VCs are not DIY-able and here's why

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VCs are not DIY-able and here's why

Postby silverbullet » 03 Oct 2019, 19:03

Another little "why its really not a good idea to try and DIY a syncro VC with instructions off youtube and stuff off ebay" type of post.
The T3 viscous coupling is a disarmingly subtle device and operates within a narrow temperature window, defined by the oil temperature of the front differential as well as the fill weight of the unit and the viscosity of the fluid used.
When cold, the VC is quite stiff and transmits a lot of torque. This is a useful characteristic because when things are cold you are more likely to encounter ice, snow, rain etc and more traction is useful, especially when first setting off.
As the front diff oil warms up, it also heats the VC and helps to bring it into its narrow operating range. A small rpm difference between front and rear axles shears the fluid in the VC and creates a small temperature increase which causes it to go into "hump" mode or otherwise "lock"

When no slip is present, the temperature drops and the VC effectively disengages (although it still transmits a modest amount of torque)
This all happens at around 120°C and at pressures upwards of 50 bar...

It is a product of pure genius and must have taken 1000's of R&D hours at SDP to perfect.

My limited experience of refurbishing 50 or so VCs has shown that fluid fill weights can vary up to +/- 10g around a nominal 290g fill of the correct viscosity fluid, which is also a special spec as calculated by VW/SDP for optimal response and predictable vehicle handling in all conditions.
(Filling precision to 0.1g is the norm and requires expensive scales that can deliver this with about 7kg on the platten)

*typo corrected*

If a VC engages too late (underfilled, excessive air volume) it will run hot and shorten its life. Too early (overfilled) and it will engage at a lower temperature than anticipated i.e. in long, slow turns such as found on motorway slip roads and could cause rapid transition from progressive understeer to sudden oversteer.

The difference across the performance graph is +/- 2g

But some guy on youtube says "stick XXXg of XXXXXXcSt in, it will work just fine"

Ooh look, a cheeky partial archive document from circa 1984 ;)
Image

Note: temperature vs. transmitted torque is valid when tested at 70rpm, which makes any bench torque wrench test pretty much meaningless as it requires ~400 degrees/second...

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Last edited by silverbullet on 04 Oct 2019, 06:56, edited 3 times in total.
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Re: VCs are not DIY-able and here's why

Postby slowcoach » 03 Oct 2019, 20:51

A fascinating post, which also outlines why I'm glad I don't have one ;-)
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Re: VCs are not DIY-able and here's why

Postby silverbullet » 04 Oct 2019, 12:29

Of course, access to factory data and industrial suppliers make all things possible...
I have four in the shop right now.Image

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Re: VCs are not DIY-able and here's why

Postby J P » 06 Oct 2019, 21:28

Which one's mine? How are the internals?
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Re: VCs are not DIY-able and here's why

Postby silverbullet » 07 Oct 2019, 06:46

J P wrote:Which one's mine? How are the internals?
Back left, I think...all of these look pretty good at first glance, no spline wear worth mentioning and the central spindles have good sealing surfaces (no scoring or rust damage)

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Re: VCs are not DIY-able and here's why

Postby J P » 07 Oct 2019, 09:48

Excellent! So the shop didn't get too stinky then :D
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