thomas.rousseau

After having read the chapter about the 993 4wd system in the book "993- the essential companion" I'm really wondering what the reasons were for Porsche to fit this on the car. I get the feeling that Porsche just did the 4wd for commercial reasons, not for performance at all.

1st: a system built around a viscous oil getting thicker when the rear and front wheels are not turning at the same speed will never be quick enough to actually help you regain control in a critical situation on a racetrack.

2nd: The possibity to improve the driveability in the winter is effectively reduced since the viscous oil then is so cold that it will take minutes of rear wheel spin before you even get the slightest torque on the front.

What is left then is if there is a built in speed difference between rear and front wheels making the viscous coupler send torque to the front always. According to the book this difference is non-existing on the Carrera4 and very slight on the Turbo.

Have anyone tried to replace the visco coupler with for example a torsendiff or something similar. A constant 4wd system of maybe 20-30 percent would maybe make it worthwile to carry the extra 50kilos the system wheighs???

/Thomas

viperbob

Here is a pic of the VC and where if lives inside the end of the tail of the tranny. As yo can see, it is not like a differential. It is more like a torque convertor in an automaitic or a tiptronic. You are correct that the 4wd system in these cars is OK at best for 400 hp under some weather conditions. It is actually detrimental for cars pushing 500+ HP. I took my drive system out, and what a great difference it made. I have since put together and sold quite a number of 2wd conversion kits. Jean actually has one as does Tarday. I know your question is more on trying to make it better, but with such limited numbers, the cost of making a new VC would be astonomical. The factory VC itself sells for $750. I fould the best thing was to save the 150ish lbs of weight. and go back to 2wd.

Rickard 993 Turbo

How much does your 2wd kit cost

thomas.rousseau

Bob,
When you say that you took the 4wd system out, what did you do then actually? Just removed the axis from the VC to the front differential or did the front diff go as well. In that case what did you do with the front drive shafts. If they are removed I guess the wheel speed sensors need to be replaced with the ones from a RWD car.
When you say that you noticed a huge difference once getting rid of the front wheel drive, what do you mean? Better cornering or better high speed stability or? Any drawbacks?

/Thomas

ttAmerica RoadsterAWD

My sentiments exactly! I've read thru google search many c4s and tt owners whom supposedly have a well functioning AWD, but not sure if it works as there have been times when they break loose at the rear,but no grip on front (snow, ice) Is it because it's too cold? Does the silicone need to be quite hot for it to bite and grip once a difference in spin happens between front and rear? If you purchase new rear tires, but keep front unchanged, does the new "taller(because its new)" tire then negate the needed tire difference (front needs to be slightly taller) to induce a 5% traction? Do both front tires turn at the same time if the rear wheels loose traction, or only one? This is too damn nebulous!!

I lifted my car on all fours last night. Tranny in neutral. rotated one rear tire....all wheels turn, then roatated other rear tire...all four wheels turn, then turned each of front wheel...all wheels turned. Turned on the car, gear in first....all wheels turn. Placed a large mirror aside the front wheel, as I accelerated in first gear I could see a rotational acceleration at the front, driver side......did the same at the front passenger side....also accelerates . Went into 2nd and 3rd gear. got rpms to about a steady 2K rpm's. Then accelerated to 4k rpms each time noted an acceleration of each front wheel. Then in 4th gear got to about 30mph, pushed the clutch in, immediately slammed the emergency break to stop the rear tires.....front wheels stopped in short order too. Seems to be OK??? Thinking this exercise should be enough to warm up the VC, Place car back on ground, lift one front tire...damn thing spins freely!.....lift the opposite wheel....same damn song. Go on a test drive...sideways in 3rd gear at 4K rpms under WOT.

So, then, if it seems to be behaving correctly, why the hell do I not have traction in the front?????? I'm thinking this German engeneering is too freaking hokie!!

Is anybody here able to induce front tire spin in wet or slippery surface?? I bet many would like to know.

Jaime

Greg H.

