This has nothing to do with the C4/C4S: "If you take an already "thick" fluid that has a HIGH volume expansion ratio with increasing temperature and "trap" it in a fixed volume hermetically sealed container and then begin heating it.

With the gas bubble you can delay the onset of increased coupling so there is little or no reaction to "ordinary" F/R drive line slippage, turning or even slightly different tire wear F/R. But once the delay is overcome the slope of the onset can be dramatic."

Can you find an example of the gas bubble theory employed in a vehicle? Lets stay with how the vc works in a C4/C4S. Here is another quote lifted from http://www.356-911.com/post1974/2004...6carrera4.htm; note there is nothing about bubbles or density change - it is not part of the equation:

Porsche 911 Carrera 4

Traction on Four Wheels

The viscous multiple-plate clutch is housed in the front axle drive unit,conveying drive power to the front axle and compensating for any difference in engine speed between the two axles. Like a multiple-plate lock, the clutch is made up of inner plates fastened to the hub and outer plates connected to the housing. Silicon fluid comes between the plates in a totally sealed compartment. As soon as there is a difference in speed, shear forces between the silicon fluid and the plates transmit torque and drive power to the front axle as required.

A further advantage, particularly in a sports car, is the low weight, all components of the four-wheel-drive system weighing a mere 55 kilos or 121 lb. This makes Porsche's dynamic four-wheel-drive system one of the lightest in the market.

and...another good link describing the device: http://www.axlefacts.com/education/v...-coupling.aspx

testing the VC operation:

Using a heavy floor jack with wheels, put a block of wood on the jack and raise the rear of the van using the skid plate. Be careful, you can bend it. The wood block, if long enough, distributes the weight across enough of the skid plate to minimize that risk. Lift both back wheels off the ground 6" or so on a smooth level parking lot, with the jack's wheels parallel to the Syncro's. The rear of the van will be moving on the jack wheels, so you need to make sure there are no obstructions that could catch the jack wheels and cause the van to fall off the jack.

Now, put a 1x1 piece of wood in front of each front tire, 2x4 may work too. You need to block the front wheels like this to be able to test whether the VC is capable of absorbing the spinning of the rear wheels without locking up and causing the front wheels to engage and climb over the wood. If you can get the rear wheels to turn/spin in the air with the clutch fully disengaged, and without the van climbing over the wood blocking the front wheels (can't be too high...1-2"), then the VC is definitely good, or the fluid is not cooked. The van should climb over the blocks as soon as you increase the engine RPMs.

It may take several tries to get the van to do this. The VC is very sensitive, and it will want to engage as you let the clutch out. Some advise using the hand brake to help slow the spin of the rear wheels ... or to start them spinning slowly at first. It is really neat when you get it to work. Suddenly, the VC is working before your eyes in a very graphic way!

That is enough research for one day...

DC4

 

+1

 

I think you would agree that it is highly desireable to have the coefficient of coupling increase at a non-linear rate, HIGH "attack" rate, as a function of the difference in rotation rate of the two sets of clutch plates. I think that you would also agree that the "shearing" results in at least some level of HEATING of the fluid. Now if you can accept that a VC fluid can be formulated, SPECIALTY FORMULATION, to have a very high expansion ratio with even a slight rise in temperature the we can come to an understanding.

But now we have a VC that might "stiffen", dramatically STIFFEN, even in a light throttle tight turn. Highly UNDESIREABLE, that, as it would result in premature drive line component failures due to over-stressing/HEATING.

So. Add a small air bubble that must be compressed to "ZILCH" before the VC fluid coupling coefficient can begin having a dramatic effect on engine torque distribution and you have an ideal REACTIVE F/R drive coupling system.

 

The bubble you speak of would get ground into a billion tiny bubbles by the tight tolerance plate pack.

 

Yes, but then all billion "tiny bubbles" must still be compressed to zilch before the VC will convey torque.

 

Allow me to add a real world scenario regarding the 'vicous coupling' -

I just had my 2002 Porsche C4S smoged at a station that did not have a 4 wheel dyno and they attempted to smog the car on a 2 wheel dyno and during the smog check a loud boom was heard, with a simutaenouls hearsh vibration of the car. That was the viscous coupling ecploding due to excessive heat becasue the front wheels were restricted from turing during the test.

The car is now at my Indi Porsche Shop being repaired.

 

Ummm...ok... you can't just write the above and not state the whole story... You're just being a tease. How about stating what happened afterwards? Did the station manager apologize, are they paying for the damages, etc. etc... tell us the story man!!!

 

Sorry didn't think all details were relevant to the topic. Yes, the station owner has accepted full liability and the car is presently at a Porsche Shop awaiting repairs (Hergesheimer Motorsports, in Lake Forest, CA.)

http://www.hergesheimer.com/

This just happened this week Monday, so the car has not been repaired or even looked at as of yet. The shop Technician was negligent in his duties and skipped the references which would have told him that he was going to smog an AWD vehicle, thus the cost to return the vehicle to original condition is being covered by the owner.

 

Directly from the Porsche Manual...
"The viscous coupling is filled with a precisely defined volume
of high-viscosity silicone oil and sealed. It is not possible
to top up oil or to check the oil level."

"If the inside and outside discs rotate at different speeds,
friction in the fluid transfers torque from the faster to the
slower discs. The discs are completely separated by the
oil and abrasion or wear therefor cannot occur."

 

It's basically a torque converter filled with fancy silicone fluid.
Like a version of the one in your uncle Ed's Buick.