rodC4S

Got my car back from the wiring harness recall and while it was there i had the dealer check my AWD and rear LSD.

All this stuff below if what the tech and i discussed, so if i am wrong so is the Porsche tech. (not unheard of, but this guy seemed to know his stuff)

On the AWD they confirmed the test i did was valid. Jack one front wheel off the ground, all other wheels stay on the ground, car in neutral.

Now, If you feel resistance when you turn the raised front wheel (should take about 5 seconds to rotate by hand) then the Viscous coupling is OK. They dont wear but they do break (case fractures). So if you feel resistance its OK. Freely turning wheel = broke VC.

At the dealer they simply lifted all wheels off and watched all 4 wheels turn when the clutch was let out in gear.

By the way, There is no LSD in the front axle.

Next the rear LSD. Here is the the scoop. These diffs are not like traditional LSD friction plate type diffs found in many other cars. They have added thrust rings with "ramps" that touch the the spider gears shafts.

When the spider gears are driven by the ring gear, the spider gear shafts try to ride up the ramps which pushes the friction plates together and causes the diff to create friction between the rear wheels (just like a clutch). When the car is under acceleration one side of the ramps is under pressure and when the car is under deceleration (through the drivetrain) the other side of the ramps is under pressure. Now, Porsche angled the acceleration side ramps differently from the deceleration side and consequently you get a different degree of "lock" under both situations.

In the standars factory diff you only get 25% lock under acceleration and up to 65% on deceleration. Again confimed by the tech. So what does this mean for testing the diff ?

Well first it means the traditional "turn one back wheel and the other will turn the same way" test WILL NOT WORK because the diff needs to be under load for the plates to lock up. Under little load it will act like an open diff and the wheels will turn in different directions. The dealer said there is a torque test for this but that it takes some time and is prohibitivly expensive. Testing at home he says you are pretty much limited to driving the car. Excessive rear wheelspin is a sign however he pointed out you are only getting 25% lock so it is possible to have the inside wheel spin a little.

So, there is my conclusion. Love to here if anyone has any different thoughts.

Rod

graham_mitchell

interesting, thanks. I don't feel I have gotten to the bottom of my particular 4wd's problems yet.

chris walrod

Thanks for the write-up. Modern open wheel race car diff operate exactly the same as different angle ramps are available to tune the car. The diff is one of the largest ***** to turn as far as chassis tuning goes. These ramp type diffs are really sensitive to clutch plate stack preload. Not sure to what degree in a street car though.

Glad to hear your AWD is good!

rodC4S

Graham,

What is your issue ?

Rod

graham_mitchell

In general, the car doesn't feel like there is any front wheel traction. And in particular, when the car was sitting on snow last winter, I had the rears spinning while the front wheels sat there motionless. That definitely shouldn't happen. I just assumed I was going to have to replace the viscous coupling at some stage. However, having just performed the 'front left wheel rotation test' we discussed, and feeling some considerable resistance, I am confused.

I just replaced the transmission fluid a few days ago. Does the viscous coupling depend on the transmission fluid or is it self-contained?

rodC4S

Graham,

Its self contained, silicone fluid. Some thoughts on the subject.

The front has no LSD so its possible that one front was spinning, also the rears only lock under load which there is not a lot of in the snow, so its poss that a rear was spining as well resulting in even less than 30% drive going to the front The only way to tell would be to get observers to watch both sides of the car. The porsche dealer tech indicated to me that even in the wet the fronts wont spin on a c4s. Next time its wet i will try mine with a couple of observers to watch all the wheels. Let you know. I do know its really hard to make the car spin anything at all in a straight line even in the wet. Its quite easy to get the back end to slide but when you hit the gas the thing accelerates and pulls out the slide so fast you need to change gear and there is not time to really analyse what is happening. My original concern over AWD and LSD was that the car would slide out so easily. I think the AWD is just very subtle and it works best under hard driving (not snow) and others have alluded to that. I will try and find a c2 to test drive and see what it feels like.

Anyone owned a c2 and a C4 wanna comment ?

Rod

graham_mitchell

I'm pretty sure that both front wheels were motionless, otherwise of course I would know that some power was being transferred to the front axle.

I suspected that the viscous clutch was self contained. Hmmm... doesn't leave me with much unless it is an intermittent problem. I will keep testing...

rodC4S

What engine rpm did you use ?

graham_mitchell

Probably peaked at 2000-2500. It was quite a while ago!

Paul

Rod

The test you describe should theoretically work, because of the preload of the Belleville washers on the disc stack. The actual problem with the test lies with the fibrous material "friction" discs in these street LSDs, which go slick within a few thousand miles making the LSD act more like an open diff. Rather than have the dealer replace these discs with the same "horse puckey" junk, go to a transmission builder who knows the availability of plasma-sprayed friction discs, which typically last 30,000 street miles.

protokultur

So, does the C4s make any useful FWD traction in snow/wet or not? I'm getting a little confused here.

graham_mitchell

it certainly should with a significant share of the torque split. Don't take my car as a typical example!

Here is an interesting article which explains the VC:
http://www.ukcar.com/features/tech/traction/viscous.htm

Randy M

Thanks for that article Graham. Though form what I understand the torque split id normally 85/15 correct?

graham_mitchell

If I understand it correctly, the torque will normally be close to a 0%-100% front/rear split. Only if there is a difference between the rolling diameters of the front and rear tires will there be any torque going to the front tires at all! But that is not as bad as it sounds - it is only during wheelslip that 4wd provides a benefit anyway.

protokultur

Oh, OK, are you going to replace your VC unit then (I don't think you can replace the rheothermal silicon fluid)? According to the advertisements it seemed like there was always some torque transferred forward (~10-15%), unless the VC is fused.