6MT Centre Differential Rebuild Project

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Re: 6MT Centre Differential Rebuild Project

Postby BillyCorgi » Wed Nov 11, 2015 11:23 pm

Hope I do not lose anybody here.....

Cryogenic heat treatment is useful in high alloy steels where, after heat treatment, residual austenite remains present at room temperatures. Having been heat treated initially through the critical part of the TTT (Time-Temperature-Transformation) curve, the austenite remains stable at room temperatures. However, the martensitic reaction can be continued where the work-piece is subjected to cryogenic temperatures (around -185*C). The residual austenite can continue the transformation into martensite improving the strength and hardness of the material.

Shot peening is a process where the surface of the work-piece is blasted with shot pellets with such force that the skin layer of the material is placed into compression. When the shot peened article is stressed, the outer shin must first negate the residual compressive stresses before going into tension. The effect is that the work-piece can deflect further (greater strain) before the surface can reach tensile force levels where a crack can occur and commence a fatigue failure. When I worked for an Australian automotive supplier the V8 gear parts and the 6-Cyl gear parts were the same. The V8 parts were put through a shot peening process that the 6-Cyl parts did not go through.

Micro Surfacing (also known as superfinishing) involves chemical or other assistance methods to achieve surface finish improvements to enhance wear performance and load bearing characteristics.

The characteristics of the viscous coupling can be changed by tailoring the viscosity of the silicone fluid. The nominal torque rating can be increased by using a higher viscosity fluid at the expense of "bind-up". Likewise be susceptibility to "bind-up" may be reduced by using a lower viscosity fluid, at the expense of all-wheel-drive traction.
Metal components within the centre differential unit are not presently failing, so pursuing technologies that concentrate on making them stronger may not be rewarding? Alternative viscosity fluids will tailor the torque characteristic of the differential. What characteristic were you thinking of upgrading?
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Re: 6MT Centre Differential Rebuild Project

Postby bigBADbenny » Thu Nov 19, 2015 9:31 pm

Just encouraging discussion around the "since its diassembled, what are the opportunities?" topic. :)
Comprehensive answer too thanks :)

Having read about the STi 5mt 8Kg and 20Kg optional diffs, I'd imagine there could be a few customers interested in a slight increase in the viscosity of the fluid, 20Kg C-diffs being understeering monsters on tarmac iirc, and 8Kg being just about right for some applications...
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Re: 6MT Centre Differential Rebuild Project

Postby BillyCorgi » Thu Nov 19, 2015 10:03 pm

Cheers! :wink:
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Re: 6MT Centre Differential Rebuild Project

Postby jp928 » Wed Nov 25, 2015 1:18 pm

As others have said, this is excellent work, and will hopefully be much appreciated. Might have been useful to have Brunsy's badd diff tested BEFORE overhaul to be sure the testing people knew their stuff. My centre is OK so far (125k), but mate's with less miles is binding slightly when hot.

Looking forward to road test results.

Could the gear clipped on the diff body be used to drive the pressure pump that is only fitted to STI 6mt boxes ? Order the viscous units all with the gear fitted would make gearbox assembly simpler?

Thanks for the work and the very readable explanations.

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Re: 6MT Centre Differential Rebuild Project

Postby BillyCorgi » Wed Nov 25, 2015 2:44 pm

jp928 wrote:Might have been useful to have Brunsy's badd diff tested BEFORE overhaul


BillyCorgi wrote:It should be noted that the CD unit provided by Brunsy3.0 was measured on my equipment when cold and before disassembly, to establish a base line.
The torque, measured cold, was about double that of the same diff once reassembled, and reassembled back to the same weight as prior to disassembly.
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Re: 6MT Centre Differential Rebuild Project

Postby jp928 » Wed Nov 25, 2015 9:14 pm

Point taken. I had meant tested by the same people who passed the rebuilt unit so you could have some confidence in their test....

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Re: 6MT Centre Differential Rebuild Project

Postby jp928 » Thu Nov 26, 2015 1:55 pm

FWIW my transmission code is on the build plate at the bottom of the B pillar ---WWEAA

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Re: 6MT Centre Differential Rebuild Project

Postby BillyCorgi » Thu Nov 26, 2015 2:13 pm

Thanks.
The Build Plate is under the bonnet in pre-facelift cars and it was moved to the B Pillar for the post-facelift cars.

This is what I have for TY856WWEAA
USDM Legact Spec B MY08-09 (also must be Australian as well - me)
1st 3.363
2nd 2.375
3rd 1.521
4th 1.137
5th 0.891
6th 0.707
Rev 3.545
TR 1.100
FD 3.900
CD Viscous (4kgf) 38913AA200
Front Diff OPEN
Rear Diff R180 - Torsen 38410AA140

I do not know much about the 38913AA200 CD unit, in ways that it is different to the 38913AA112 CD unit.

