mfanatic325

Posted this in the Wheels section, but I guess not many people visit it. Going to see if there's any luck on this main 996 forum:

I'm considering purchasing aftermarket wheels/tires for my 2001 996TT.
From the preliminary research I've done on the forum thus far, I know that the OEM sizes are as follows:
Fronts: 18x8 +50 with 225/40/18 tires
Rears: 18x11 +45 with 295/35/18 tires

I'm in need of assistance in confirming whether or not the following aftermarket set will fit:

Aftermarket specs currently being considered:
Fronts: 19x8.5 +50 with 235/35/19 tires
Rears: 19x11.5 +60 with 305/30/19 tires

Macster

Can't tell you if they physically will fit but the tire sizes are imcompatible with the Turbo AWD system.

The front tires make 792 revs/mile the rears 770 revs/mile. Turbo wheel/tire sizes want to be selected to have the rear tires turn a bit faster, more revs/mile, than the fronts so torque is transferred to the front diff/front wheels/tires.

Those are not the wheels/tires you are looking for.

mfanatic325

Appreciate it!

Assuming the wheels physically fit, what would you recommend the tire sizing to be at, to get them closer to the slower/faster (front-to-rear) revs/mile bias.\?

kmagnuss

But isn't that the tire size that's stock on the 997TT's? I know exactly jack squat about them, so maybe they have a different transmission/differential system.

To answer your wheel question though... no... they will not fit. They wont' even mount up... they're set too far inward in the rears.

mfanatic325

I'm willing to use spacers in the back, but wondering if I'll need 10mm or 15mm...

And yeah, I THOUGHT the tire sizing might be okay, but perhaps not for AWD. It's coming off of a non-Turbo 997, so it could very well be wrong sizing.

mfanatic325

So I reached out to the wheel manufacturer, and they confirmed that the fronts will be fine, but the rears will need different tire size, like a 315/25/19 in order to run properly with my 996TT's AWD system and all.
Furthermore, they say that I'll need approximately 15mm spacers for the rears.

Still not 100% sure whether or not I can fit the wheels under the fenders if I essentially have 19x11.5 +45 offset, nor with 315-width tires, but I'm willing to do fender rolling if need be to make everything clear.

kmagnuss

FYI my wheels are the following... on a 996tt
Champion Motorsport RS171
8.5x19” ET53 and 11.5x19” ET44 (edited to correct)

I'm running 235/35's and 315/25's with no transmission issues...
I'm also running 15mm spacers up front and 10mm in back and it sits perfect IMO.

Sorry... I had my ET's messed up when I replied about your wheel sizing working or not before. I'm not smart.

The biggest problem with 315/25/19's is that you are limited to Toyo Proxes (or similar), Nitto Invo, and once in a blue moon you used to be able to find PS2's (haven't seen them lately). I'm not aware of any other tires in that size.

mfanatic325

Would you mind posting pix of how your setup sits/looks? I definitely want to gauge whether or not the setup I'm considering can work for the rears.
I'm prepared to do 15mm spacers for the rears, if not more, along with the 315/25/19 tires. Including fender rolling as needed.

Macster

The 996 Turbo/C4S and 997 Turbo AWD systems are very different.

The 996 Turbo (and C4S) use a viscous fluid coupling that relies upon the rear wheels/tires turning more revs per mile -- but not much -- than the front wheels/tires. This speed difference heats up the viscous fluid in the coupling -- working temp ~300F or higher! -- and this viscous fluid then transfers torque to the front diff. The range is from 5% to 40%. I do not know at what speed 5% torque is transmitted. My info is the 40% number occurs at around 150mph.

The 997 AWD system uses a clutch pack that is controlled by a signal from the DME. It is similar to a multi-plate wet clutch. I do not know the max. amount of torque that can be transmitted from the rear to the front. But the transfer is nearly infinitely adjustable from zero to whatever the max is and can be active even from a dead stop.

One can watch videos of 997 AWD car launches and see the front wheels/tires spin from a dead stop. No way one will see that from a 996 Turbo and its AWD system. In fact one can see the rear tires spinning furiously -- in the snow for example -- while neither front wheel/tire is spinning at all and in fact they can both be nearly stationary.

(There's a video of this in another thread in this 996 section. I've seen this before in other videos, too.)

My Porsche tech buddies tell me this is a good way to burn up the viscous coupling. (Another way is to get the rear tires stuck and instead of rocking the car free spinning the rear tires until the viscous coupling/front diff gets cooked.) They get cooked because the viscous fluid gets super hot from the huge speed differences between the rear wheels and the (nearly) stationary front wheels and this heat can get high enough to destroy the plates in the coupling or overheat the bearings in the coupling or even overheat the fluid in the front diff and ruin something in the front diff (bearing, seals, gears even). The viscous coupling though sealed is part of the front diff.

(Improperly towed cars can suffer a failed viscous coupling/front diff too. All 4 tires/wheels have to be off the ground.)

kmagnuss

I edited my ET on the rears above... I had them confused with the narrow body fitment, which was the 62... they're actually 44. So with the spacers it's a net 34.

Yes... I'll snap some pics tomorrow when it's light out.

kmagnuss

Great explanation Macster! Thanks!

mfanatic325

Awesome, please see if you can capture the dead-on rear shot vs. rear 3/4 shot vs. side-profile shot vs. dead-on front shot. I'm curious how far with net 34 offset and 11.5" with 325s sit. Thanks!

mfanatic325

Ugh, due to having to factor in cost of getting new rear tires, and having to use 15mm+ spacers for the rears, I decided against going with this set.

Thanks all for the help!

kmagnuss

Here's one anyway:

craig001

I have a set of 17mm H&R spacers with bolts in my garage. I ended up using 14mm instead with my Kinesis K19 rims on my C4S, PM me if interested.