I'm looking at purchasing an 02 996tt and have a question with regards to the possibility of damage to the viscous clutch differential. The car in question has low mileage (around 12K) but the owner has been running 19" wheels with the incorrect set up of 235/35/19 front and 305/30/19 rear resulting in a taller rear tire ( about .7 inches taller than front) He has put only about 2-3K miles on the car with this incorrect set up.

My question is, what is the likelyhood of any damage to the viscous differential due to the above set up? The car has not been tracked and has just street miles. How easy is it to tell on a PPI if the diff has been damaged? Will it be noisy? Dirty fluid?

I've read the various opinions on this forum it makes it sound that the minute you put slightly different size front and rear tires on the 996tt that it blows up. Is all this just conjecture and heresay or has anyone actually experienced damage to the diff due to the above set up with a slightly taller rear tire compared to the front. I like the car I'm considering but at the same time I don't want to buy into a can of worms. Any valid substanciated opinions / experiences are welcome.

 

Stay within 4% and you should be fine on a 996.

235/35 19 and 315/25 is most trusted.

Please call me, I'll be happy to help you with it.

 

So 4% of 25-26" is around 1", so .7 is in spec according to Damon.

 

I had that same set-up on my car when I bought it, and I know for a fact the prior owner had these "incorrect tires" on the car for about 15k miles. I replaced them when I got the car with the standard 19-inch tire sizes, and have had no detectable issues, vibrations, problems, etc.... I think you'll be fine, but I would replace them just to be safe.

 

Thank you. I just got off the phone with Stanley at GBox and spoke to him at lenght about it. It seems to me that these guys have a lot of experience with Porsche gearbox and diffential issues. I'm not quoting him but this is what I got from the conversation. He told me that its not the differential that gets damaged but rather the viscous coupler which is pretty useless anyway. His take on it is that the AWD set up from Porsche on the 996tt is very limited since only about 50 lbs of torque gets transferred to the front even though that seems low to me. The way I understand it is that there is really no way to tell if the viscous coupler is transferring much torque to the front unless you were to put it on a dyno. He said that almost all the viscous couplers he has seen have heat damage because any time the wheels are turned while driving you are causing the tires to rotate at slightly different speeds. The 996tt is essentially a RWD car unlike the 997tt. He told me that since that 996tt puts so little torgue to the front wheels its really impossible to tell is the coupler is working for the most part or not.

 

I would be buying the car with the stock oem 18s and stock tires. I was hoping that this set up would not damage anything with only 2-3k miles on it but wanted to get some other opinions on it. Thanks....

 

I concur. This AWD system is designed to work at a variety of speeds in both turns and a straight line- turning alone causes a differential between both sides of the coupler. They chose a viscous transfer sys specifically because the car needs to remain flexible- if it was a locker ctr diff you would have to take special precautions like exact tire ratios. I honestly dont think 1 rotation per 100yds or whatever .7" equates to would be a major issue for this type of viscous coupling. Now if it was a LSD style transfer-case w/ clutch packs I would feel differently.

I'm an engr and I say purchase as is.

 

It is diameter that matters. buy it and if it fails, buy my spare diff

 

I was in the same boat a few years ago. I bought my 02 with just under 13K on it with 997tt rims and tires. The prior owner only had the new 19" rims on the car for 2K miles. I have about 7K miles on my car now with the 19" setup and I haven't had any problems.

 

I just bought a set of 19's from a local guy and noticed immediately that he had the wrong tires sizes. They are 19x8.5 and 19x11 and he is running 245/35/19 and 295/30/19. Will this cause an issue? My biggest concern is the fronts, but thought I would throw it out there for you all (the experts) to weigh in. Thanks ahead of time!

 

I doubt it's damaged. These cars transfer very little torque to the front and only IF the rears are spinning. You will not see front tires spinning. Not even in ice or snow. look up any number of you tube videos and you will be hard pressed to see the front tires spinning.

I have had my car in RWD and AWD mode and at higher torque levels AWD definitely makes the car feel very stable.

There are so many RWD conversions that you can get a viscous diff for almost nothing if you're patient and diligent.

I would definitely use this as a bargaining chip.

 

Sage advise.

 

That's the BEST solution......buy the car