6MT Centre Differential Rebuild Project

[BillyCorgi]

Small Update.

As of close of business 31st December 2022 CorgiWerx has rebuilt and shipped 1,205 Subaru Centre Differentials

[TFish]

Hi all,

I've got some drivetrain symptoms and just rebuilt the rear diff and found nothing wrong. The tailshaft CVs are a bit stiff outside of the middle and I'll probably look to replace those. Centre bearing is smooth.

2007 Liberty GT-B 2.5T 6spd man. It's got a Corgiwerx centre diff swap about 20-30,000klms ago but has also done about 15 khanacross events and obviously been driven hard.

I'm looking for some guidance on testing the centre vicious diff on jack stands as the point of this thread is that there is no information available about it, just replace it again.

So basically, I'm thinking of getting the thing back together and then running it in 1st on the stands until the centre diff warms up and seeing what happens between the front and rear. That's all I've got so far.

Cheers,
TFishy

[bigBADbenny]

Driving the warmed up car in a car park doing slow full lock figure 8’s will trigger viscous centre diff BANG BANG BANGs.

Its the wheel speed difference changes that provoke the binding of the viscous centre diff with failed viscous fluid symptoms.

Chasing NVH issues can be informed by noting the frequency of the noises, do they conform to engine, gearbox output or wheel speeds or conform to body changes in body direction: steering and suspension, or combinations thereof.

Using contact microphones or piezoelectric transducers with switching, amplification and or recording/datalogging can be helpful for difficult to diagnose NVH issues.

[TFish]

Ah right, thanks. That doesn't sound like my issue, along with other symptoms I'm get a delayed bulldozer effect when braking into corners. I'm braking and get the speed about right about 1 sec before the corner and then as I enter the corner extra drive appears. There's a delay where it's smooth and then KICK

[bigBADbenny]

If it’s an engine braking transition, or a power on/off situation, that can be a mount issue: engine (usually LHS?), pitch stop, rear diff to name a few possibilities.

For the engine mount it’s possible to load the engine whilst braking, stationary, if the lhs rises by more than 10mm (iirc) then that mount is a potential culprit.

As are any stock relieved (with holes or silicone for nvh) bushes as found at the pitch stop, lca bushes and diff etc.

[BillyCorgi]

CENTRE DIFFERENTIAL UPDATE…..
It has been a while since we have discussed any ongoing product development with the centre differentials. The present delivery status is 1,328 refurbished centre differentials shipped as at close of 30th June 2023.

The 6MT centre differential for the Spec-B gearbox is the product we commenced with and remains our premier product. It is the best of the Subaru centre differentials with major components being case hardened and nitride. They are highly resistant to wear compared to the 5MT and 6MT Cable Shift gearbox centre differentials with non-heat-treated components.

The 6MT Spec-B units do have one Achilles heel that is the bronze plated thrush washer behind the side gear. Normally, when the centre diff is new and in good condition the thrust washer is fine, However, when the centre diff commences its failing process, if the damping fluid expands the lubrication clearance closes, the lubrication oil is squeezed out. If the ailing centre diff is operated for too long a duration is this condition the side gear makes metal to metal contact with the thrust washer and the thrust washer gets torn up. See images.



The first image shows three thrust washers. The left most thrust washer is from a failed centre diff, however the thrust washer is in very good condition. The centre and right most images are thrust washers that have come from centre diffs where the centre diff was not replaced promptly, and operator abuse has led to the thrust washer destruction. The forces within the ailing centre diff have “spun” the thrust washer, torn off the lock-tabs and scored up the contact surface.

We are in the process of developing and trialing our own replacement thrust washers made from 65MN Spring Steel, similar in design concept to that used in the 5MT and 6MT Cable Shift centre diffs. A prototype unit has been hand made and is about to be installed into a vehicle for trials. See the second image. We hope that will lead to a source of replacement trust washers for damaged units or a source of improved thrust washers should they prove more durable.

[BillyCorgi]

SOME CENTRE DIFFERENTIAL APPLIED SCIENCE

Time for a follow up on some Centre Differential research.....
The Subaru viscous centre differential is essentially a viscous coupling mounted in a differential gearset. Its purpose is to transfer drive torque across the differential gearset when traction is lost.
Scouring the internet, papers on viscous couplings can be found. The basic formula for the viscous torque created by the viscous coupling is

Tq = n.pi.ps.vs.w.(r-in^4 - r-out^4) / lambda

where
Tq = viscous coupling torque
n =, number of plates
pi = Pi or 3.142......
ps = density of the damping fluid
vs = kinematic viscosity of the damping fluid
w = rotational speed difference between the stationary and rotating plates
r-in - outer radius of the inner plate
r-out = inner radius of the outer plate
lambda = clearance between the plates

So basically the torque rating of the centre diff is a function of....
the number of plates, the gaps between the plates, the density and viscosity of the damping fluid and the second polar moment of area of the annulus of overlap of the inner and outer plates.
(Still with me?)

