Forester SG Manual Diffs

Hugh

Hugh

Forum Member
Joined
Mar 24, 2021
Messages
48
Location
South Australia
Car Year
MY06
Car Model
SG Forester X
Transmission
5MT w/ DR
Hey everyone,

Just trying to figure out the centre diff subaru AWD system.

For my SG 5MT Foz:
It has the Viscous LSD centre diff right?
If one wheel loses traction, more power will be sent to the opposite axle through the centre diff.
What happens if two opposite wheels have no traction in this system?

Cheers
 
If you get diagonal or front wheel spin on an SG that's where you will stop. The rear has an LSD diff. The centre diff distributes power front to rear not to one side or the other.
 
But you can make that manual be better or not?
 
Kevin said:
If you get diagonal or front wheel spin on an SG that's where you will stop. The rear has an LSD diff. The centre diff distributes power front to rear not to one side or the other.
Click to expand...
Just for clarity, the centre viscous diff in an SG is also a vLSD.
 
Does that works well?
 
All EJ centre diffs are viscous LSDs at the very least. Then there’s the upgraded/computerised DCCD setup.

The centre VLSDs have different ratings. Typically they’re 5kg. STi got 20kg and there’s a custom 12kg unit out there too from what understand.

Cheers

Bennie
 
How well do these vLSDs centre diffs perform?
 
Autos are another kettle of fish from what I understand @Kevin. Unless their design changed dramatically the auto is full time front wheel drive with the electronically controlled clutch packs to engage the rear wheels. This is why the centre diff lock switch mod can be done. Can’t do that trick with the manual gearboxes.

@Hugh - the stock centre diff goes alright. Just about everyone with a manual lifted Subaru is using them. You’ll learn what overloads them allowing slip to (typically the front end) one end over the other, and you’ll learn how to avoid this.

Cheers
 
El_Freddo said:
Autos are another kettle of fish from what I understand @Kevin. Unless their design changed dramatically the auto is full time front wheel drive with the electronically controlled clutch packs to engage the rear wheels. This is why the centre diff lock switch mod can be done. Can’t do that trick with the manual gearboxes.
Click to expand...
That's not so, Bennie. Our SH auto is full time AWD.

This is an old (and wrong) statement, endlessly repeated throughout the internet.

e.g. See here,

El_Freddo said:
@Hugh - the stock centre diff goes alright. Just about everyone with a manual lifted Subaru is using them. You’ll learn what overloads them allowing slip to (typically the front end) one end over the other, and you’ll learn how to avoid this.

Cheers
Click to expand...

Unlike a mechanical LSD, a vLSD is extremely smooth. You are very unlikely to notice its operation, except in extreme conditions.
 
I agree. All AWD Subarus since the 80's are fulltime awd. I'm pretty sure our 1982 1600L wagon had 2wd, 4wd & 4wd low, but that's from a 30+ year old memory..
 
The following quote is from here (post #95 in link provided by Ratbag):


ABFoz said:
The FWD impression is when the centre diff wears out due to donuts, uneven tire wear, etc. Where the wet clutch isn't as effective in holding torque anymore. That's the extreme, though, as the Transfer Control Duty solenoid fails first and when binding is felt on high traction surfaces, the owner should replace the the solenoid immediately to save the centre MPT diff.
Click to expand...
^This. Can’t be a “proper” centre diff if there are wear items to the point that you can get power only to the front wheels!

How else would the 2wd fuse under the bonnet work?

From what I can see in the parts diagrams from a Gen1 AWD auto Liberty and my MIL’s 2002 SG foz there is not a mechanical centre diff, just a set of clutch packs for the rear output. The front diff pinion shaft is directly driven from the drive gear clutch pack layshaft from what I can make out with different component diagrams. An exploded diagram of the whole transmission internals would be good to reference but I don’t have ready access to it if one exists. Also, the name is a giveaway: MPT = Multi Plate Transfer. Not a true mechanical differential.

So it seems from what ABFoz said, I had the operation of these rear clutch packs backwards or was getting my message across incorrectly. The Auto AWD system is not reactive, rather it “deactivates” full lock as needed depending on a bunch of variables. This eliminates it as a reactive system in my mind - but the fact remains, full drive to the front wheels remains when the rear drive clutch packs are so worn out that they provide little to no drive to the rear wheels. Hence why you can do the “diff lock switch” mod on autos!

Yes, I get that your SH could be different. It probably has a VTD that has a mechanical diff component that is a centre diff with torque control similar to the MTP setup. VTD = Variable Torque Distribution. Many things changed in the SH! I’d need to grab a VIN to checkout the rear transfer and extension diagram to see what’s there exactly.

This is a pic of the VTD unit:

subaru-vtd-differential.jpg


1624069282249.jpeg

^ MPT on left, VDT on right.

Cheers

Bennie
 
Kevin said:
If you get diagonal or front wheel spin on an SG that's where you will stop. The rear has an LSD diff. The centre diff distributes power front to rear not to one side or the other.
Click to expand...
Hmmm - maybe I misunderstand how these things work ...

I thought -
- front right starts spinning at say 1000rpm but car is stopped - left front is stationary
- drive to front diff must be turning at 2000rpm (half the speed of front right - thereabouts - depending on front diff gearing) - drive to rear is zero rpm at this moment.
- centre vLSD start working - sends 2000rpm to rear diff
- rear LSD ensures that both rear wheels turn and depending on traction, car starts to move.

Various YouTubes seem to demonstrate this?
 
That would make sense. This from the "Technical Description" would seem to confirm that style of operation:
"When wheel slip occurs a rotational speed difference between the front and rear axles is created and the viscous
coupling automatically matches the torque to grip, in order to restore maximum traction.
The nerve centre of the limited slip differential is the viscous coupling, which is a housing containing a number of
inner plates connected to the rear wheels and outer plates connected to the front wheels. The housing is also filled with
silicone fluid and its viscosity increases with temperature. When wheel spin occurs, there is a rotational speed difference
between front and rear axles and as a result the inner and outer plates also turn at a different speed.
This causes an increase in the temperature and viscosity of the silicone fluid due to the shearing of the fluid between
the plates. The increase in viscosity makes the silicone fluid more difficult to shear and hence torque is transferred from
the slipping axle to the axle which still has traction"
 
Last edited:
Yep. Some weird fluid that gets thicker with heat apparently. Same principal in viscous fan couplings.

Good description @Kevin - was that in the owner’s manual or somewhere in the FSM?

Cheers

Bennie
 
It's called a non-Newtonian fluid.

I have some photos of the inside of the centre diff. One that was wrecked by doing donuts ...
 
Ratbag said:
It's called a non-Newtonian fluid.

I have some photos of the inside of the centre diff. One that was wrecked by doing donuts ...
Click to expand...
Here's the photo. You can make out the shear plates through the fluid, which should be a clear colour ...

E-30_JAK_2014-_5022459_E.jpg
 
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