vogz

I am running 245/35-19 fronts and 305/30-19 rears. (Pilot Super Sports)

Fronts are 25.8in and rears are 26.2in which calculates to a 1.6% difference with rears being taller.

I have noticed zero adverse effects from this setup vs. my previously stock sized 18in setup.

The car gets more traction and handles better than before and doesn't understeer more or less. The traction and handling advantages are probably mostly due to running a PSS tire vs PS2 before and being slightly wider.

jumper5836

225/40/18 front 265/35/18 rear (winter setup)
225/40/18 front 295/30/18 rear
235/40/18 front 295/30/18 rear
235/40/18 front 295/35/18 rear (alt winter setup)
235/40/18 front 315/30/18 rear

Never had issues with AWD with any of the these sizes.

TheKane

Interesting feedback everyone. Thanks!

Since the thread migrated to winter setup questions/opinions. I thought I'd share my setup for 5 winters:

I run 18 inch rims that are matt black from the TireRack. They don't seem to sell them anymore, but they looked very similar to the turbo twist wheels and they even had Porsche center caps (also in matt black) as an add-on. They are heavy, as they are less delicate, something I wanted for our poor winter roads.

I have Nokian Hakkapeliitta 7 (studded) and swap out the wheels/tires with my HRE whenever I drive up to the snow (20-30 times per year). I also did a 4K mile ski trip through Canada and the Icefields Parkway in the middle of winter. It's a great combination. Numbers are:
Front - 225/40-18 at 25.1 diameter
Rear - 255/35-18 at 25.0 diameter

Combined with the guard 60/40 lsd and PSM off, the car is the best snow car I've ever driven.

Carlo_Carrera

Perfect snow/winter sizes IMHO.

strathconaman

I have been running the following Super Sports on my C4S (RPM - Revs per mile, as per Michelin's Specs)

235/40R18 - 819 RPM
285/35R18 - 804 RPM

Which is the same RPM difference as Vogz:

245/35R19 - 807 RPM
305/30R19 - 792 RPM

After reading all the discussions on the 996 AWD system, as far as I can tell, the front diff works by the difference between the RPM front to back. What I can't find is any reasoned explanation as to why the diff doesn't work (or doesn't work as well) when the rears are larger than the front.

From my experience, going from stock tire sizes to my current set up has produced slightly more over steer (and slightly more fun). But that is what I would expect with more rubber up front and less in the rear.

But the Super Sports are such a good tire, if someone could convince me that I was going to damage the diff by running larger rear tires, I would remove the whole AWD system rather than go back to PS2s.

Nzo

I believe when the rears spin faster they create more heat in the center diff which increases the viscosity which transfers more power to the front.

Third-Reef

With rears turning faster the front wheels are pulling the car. With rears turning slower the car is pushing the front wheels. The center viscous coupling can't tell the difference.

Berra

Super Sports are good but they aren't that much better than the PS2.

strathconaman

I hate to appeal to authority...I have a three logical fallacy per day limit.

On pages 258-259 of the 996 Essential Companion, the operation of the diff is described. The front diff operates by the fluid increasing viscosity with temperature. With the stock tires (a RPM difference of 3 RPM) it warms up to 5% power transfer after 45 seconds of driving. It increases to 40% power transfer at about 155mph...

Note that the same diff is used in the C4, and the stock tires (225/40R18f 285/30R18) have an RPM difference of 11.

996 Essential Companion goes on to say that there are no warnings in the owner's manual about tire sizes. There is nothing in the factory manuals.

My conclusion from all of this is that slightly larger rear wheels on the 996 C4/C4S/Turbo will affect handling as described above, and likely will cause the differential to reach 40% power transfer before 155mph, because of the higher temperature within the differential.

This may lead to the failure of the diff from overheating, but for a car that can run 155mph continuously until it runs out of gas, it doesn't seem likely. It also doesn't seem likely that it will cause the end of the world, your dog to run away, or any other ill effects.

Third-Reef

I have been running with larger rears for a year now, about 13k miles and living to tell about it ( 225 45 18 front and 295 35 18, came on the car) it did good on my track day at Laguna going to change to stock size micelin PSS soon and i will report back with any significant changes.

Nzo

Larger rears will get to 40% power transfer later than sooner. Larger means fewer revolutions per mile compared to the front.
For what its worth Ive used both 315 (positive) and 335 (negative) rears with 245 fronts and noticed no difference in handling.

Macster

Not sure how you reach your conclusion. Larger rear "wheels" (tires) will decrease the speed difference of the two sets of discs in the viscous coupling. Thus the fluid obtains its critical temperature -- the temperature at which it is able to transmit torque from the discs driven by the rear diff to those connected to the front diff -- slower than it would with smaller rear tires. And at all speeds the fluid temperature difference will be less than it would be otherwise and thus the amount of torque transmitted from the rear diff to the front diff will be less.

