Okay ive read the threads but I gotta ask specifically, has anybody actually fried their center diff using wheels and tires of unequal diameters on their 996tt .... No theory, hearsay or perpetuation of stuff ... Who as experienced themselves or had friends experience this issue? I'm trying to better understand probability or frequency and I'm curious as ive seen things like IMS failure issue on 996 cars be discussed to the point where it sounds like huge percentages of these cars are having cataspraphic engine failure yet when I chat with the top Porsche wrenches in my major metro market area they all are clear on how truly infrequent the ims failure issue actually occurs ... So before I dismiss the idea of bolting on those great looking wolverine 997 tt wheels on my 996tt I wanna hear from those who know directly about center diff damage probability ...thanks!

 

It has to do with the tire sizes. As long as you run 235/35/19 (or 225/35/19) front and 315/30/19 rear on the 997tt wheels, you will not have any issues.

 

Does anuone know the specifications of 996TT and 997TT wheels? Are the first lighter cause they are 18"?

 

I had ran different diameters between front and back for about 7k miles and then changed them back to the same diameter. No diff problems.

 

Well maybe not now or yet!!!!!!!!!!!

Having the same rolling Diameter tires on any AWD / 4WD is very important. You may not have had a major failure but you did cause undue wear and tear on the driveline that could cost you down the road.

 

It is not a theory or myth. Using tires of un equal size is bad...... Period, Dot, THE END!!!!!!!!!!!
I can explain it in a pretty simple manner... I am going to use even numbers for ease!!!!
Let say your car has axle drive ratios of 4/1... That means for every 4 times the drive shaft from the center diff goes around the axles the wheels are mounted on go around 1 time...
So you have a big wheel on the back, it has a circumference of 5 feet
You have a smaller wheel on the front, it has a circumference of 4 feet.
In one mile the big tire will go around 1056 times
In one mile the small tire will go around 1320 times
So with axle ratios of 4 to 1 being constant you get where this is going.
The center diff that is driving both the tires goes around 4224 times and the rear tires should have gone around 1/4 of that = 1056; the front should have gone around 1320 revs.... So that is a difference of 264 revolutions between the two tires.
Now ask yourself, what ate up that differance,??? Were the tires spinning or did the mechanical components of the engine and drive train absorb that difference.... If so what was the result of it; is there something in the system that can absorb minor differences.... If so what is the result of that.....
In short, when the diameter difference is to much for too long something goes Kaboom!!!!!!!!!!!!! Now if it was only the pavement behind me being ripped from the ground that would just be plain cool to watch... BUT IT IS NOT!!!!!!!!!!!!!

The drive line of your car pays the price.

 

Do a search on this. The general consensus is that it will work but will require spacers on the rear and buying correct tire sizes (i.e- not the 997 sizes)

 

May I suggest to All that I'm guessing that the answer here isn't ...it's bad don't do it... But probably that at some degree of variance it will cause damage sufficient to require repair .... In other word guys ... It's a dial not a switch so to speak... And I'm trying to determine if anybody has first hand experience with a center diff failure and what the variance in diameter was and was damage done racing where you're "at speed" and engaging AWD or just street driving ...

 

If you buy the right tire sizes, there's no reason to think that this wheel would cause any more damage the other piles of 19" wheels guys are throwing on.

 

I don't have spacers in my setup. I only thought the spacers were to make the tires look more flush and served no function with the rotation of the tires. Before I made the switch, I did all the research I could and basically came to the conclusion that you have to keep tire rotations as equal as possible between front and rear (<1% difference) and you had to keep the height below a certain number, i forget what it is but its in the other threads. As long as you follow those rules you should be golden.

As a side note. I can attest to the harm of just throwing on random tires. My previous car, an e92 335xi blew its diff one random day while I was making a left turn. I thought it was just a manufacturing fluke for a long time until it dawned on me that I wasn't using the proper wheel and tire setup. the diff blew at about 17k miles if I remember correctly at which point I probably had put about 10k miles on the wrong size tires.

 

This topic has been talked about several times before, each time everyone is saying how you should stick to the original sizes and overall diameter...
How about mentioning some tire brands/models that meet those specifications?
For example what tires (brand, model, size) will be good and won't damage the diffs for the stock 18" and what tires for 19" rims.

 

i have been runnimg oem 997 turbo rims and tires no spacers for 8000 miles on my 04 turbo with no issues. use your own decresion.

 

I'm running 235/35/19 front and 315/25/19 rear. Overall rotational difference is less than 1%

 

What brand and model? Some tires of the same size have slightly different diameters. By the way what are the stock 18" tire diameters?

 

Nitto Invos.