So, as the physical dimensions and the number of plates, etc of the viscous centre diff are fixed by the unit's design, and as the density of the polydimethylsiloxane damping fluid is basically the same across the range of useful viscosities (rho = 0.97), the torque rating of the viscous centre differential is basically a linear function of the viscosity of the damping fluid.
For the 4kg rated factory specification of the standard centre differential the kinematic viscosity of the damping fluid is about 100,000 cSt. The rest is simple math.

[BillyCorgi]

"If you know, you know!

The magic centre diff torque formula.



https://www.youtube.com/watch?v=CwZ6XCQc5WU

[BillyCorgi]

Centre Differential Analysis.....

So a young gentleman on club.liberty.asn.au Facebook page posed this question.....
"Brand new tires on the rear axle & completely worn tires that need replacing on the front axle.
Does driving with this arrangement damage the viscous coupling & gearbox?"

Here is my response.....

"Hi, This is Peter from CorgiWerx, the Subaru Viscous Centre Differential Expert
For those playing along at home lets do some analysis....
Let's take a Gen4 Liberty 2.5NA running 215/45R17 tyres and a 3.9to1 final drive.
The rear tyres are new and the front tyres are worn out.
The vehicle in question is doing a steady 60kph.
Th rolling diameter of the rear tyres in 625.3mm, the tread depth for these tyres is 8mm so the rolling diameter of the worn front tyres is 609.3mm.
At a speed of 60kph, the rear tyres are rotating at 509.05 rpm and the front tyres are rotating at 522.42 rpm.
The rotational speed difference is 13.37 rpm.
With a final drive ratio of 3.9to1 the speed difference across the centre differential is 52.13rpm.
For a standard OEM centre diff with a 40N.m @100 rpm rating the centre diff torque will be 20.85N.m.
The 52.13rpm equates to 5.46 radians per second.
Power = Torque x Rotation Velocity or in other words, the heat generated within the centre diff will be 114 watts.
Now, I do not know when it is the last time you grabbed hold of a 100watt incandescent light bulb, but they get fecking hot!
It can quickly overheat the viscous fluid.
Analysis: Not good for the centre diff"

[subyroo]

Centre Differential Analysis.....

So a young gentleman on club.liberty.asn.au Facebook page posed this question.....
"Brand new tires on the rear axle & completely worn tires that need replacing on the front axle.
Does driving with this arrangement damage the viscous coupling & gearbox?"

Here is my response.....

"Hi, This is Peter from CorgiWerx, the Subaru Viscous Centre Differential Expert
For those playing along at home lets do some analysis....
Let's take a Gen4 Liberty 2.5NA running 215/45R17 tyres and a 3.9to1 final drive.
The rear tyres are new and the front tyres are worn out.
The vehicle in question is doing a steady 60kph.
Th rolling diameter of the front tyres in 625.3mm, the tread depth for these tyres is 8mm so the rolling diameter of the worn front tyres is 609.3mm.
At a speed of 60kph, the front tyres are rotating at 509.05 rpm and the front tyres are rotating at 522.42 rpm.
The rotational speed difference is 13.37 rpm.
With a final drive ratio of 3.9to1 the speed difference across the centre differential is 52.13rpm.
For a standard OEM centre diff with a 40N.m @100 rpm rating the centre diff torque will be 20.85N.m.
The 52.13rpm equates to 5.46 radians per second.
Power = Torque x Rotation Velocity or in other words, the heat generated within the centre diff will be 114 watts.
Now, I do not know when it is the last time you grabbed hold of a 100watt incandescent light bulb, but they get fecking hot!
It can quickly overheat the viscous fluid.
Analysis: Not good for the centre diff"

I'm no mathematician Billy, but something is amiss in that bold red text.
Please correct me if I am wrong here, but I think the first lot of front tyres should be the new ones... Y/N?

[BillyCorgi]

The Rear Tyres were new.

[subyroo]

The Rear Tyres were new.

Bloody hell!, I can't even type correctly, and here I am having a dig at you for the same thing.

[nvmylh]

But if that light bulb was in 60km/h airflow, it wouldn't get that hot?? haha

[BillyCorgi]

But if that light bulb was in 60km/h airflow, it wouldn't get that hot?? haha

The light bulb is in the gearbox which generates its own heat!

[BillyCorgi]

CorgiWerx has reached the milestone of 1,500 Centre Differentials

Alessio Magaraggia was recently the winner of the CorgiWerx "Golden Ticket" promotion to celebrate the 1,500th CorgiWerx Centre Differential



Congratulations Alessio!