Cheers!
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Re: 6MT Centre Differential Rebuild Project

Postby jp928 » Thu Nov 26, 2015 3:07 pm

Ok, so the plate moved at facelift change! I think I must have the same transmission chart that you are working from, as I see all the same data.

So the difference between the...111/112 and ...200 Viscous is so far unknown ? Hope mine doesnt give up too soon!

thanks
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Re: 6MT Centre Differential Rebuild Project

Postby BillyCorgi » Thu Nov 26, 2015 3:24 pm

jp928 wrote:So the difference between the...111/112 and ...200 Viscous is so far unknown ? Hope mine doesnt give up too soon

Looking forward to seeing my first 38913AA200 CD unit soon!
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Re: 6MT Centre Differential Rebuild Project

Postby Stevo3RB » Tue Dec 08, 2015 1:03 pm

Billy - you da man...

Have read this topic with interest as I've just started experiencing a slight clunk, which is more noticeable in my work car park with smooth concrete floors. I had my centre diff replaced at about the 150K mark and am now up to 217K so I would be a little disappointed if it were the diff again. But it's good to know the option for an exchange unit is available.

Question (and I may have missed this as I sat and read the whole thread at once) but what are the advantages and disadvantages of a higher viscosity fluid in the centre diff? Am I right in thinking that tuned/worked H6's would benefit from a higher viscosity whilst stock engines would be fine with standard?

I am going to do a few other checks to try and pinpoint the clunking but suspect I may be touch if the diagnosis is the diff...
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Re: 6MT Centre Differential Rebuild Project

Postby BillyCorgi » Wed Dec 09, 2015 10:28 am

Stevo3RB wrote:Question (and I may have missed this as I sat and read the whole thread at once) but what are the advantages and disadvantages of a higher viscosity fluid in the centre diff? Am I right in thinking that tuned/worked H6's would benefit from a higher viscosity whilst stock engines would be fine with standard?

Good question. Unfortunately there is no direct straight answer.
If anyone would like to add to what I am saying here, please do so. I do not have any mortgage on correctness.
First, lets look at the steering geometry for Ackerman steering.
Image
free upload image
When turning, the wheels all try to turn about the centre of rotation.
Neglecting slip stiffness of the tyre (another subject for later) when the car is turning slowly as in a car park, the rear tyres will try to turn through a tighter radius that the front tyres.
As the car moves, the tyres will all try to roll through different lengths, and this is where the differential action comes into play.

If the centre differential is "locked" then the front wheels will try to pull the steering back to centre, forward, The vehicle will try to understeer.
As the viscous coupling becomes stiffer (high transfer torque) the affect of this centering forward understeer increases.
The tyres must transfer this grip through the transmission to turn the viscous coupling or otherwise the tyre slip, skip, jump, and the transmission goes bang.
Noises that you have associated with a failing centre differential.

So, cars with bigger stickier tyres will or should cope with a higher torque centre differential more than cars with more standard grocery-getter-car type tyres.

A higher stiffness viscous coupling will work on track with good quality tyres, etc.
The "however" is if this is a daily driver, then you would need to look out for the tight round-about-turn in drizzly conditions where the front tyre may let go unexpectedly when there is less than normal friction between tyres and road surface to turn the viscous centre differential..

Somewhere in between the two is a desirable option?
The standard centre diff will be set on the lower torque side so that a below average driver does not come "a cropper".

This all goes out the window when the vehicle is used in gravel (say, a rally car) which will benefit from a much stronger centre differential and ensure that the tyres are pulling the car in the direction in which the tyres are pointed, irrespective of the direction the tyres are pointed in.
Hope that this starts to address the question?
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Re: 6MT Centre Differential Rebuild Project

Postby Stevo3RB » Wed Dec 09, 2015 2:40 pm

BillyCorgi wrote:
If the centre differential is "locked" then the front wheels will try to pull the steering back to centre, forward, The vehicle will try to understeer.
As the viscous coupling becomes stiffer (high transfer torque) the affect of this centering forward understeer increases.
The tyres must transfer this grip through the transmission to turn the viscous coupling or otherwise the tyre slip, skip, jump, and the transmission go bang.

So, cars with bigger stickier tyres will or should cope with a higher torque centre differential more than more standard grocery-getter-car tyres.

A higher stiffness viscous coupling will work on track with good quality tyres, etc.
The "however" is if this is a daily driver, then you would need to look out for the tight round-about-turn in drizzly conditions where the front tyre may let go unexpectedly when there is less than normal friction between tyres and road surface to turn the viscous centre differential..

Somewhere in between the two is a desirable option?
The standard centre diff will be set on the lower torque side so that a below average driver does not come "a cropper".
Hope that this starts to address the question?


