels and the rear wheels to move at different speeds.

I tested it in a big parking lot in a heavy rain, and my stopping distance (maxinmum brakes before skidding) was 20-30 percent shorter. It also seemed to be more stable while cornering, but that's subjective. It is also noticably less squirrely at freeway speeds during the rain in 4WD.

Yep, and that's why I specified AWD. That does need a centre diff. The viscous coupling acts to make the mechanical centre differential a limited-slip unit.

As you describe well, 4WD is a different beast. May or may not have a centre differential.

Larry Van Wormer

Sorry, didn't read the first post right. I was talking about the AT. I apologize for the confusion.