The max. transfer of torque will occur at a higher speed, something above 155mph.

If the larger rear tires are too large the discs driven by the rear diff will spin slower than those connected to the front diff and the transfer of torque will be from the front discs to the rear discs. Since the front tires are less loaded than the rears under some conditions this will exert some braking effect on the front tires which could lead to handling problems which I suspect would be exacrebated in marginal traction conditions or when operating at high speeds during cornering, such as would be the case on the track.

Regarding any tire size call out in the owners manual regarding tire sizes, I don't have my owners manual handy to refer to.

But I do have a Turbo factory manual handy and in it tire sizes (summer and winter) are called out:

Also, in the section covering tires there is this:

If new tyres are to be mounted or the tyres of one axle are to be replaced, tyres of the same make and with the same specification code must always be used on each of the two axles. If tyres are replaced on one axle only, the different tread depth than on the other axle can cause a noticeable change in the familiar handling. This is especially the case if new tyres are mounted on the rear axle. This effect decreases with increasing tyre mileage.

Porsche is pretty much in the camp of how to put this? Encouraging? all servicing to be done at/by a dealer. The owners manual is not a comprehensive reference for those who want to do their own servicing.

And this includes tire selection. The dealer using its parts database know what tire sizes to fit and these will be the correct size to in the case of the 996 Turbo to ensure the viscous coupling will not be subjected to over speeding or under speeding due to incorrect/incompatible rear and front tire size differences.

strathconaman

This doesn't logically follow. If the size difference is greater than "stock" the speed difference between the plates will be greater. Unless you have some calculation that includes the final drives, which appear to be different front to back. My reading suggests it is the friction between the plates that causes the temp to rise, increasing the viscosity and transferring power forward. If there was no difference in speed between the front and rear tires, the diff would never transfer any power.

This description sounds like what would happen if the front diff worked like a torque converter. I am not sure that is the case. Does the diff operate by the momentum of the fluid, or through the change in viscosity?

Macster

Both diffs have the same final drive ratio.

The viscous coupling requires a difference in speed between the discs connected to the rear diff and those connected to the front diff. This difference in speed raises the temperature of the viscous fluid making it, well, more viscous. This viscosity of the fluid then transfers torque from the faster turning discs to the slower turning discs.

There are a couple of critical points: One is the difference in speed doesn't want to be too small. If the speed difference is too small the fluid doesn't get as hot as it should and the amount of torque transferred is reduced. It could even not happen at all if the fluid doesn't get above some critical temperature at which its viscosity becomes enough to transfer torque.

Nor does the speed difference want to be too large. If the speed difference is too large the fluid gets too hot. The fluid already runs quite hot. IIRC the operating temperature of the fluid is in the 350F range. I do not know what happens to the fluid if it gets too hot. Does it break down, crystalize, become abrasive, which could shred the discs? Who knows? I know I don't want to find out the hard way.

This heat has to go somewhere. Since the coupling is part of the front differential the heat goes into the front diff. The diff design is intended to bath the coupling housing with diff fluid which helps remove the heat. The front diff has some cooling fins which help dispose of heat as the car moves down the road. This helps keep the fluid temperature stable albeit at its rather high working temperature.

There is yet one more critical point. That is the rear wheels/tires must turn faster than the fronts. This is so the discs driven by the rear diff spin *faster* than those connected to the front diff so the torque is transferred from the rear to the front.

The viscous coupling is not serviceable and I have seen no pictures of one disassembled. I have seen drawings (X-ray like view) (though I can't recall where now) and this is what I remember. The discs do not operate like a torque converter in an old style automatic transmission. I have the impression they are plain discs, featureless. I do not know what the disc material is.

The viscous fluid is the used to transmit torque via the friction that it has with the discs being driven by the rear diff and with the friction it has with the discs connected to the front diff.

The "trick" to fitting tires to these cars is to stay with sizes that have the rear wheels/tires making more revs per mile than the fronts and that the differences in revs per mile are within the limits of those of the factory sanctioned tire/wheel combinations without having the rear wheels/tires make any more revs per mile than any of the factory sanctioned wheel/tire combinations.

strathconaman

The "porsche recommended" snow tire sizes (225/40R18 and 265/35R18)

Dunlop SP Winter Sport 828 RPM Front, 826 RPM Rear
(I can't find RPMs for the Pirelis)

So, what you are telling us, is that Porsche specs winter tires for these cars, that don't make the AWD system work properly? Or at the very least, make the AWD system less effective?

http://files.porsche.com/filestore/d...or-996-986.pdf