Yes, that answers my question...

As I A) still have stock wheels, B) use the car as my daily, and C) am highly unlikely to take my car to the track, I'd likely look at stock fluid.

However, with the intention to upgrade to bigger wheels and grippier tyres the "somewhere between the two" (if such a fluid type existed) might be more appropriate.

Thanks for that. Definitely food for thought!
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Re: 6MT Centre Differential Rebuild Project

Postby BillyCorgi » Thu Dec 10, 2015 12:44 pm

Let's discuss tyre dynamics, and then the interaction between the tyres and the viscous coupling centre differential.
Let's study the attached diagram.
Image
image upload no resize
Suppose the same car, of the same weight, travelling the same speed is fitted with low grade budget tyres. Diagram on the left.
The driver wants the car to take a turn in the direction of the black arrow.
To generate sufficient side force to move the momentum of the car in the desired direction, the driver needs to input a steering angle to the blue line.
The angle between the direction the tyre is pointed and the direction the tyre moves is called the slip angle.

Now let's consider the same car now fitted with high performance tyres. Diagram on the right.
To make the same manoeuvre to turn the car and change direction, the driver requires a smaller input to the steering wheel.
As the tyre is more grippy, the angle between the direction the tyre is pointed and the direction the tyre moves is less and it has a smaller slip angle.

So, there is a attribute called "slip stiffness". This is a measure of side force generated per angle of slip.
A high performance tyre will generate higher level of side force or turning force per degree of slip, than a budget tyre.
The high performance tyre is said to have greater slip stiffness.

So when you fit those high quality high performance tyres to your car, you notice that the car becomes more responsive, it turns in quicker, less steering angle input is required to turn the vehicle and ultimately higher speeds through corners are achievable compared to the budget tyres. (Higher speeds in turn created higher centripetal forces requiring higher side forces from the tyres)

How does this effect the viscous centre differential?
The tyre has a limit in the frictional force it can excerpt with the road surface. Beyond that, the tyre brakes traction and slide occurs. Once slide occurs the tyre cannot control the direction of the car.
This diagram represents the maximum force the tyre can exert upon the road surface in any direction.
(If the diameter of the circle represents FORCE, then High Performance Tyres have a larger diameter compared to low grade budget tyres and a wet weather circle would be smaller than a dry weather circle)
Image
image hosting
Direction A indicates the maximum force that the tyre can provide to propel the car forward. In this position of maximum traction there is no force available to change the sideways direction of the car and turn the car.
Direction B indicates the maximum force that the tyre can provide to slow the car. In this position of maximum braking force there is no force available to change the sideways direction of the car and turn the car.
Direction C & D indicates the maximum turning force that the tyre can provide to change the direction of the car. In this position of maximum side force there is no force available to change the velocity of the car.

Direction E indicates a turn under power. Some force is generated to accelerate the car forward and some force is generated to change the car's direction. The sum of the two forces being at the limit of the contact force between tyre and road surface.

With a very stiff viscous coupling centre differential, and when the tyres rotate at different speeds requiring differential action, say during a turn, force is required between tyres and road to turn the viscous coupling. If this force is too high, because the rating of the diff is very high, or the centre diff is on its way out and failing, then forces required to turn the viscous centre diff can become so large that there is insufficient forces left to change the direction of the car. The force diagram moves to position F.
Image
image upload no compression

This is one reason why the 20kg.F STI viscous centre differential is renowned for making daily driver road cars "understeer like a pig"
The 20kg.F centre differentials work great when the suspension is especially set up to compensate for them and also on off-road rally surfaces. On Gravel the tyres are continually braking traction with the loose road surface so the static friction diagrams we have been examining no longer apply. Maybe that is a discussion for another day.

So, Stevo.3RB, the thicker silicone fluids exist. Certainly think about tuning the stiffness of the centre differential, however do so understanding the dynamics of the handling of the car that will be affected.
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Re: 6MT Centre Differential Rebuild Project

Postby Daemos » Sat Dec 12, 2015 11:25 pm

BillyCorgi, I am interested in actually purchasing one for my 02 WRX STI, all info points to the fact that my transmission takes the same part number as discussed 38913AA111 which was then superseded by 38913AA112.

I however, do not have a centre differential to exchange, as I am currently running a cusco tarmac gear, however, would I still be able to purchase one (obviously not getting any $100 core refund)?

I am also curious to get something faster reacting than the 4KG diff, I know a 20KG fluid would not work on the street..

Based on my research people have said a 12KG works pretty well in the 5MT see: http://www.perth-wrx.com/vb/general-sub ... -diff.html

And subaru do sell a 10KG for the 6MT... see: http://www.rhdjapan.com/sti-differentia ... preza.html

I'm curious to know your thoughts BillyCorgi!
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