Rob996

Please pay special attention to AWD system limitations, reference to speed..

The following is an excerp from AutoZine Technical

Viscous-Coupling differential
Viscous Coupling center LSD is commonly used in many simple 4WD systems. One of the earliest examples was Volkswagen's Syncro system.


Inside a viscous coupler as shown in the right hand side picture, there are many circular plates positioning very close to each other. Both drive shafts connect to roughly half of the plates in an alternating sequence as shown. The sealed differential housing is fully contain of a high viscosity liquid, which has a strong tendency to "visco" those plates together.

In normal condition, front and rear axles run at roughly the same speed so the plates and viscous liquid are relatively stable to each other. When tyre slip occurs in one of the axle, that means the alternating plates run at different speed, viscous liquid will try to visco them together. As a result, torque is transferred from the faster driveshaft through the liquid to the slower driveshaft. The greater the speed difference, the larger the torque transfer. As a result, limited slip function is implemented.

Characteristic of Viscous Coupling center differential
Note that Viscous-Coupling LSD is a speed-sensing device: under no-slip condition, no torque will be sent to another axle. Whenever slip occurs, theoretically up to 100% torque can be sent to any axle, depending on the traction difference between front and rear axle. Therefore it is a part-time 4WD.
Being a part-time 4WD, it does not have the neutral steering of a permanent 4WD can obtain. For cars based on rear-wheel drive models, such as Porsche 911 Carrera 4, this is not a real problem - as normally the car runs like a RWD car thus is capable to deliver the desirable throttle oversteer . However, for other front-wheel drive-based cars like VW Golf Syncro and Volvo 850 AWD, the part-time 4WD can do nothing to correct their understeering manner. This is the first disadvantage.

The next problem is the delay before the 4WD get into effective. Since viscous liquid is not a fixed medium (unlike gear), it takes time and speed difference to be effective. The function between speed difference and torque transfer is an exponential function - that means in the early stage of slip, torque transfer remains near zero.
To cure this problem, most manufacturer varies the final drive ratio such that introduce a slightly speed difference even in normal condition. As a result, the car actually runs with 95:5 torque split between front and rear. This shorten the delay time. However, it is still impossible to match the pure mechanical Torsen LSD.

It might be less effective than Torsen system, but it is certainly the cheapest, so we can find it in many mass production 4WD cars.

Rob996

The following confirms my understanding of the C4 AWD system limitations, please refer to the underlined section below.



Viscous Coupling center LSD is commonly used in many simple 4WD systems. One of the earliest examples was Volkswagen's Syncro system.


Inside a viscous coupler as shown in the right hand side picture, there are many circular plates positioning very close to each other. Both drive shafts connect to roughly half of the plates in an alternating sequence as shown. The sealed differential housing is fully contain of a high viscosity liquid, which has a strong tendency to "visco" those plates together.

In normal condition, front and rear axles run at roughly the same speed so the plates and viscous liquid are relatively stable to each other. When tyre slip occurs in one of the axle, that means the alternating plates run at different speed, viscous liquid will try to visco them together. As a result, torque is transferred from the faster driveshaft through the liquid to the slower driveshaft. The greater the speed difference, the larger the torque transfer. As a result, limited slip function is implemented.

Characteristic of Viscous Coupling center differential
Note that Viscous-Coupling LSD is a speed-sensing device: under no-slip condition, no torque will be sent to another axle. Whenever slip occurs, theoretically up to 100% torque can be sent to any axle, depending on the traction difference between front and rear axle. Therefore it is a part-time 4WD.
Being a part-time 4WD, it does not have the neutral steering of a permanent 4WD can obtain. For cars based on rear-wheel drive models, such as Porsche 911 Carrera 4, this is not a real problem - as normally the car runs like a RWD car thus is capable to deliver the desirable throttle oversteer . However, for other front-wheel drive-based cars like VW Golf Syncro and Volvo 850 AWD, the part-time 4WD can do nothing to correct their understeering manner. This is the first disadvantage.

The next problem is the delay before the 4WD get into effective. Since viscous liquid is not a fixed medium (unlike gear), it takes time and speed difference to be effective. The function between speed difference and torque transfer is an exponential function - that means in the early stage of slip, torque transfer remains near zero. To cure this problem, most manufacturer varies the final drive ratio such that introduce a slightly speed difference even in normal condition. As a result, the car actually runs with 95:5 torque split between front and rear. This shorten the delay time. However, it is still impossible to match the pure mechanical Torsen LSD.

