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Guest Message by DevFuse

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The difference on the AWD system.

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4 replies to this topic

#1 hs2


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Posted 13 February 2004 - 12:22 AM

I heard that, on Legacy Outback, the system used on AWD is different between the automatic and the manual transmission cars. The automatic version is more like FWD base with torque provided to the rear as required. The manual version is more like 4WD base with the modulation of the torque between front and rear.

Is that true?

I am also curious about the cons and pros about the difference.

What do you think?

I think the automatic version would be better on the fuel economy and the manual version would be better on the traction. (???)

#2 Commuter


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Posted 13 February 2004 - 07:59 AM

I only have a moment, but essentially you are on track.

The auto's bias the torque to the front (90/10 in the late 90's Legacies) and use an electronically controled clutch pack as the center diff. The manuals use a viscous coupling that provides 50/50. This does not include the WRX vehicles or the VDC vehicles which are somewhat different again.

In all, I think Subaru has about 4 different AWD mechanisms on the go these days.


#3 joybell


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Posted 13 February 2004 - 10:08 AM

Isn't the automatic AWD electronically controled, and the manual AWD mechanically controled? And isn't there a difference in the way the AWD works when the wheels slip? For the automatic, power is supplied to whichever wheel has the most traction (front or back and diagonal), and the manual power is supplied only back to front and front to back, not diagonally? I'm still not sure if this is correct. In any case, a mechanically controled AWD system as in a MT sounds more durable that the electronically controled AWD in an AT, although in theory the AT would have more traction.

#4 ferret


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Posted 13 February 2004 - 10:27 AM

Not including VDC or WRX, you are correct on Subaru AWD.

Automatic: the TCU (Transmission Control Unit) has rotation sensors on the FWD input shaft to the front differential and another on the output tailshaft to the rear differential. When it detects a difference, a current is applied to the center differential fluid causing it to 'gel' and effectively lock-up. 2 things to note here. The Automatic IS in Front wheel drive in most conditions. Lockup drives front and rear. Front differential is an 'open' differential so only 1 front wheel has power applied. NO LOCK-UP Differential in the front. Rear on MOST models is also an Open rear so only 1 wheel drives here also. Some models (Forester-S) get a lock-up rear. So with a Lock-up rear differential you effectively can drive 3 wheels at once. But most Subaru's drive only 2, 1 front and 1 rear.

The Standard 5 speed has a mechanically lockup front/rear differentail in the transmission tailshaft. This has been said to provide a 50/50 split. Once again it drives the wheel with the least resistance. Once it has a speed differential from front to rear, the silicone sealed fluid 'shears' and heats quickly to allow minimum slip from front to rear. The front and rear differentials work as with the Automatic, that is Most Subaru's drive only 1 front and 1 rear in AWD situations.

Hope these explainations help in your understandings and discussions.:)

#5 meep


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Posted 13 February 2004 - 06:46 PM

I can personally attest that the all wheel drive in the manuals is quite effective. I have a broken loose both front and rear wheels during sharp acceleration on wet roads, and the ice and snow performance is equal to the older Subaru's part-time four-wheel-drive performance.

To clarify, transmission output is sent to a 3rd differential, envisioned as if it were in the middle of the car between the front and rear. Equal torque is applied to both ends in a no-slip situation. The viscous fluid does not permit a whole lot of speed differentiation between the front and rear and in my experience was very effective even in off road situations that were perchance a bit too daring for the car's design.


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