Doesn’t a Decoupler make more sense than the outdated viscous-coupling?One of the last vehicles that came with the viscous-coupling was the “Porsche 911 Turbo” which was produced until 2008. According to the development engineers of Porsche, a controlled multi-plate coupling would have needed more weight and installation space than the viscous-coupling which was used. Furthermore the soft engage characteristic of the viscous-coupling allowed for the powertrain to be dimensioned in a more compact way. So from a mechanical viewpoint the well-tried system of the Viscous-coupling is definitely not obsolete.
However the biggest advantage of the Viscous-coupling is also its greatest weakness: Namely that it is a self-sufficient, self-regulating system which also works without complex electronics. In consequence this also means that you need to make a big additional effort if you want to combine it with electronic control systems. When it comes to this, the ABS brake system doesn’t cause any problems here, however electronic stability systems with various acronyms such as ESP, DSC, PSM and similar do cause problems. This is because brake interventions that are meant to be selective get transferred automatically to other tires as well by the viscous-coupling. Because ESP has been made mandatory for all new cars in 2011, we can expect the viscous-coupling to vanish as an all-wheel drive system.
I pursued this question in detail in my Bachelors Thesis.
So what is the best all-wheel drive system then? Haldex? Torsen differential? xDrive?
To tell you right away: I personally have retrofitted my Syncro with a Decoupler and I would no longer want to go without it, also for the simple reason that my favorite specialist garage doesn’t own a brake-test stand for 4WD vehicles. Removing the cardan shaft every year before the technical inspection can get pretty tedious over time. Moreover I use the Decoupler so I can switch off the 4WD when I’m on a highway with good grip driving at speeds of more than 90km/h (56 miles/hour )- to spare the Visco. Last but not least the Decoupler is more practical in case the tow service is only able to raise the car in order to tow the car.
You may have already noticed that I mostly use the Decoupler to switch off the 4WD. That’s because I want to be able to benefit from the advantages of a permanent all-wheel-drive system even in cases of changing road conditions. During the winter, when snowy and icy roads alternate with grippy asphalt, the viscous coupling can demonstrate its strength. The same goes for wet roads or slippery foliage in the fall.
The widespread praxis to combine a hardened viscous-coupling which has sucked oil with a Decoupler requires roads with low friction coefficients. This is done to avoid tensions and therefore costly damages to the transaxle. However this mean that the 4WD can only be used off-road and let’s be honest, how many kilometers do you drive off-road and how many on the road in a Syncro?
Ranking the all-wheel drive systems in the sense of setting up a technological development chain (placing electronically controlled systems on top) is possible but useless. For instance mechanically simple all-wheel-systems with a decoupler and mechanical diff locks have a higher climbing power than modern designs with electrical controls- they all can’t exceed the simple system with a lock-out value of 100% .
Therefore the best all-wheel-system is the one that fits your specific needs. If the Syncro is used solely in sandpits, a Decoupler with a solid shaft (or better a stiff VC) is sufficient. The same system won’t make you happy though if you mainly drive on roads, because there you have to use the two-wheel drive 95% of the time. And electrical devices (ESP) need electrically controlled 4WD systems, including all advantages and disadvantages.
Why does viscous-coupling hardening appear so often in the T3-Syncro?
At temperatures around 150°C / 300°F (local hot spots) the silicon oil reacts with the oxygen from the air in the viscous-coupling. This oxidation process is well known and results in a gradual thickening of the silicon oil. For example, unequal worn tires are responsible for a permanent increased temperature level and therefore an early aging viscous-coupling.
Degassing the silicon oil and the subsequent filling with CO2 increases the temperature resistance of such a viscous-coupling to around 200°C / 392°F. Thereby the aging of the silicon-oil is practically stopped. An on-road test over 10.000 km with unequal worn tires showed no wearing of the tested viscous-coupling(!).
The much more heat-resistant CO2 – Viscous-Coupling can handle unequal worn tires, high speed or intensive off-road use much easier.
Isn’t the "oilsucking" a result of an overheating viscous-coupling?
First of all we have to distinguish between a gradual hardening and an extreme hardening. Gradual hardening is a result of aging silicone oil (oxidation), which leads to the thickening of the fluid. Silicone oils with high zero viscosity (of up to 300k) are more vulnerable to this and were therefore only used in the early days of the test phase of the viscous-coupling.
The more common problem, namely that of extreme hardening, is a result of a construction fault. The extreme hardening of the coupling only occurs when the viscous-coupling has sucked gear oil from the front transfer case. The problem of “oilsucking” was discovered at the end of the 1980s at Steyr-Daimler-Puch (SDP) and was consequently investigated in a special Diploma thesis.
The remarkable conclusion was the following: At wintery outdoor temperatures, a static vacuum forms in the viscous-coupling due to the high thermal expansion of silicone oil. This vacuum is enhanced during start-up. Because of the vacuum the viscous-coupling sucks in portions of gear oil in the cold-running phase and breaks over time. This problem does not depend on the mileage. However the results of this investigation came too late in the summer of 1990. Therefore SDP did not make any further effort in solving the problem.
Our solution: A slight excessive pressure, applied to the lid of the viscous-coupling with help of a special valve, prevents the occurrence of an unwanted vacuum. However that means that in comparison to the original SDP-factory adjustment, we need to use a different filling quantity and an adjusted level of viscosity. Yet with a test stand, working out a new adjustment is a solvable task.
Doesn’t the special valve in the Visco lid cause a dynamic unbalance?
In various VW-Bus forums you’ll find the assumption that silicone oil is pressed out of the Viscous-coupling into the transfer case as a result of overheating, and that later in the cooling process, gear oil is sucked in. Yet only the first part of this rumor is true.
The truth is that the damage pattern of the inside-out quad-ring (or X-ring) does exist, however those viscous-couplings are always measured as “empty” on the test stand. A Viscous-coupling which cannot change into Hump-mode anymore due to abrasion can indeed overheat. In this case the inner pressure of the viscous-coupling increases uncontrollably, and before it reaches the burst pressure of 100 bar the silicone oil is discharged into the transfer case. Then the viscous-coupling doesn’t produce any noticeable propulsion anymore- the Syncro is now permanently in 2WD-mode.
Roughly estimating, statistically there is one case of an overheated viscous-coupling for every nine cases of hardened ones.
But the test stand is upside down; doesn’t that inevitably lead to different measured values?
The special valve, just like the conventional screw plug (+ copper seal ring), weighs about 7g and the centre of mass has virtually the same position. That’s why we expected no problems from exchanging the screw plug. A measuring on a balancing machine has confirmed this notion.
At VW-Kern, do you test each viscous-coupling in gear oil?
With dynamic pressure ratios of around 40 bar (580 PSI), gravity is all but negligible. Also SDP had commissioned a Dissertation in which the various forms of plates were examined in this configuration.
The big advantage of this construction is that the viscous-coupling can be tested while immersed in gear oil – just like it happens in real life in the front axle drive.
Test runs in gear oil are naturally complicated and therefore we use them mostly for defining basic tunings and for leak tests of new materials. As soon as the new basic tuning is found, those test runs aren’t necessary anymore.
For research purposes we also do longtime test runs under the influence of extreme conditions (low temperatures, vibrations) in gear oil.