I wish DJ was still hanging out on Rennlist. He used to track a C4S and I understand he is a pretty good driver. He told me the 4wd system saved his a** on more than one occasion.

Greg H.

fc-racer

If you live in a wet climate, the AWD system is awesome. When I lived on the west coast and drove my 91 Turbo, I had MANY scary moments, quite often when I least expected it, even with good tires. The AWD in the 993 lets you get away with a loit, while still giving the great RWD feel we love so much. In addition the VC system is not as bad as people make it out to be. On my racecar, I switched to a torsen centre differential and while it does allow more power to be put down, the feeling it gives is far more abrupt than the 993's system.

911/Q45

I've not gotten spin in the wet(too cautious to drive that hard), but I've definitely felt it spin the front(and rear) tires at the track when cresting a hill at full power and the suspension gets light.

ScottMellor

If I nail the gas at a slow roll in the wet, all 4 tires spin. Michael Schatz explained to me that it only takes about 1/4 of a revolution of the rear wheels for the VC to engage. I think for optimal operation, your rear tires need to be just a tiny bit smaller than the front to keep it pre-loaded. Jim Brady has some good insight and research on this topic. Of course, his car is "Almost Stock".

Felix

I'm sure the AWD was done for the majority of street drivers who would get into trouble with 400+ bhp and twin turbo levels of torque, RWD and no PSM or its 1995 equivalent. The 993tt wasn't meant as a track car; that's what the RS and GT2 are/were for. No AWD on either of them!

Jean

The difference in drivability is night and day. Yes you remove the torque tube, front diff, front axles, change the shifter assembly etc..
As far as grip, I doubt 2WD vs. 4WD will make a difference on high HP cars, you will loose grip no matter what. For a high HP car you need to install a race LSD such as the Motrosport or Guard etc.. with a 40/60 locking at least if you want grip on the track.
You need to improve/alter your suspension geometry as well to make it more suitable and balance over/understeer.
Not a difficult task, just one that needs patience and determination to get it 100% right and benefit from the advantages of 2WD.
If you are worried about resale value or drive just on the street, I would advise not to go there at all.

Felix

Heat is not required for the viscous coupler to work. Heat is generated when there's a speed difference between the input and output flanges but that's OK, to a point. Lots of heat is generated when there's lots of slip. Too much heat and permanent damage occurs. In street driving the coupler should survive but probably not on the track or with high horsepower, both situations where wheelspin is much more common.

ttAmerica RoadsterAWD

Hmmm. I have read that the hotter the silicone (to a point?), the thicker the fluid, hence the stronger the grip. Since it happens to be winter, I'm thinking maybe my problem is exacerbated by the fact that it's cold. People whom don't feel and confirm no front tire peelout have seen this in cold climates, ie, snow and ice.

Jaime

thomas.rousseau

According to the book "993-the essential companion" the visco oil starts gripping when it reaches a temp. of 160 C, so it sure needs heat.
The only thing I could think of (for me to think this works at all) is that the VC actually provides torque to the front all the time (induced by a delibarate speed difference between front and rear).
Then I could imagine that if the visco oil is heated up just by normal driving, it could actually rather quickly send more torque to the front if the speed difference suddenly increased.
According to the book though they claim that the built in speed difference is to small to create even the slightest torque on the front by just normal driving.
An interesting test would be to take the car out for a smooth calm driving during a couple of minutes. Then place the rear wheels on something slippery (like dyno rollers) and measure the torque on the front as function of time. Within 1 second full torque on the front should be delivered if this system shall any effect on driving feel (except during very slippery conditions where probably the VC gets heated up much more by repeated rear wheelspins).

ttAmerica RoadsterAWD

After last nights "execise", I almost lifted the rear with a rolling jack, lined up straight, rear wheels off the ground and carefully engage 1st gear to see if it would crawl. But, I chickened out as thought of my partners back in the ER asking me what I was thinking. Theoretically, leaving front on the floor and lifting the rear by the engine on a rolling jack and slowly letting out the clutch should induce a "frontweel" drive.....

Jaime