It might be less effective than Torsen system, but it is certainly the cheapest, so we can find it in many mass production 4WD cars.
Advantage: Cheap and compact
Disadvantage: Part-time 4WD only. Normally feels like 2WD.
Who use it ? VW Syncro, Lamborghini Diablo VT, Porsche 993/996 Carrera 4 and Turbo, Volvo 850 AWD etc.

wwest

The Porsche C4 transmission output drive shaft is ALWAYS connected to the rear diff'l, SOLIDLY connected. ANY torque coupled to the front diff'l via the VC is deducted/subtracted from the rear drive. The best torque distribution you might possibly attain would be 50/50.

"..visco them together.."

See post #11 for a breakdown of the operational aspects of "visco".

WCE

When I had my 996TT, I noticed the front wheels wouldn't even spin on ice. I did some research at the time and basically came to the conclusion these VCs wear out and depending on how your car's been driven and mileage, you very likely have nothing but front end ballast after about 50k miles!

smackboy1

I used to have a C4 and also Audi, MB 4matic, Range Rover and other 4X4 SUVs but I now drive a RWD and I can offer these observations:

It's possible that the VC is in need of repair e.g. if someone strapped it to a 2WD dyno and ran it for a while with the front wheels strapped down. It's also possible that it's working as intended. It may be that there is torque transferred to the front axles, but just not enough to overcome friction to spin the front wheels.

The Porsche VC AWD is a road going system and is not really designed for offroad or to get the car out if it is buried. It does not have the traction of a 4X4 system, or even Quattro. The whole point was that Porsche wanted an AWD system that drove like RWD. IMHO Porsche used the VC center diff for a few reasons: cheap, mechanically simple, lightweight, maintenance free. The principle of the VC is like using an electric fan to blow a windmill. It's actually not the best way to maximize traction and nobody would use it on the Rubicon Trail. There is torque transfer, but there is no solid physical connection between the front and rear axles. This VC cannot be locked. Combine this with the open rear diff and low ride height and it's easy to see how the car can easily get stranded.

It should be no problem to confirm the front wheels are getting some power. With the steering wheel turned hard over, either: a) at low speed sliding around a slippery parking lot (autocrossers know what I mean) or b) pulling out of a driveway when the front wheels have traction but the rear wheels do not, there should be enough feedback through the steering wheel and the motion of the car to confirm that the front wheels are pulling the car. It's not instantaneous, it can take a slight delay for the fronts to pull while it "senses" the rears lose traction, but there should definitely be more of a 4 wheel drift sensation compared to the oversteering pushing sensation of a RWD. Does that make any sense?

fly4beer

Since PSM will cut/reduce the throttle in a rear slip condition, how will the VC ever hardens?

Does that mean that with PSM on all a C4 gets on the front wheels are the 5% torque defined by the difference between front and rear tire diameter? (rear tires being smaller in diameter resulting to higher rear wheel revolutions than front, resulting to higher revolutions of the shaft from trans to vc than front wheels to vc, revolution difference of the shafts hardening the viscous liquid to the level of about 5% torque to the front)

rb101

I think the PSM will only cut the throttle if it senses rotation of the car to help keep the car from going into a spin. The only time I've ever noticed the throttle being cut on me is when I make a hard u-turn.

Rick
99 996C4
87 944S

Psycho Sid

OK I think we've spun of our axle... talk about a lot of opinions and no real solutions.

That said I'm with wwest, the wheel sensors detect the difference in wheel rotation and will use the vis. dif. to compensate if the rears are spinning and the fronts are not.
So whether your in motion or just digging yourself in your 4x4 system should kick in.

In other words, your vis. dif. is busted...

One of the main reasons why this happens is because owners change their tires without paying attention to the very specific overall wheel+tire diameter related to the 4 wheels drive system.
Stock there is 1mm difference between the front and rear wheels (section width) on the 996 C4S/C4 if you where to put a tire or wheel sizes where the overall height/diameter is of by more than 3mm then effectively your front and rear wheels are running at different rotations per km and this will screwup your vis. dif. GUARANTIED and without giving you any fault indications!

So if that's the case it's an easy fix to make sure it doesn't happen again (right size tires), but a costly one to fix (replace) your vis dif, I know that totally sucks!.

Check out this site and you'll see what I mean.
http://www.wheelsmaster.com/rt_specs.jsp

Good luck !

Cheers,

Sid.

s_kelly

Any sealed bearings in that there viscous coupling?

wwest

Sealed as in being inside the VC housing, yes, 2.

Xeattle

Anyone ever remove and replace their viscous drive line on a 996 C4?

Any photos and can you drive around with it removed? I think it is pretty easy to remove but I'm not sure until I pull all the underbelly plates off to have a look.

If the viscous drive is worn out I don't see any harm in leaving it removed until a new unit can be ordered.

I think this part is about $1000 but I'm not certain. I want to pull the part number off the unit this weekend so I can be sure to order the correct one.

Is the 997 viscous driveline any different? Can we run a 997 viscous on a 996 or are the parts one and the same.