Nzo

Would explain why so many people say the car is bad in the snow.

Macster

Apparently. Porsche is apparently limited by the small market/selection of snow tires and the small market demand for Turbo snow tires. The end result is there are no sizes available that deliver the desired revs/mile differences between the front and tire tires.

The AWD system would not be very effective even with snow tires fitted that resulted in the ideal revs/mile differences.

There is no torque transmitted from a dead stop so if the rear tires do not have enough grip to get the car moving they'll just spin. The fronts won't receive any torque because the speed difference between the plates in the viscous coupling the excessive speed difference over stresses the fluid and instead of having it transmit torque from the discs driven by the rear axle to the discs driving the front axle just sheers the fluid which increases its temperature and if (as the Porsche techs cautioned me) just overheats the fluid to the point it runs the coupling/front diff.

While according to Porsche 40% of the torque can be transmitted to the fronts axle this occurs at around 150mph. I dare say even if one had snow tires that were rated to this speed grip to maintain that speed would be the least of one's worries.

Essentially with the style of AWD the Turbo is essentially a rear wheel drive car at pretty much legal highway speeds. Assuming the amount of torque transmitted is linear if at 150mph 40% is transferred then at 75mph roughly just 20% would be transferred. The times I've driven 75mph in my Turbo and I drive it at this speed a lot (hours per day) on my long distance road trips has been on dry pavement and whether there was 0%, 20%, 40%, or even 100% torque transferred it would have not made one iota of difference.

The one time I had my Turbo in the snow -- on summer tires no less (thankfully new summer tires) -- my max. speed was probably 30mph and often less. There was no concern about keeping the car moving but I was very concerning about having to stop the car. Thus I allowed plenty of distance between my car and the big rig ahead I was following through the snow in case the big rig did an emergency stop I would have plenty of time to react and bring my car to a stop and thus (hopefully) avoid a snow storm pile up.

Have to add that the more I think about the Turbo AWD system and the better I understand how it works and what it needs to work and under what conditions it works the less impressed I am with it and the less I like it.

I hope it doesn't come to this but in the event the front diff goes out I will seriously consider deviating from "factory" and instead of replacing the front diff instead have the tech remove the cardan shaft, the front diff, the half shafts, and fit GT3 front spindles/hubs and convert my Turbo to a simpler RWD only configuration.

jumper5836

^ I had had two experiences with all the power being sent to the front being transferred, once on a sandy road from a stop, I accelerated and the rear slipped enough that once it got going all the power was being sent to the front wheels. I thought I blew the engine it had no power, as soon as a let off the throttle and got back on it the power came back. There was no way I was going 155 mph from a stop

Next time was at the track, my left rear tire hit coolant on the exit of a corner and when I accelerated the psm light and power was down again. It felt exactly the same as the first time I encountered it. Lack of power which you could play with the throttle but not full power until backing off a bit then it reverted back to normal. Again the speed coming out of the corner was about 100 kph not even close the 155mph

The AWD system does work, it just does a good job of not feeling like a AWD during normal driving. It really helps when you really need it though.

jumper5836

The general rule is for front and rears must be within 3% of each other's rolling diameter for AWD PSM to work properly.
The Porsche recommended specs for winter tires are 0.9%


just to throw out a few more scenarios

Specification Sidewall Radius Diameter Circumference Revs/Mile Difference
225/40-18 3.5in 12.5in 25.1in 78.8in 804 0.0%
265/35-18 3.7in 12.7in 25.3in 79.5in 797 0.9%
275/35-18 3.8in 12.8in 25.6in 80.4in 788 2.0%
285/30-18 3.4in 12.4in 24.7in 77.7in 815 -1.4%
285/35-18 3.9in 12.9in 25.9in 81.2in 780 3.1%
295/30-18 3.5in 12.5in 25.0in 78.4in 808 -0.5%
295/35-18 4.1in 13.1in 26.1in 82.1in 772 4.2%
305/30-18 3.6in 12.6in 25.2in 79.2in 800 0.5%
305/35-18 4.2in 13.2in 26.4in 83.0in 764 5.3%
315/30-18 3.7in 12.7in 25.4in 79.9in 793 1.4%

Specification Sidewall Radius Diameter Circumference Revs/Mile Difference
235/40-18 3.7in 12.7in 25.4in 79.8in 794 0.0%
265/35-18 3.7in 12.7in 25.3in 79.5in 797 -0.4%
275/35-18 3.8in 12.8in 25.6in 80.4in 788 0.7%
285/30-18 3.4in 12.4in 24.7in 77.7in 815 -2.6%
285/35-18 3.9in 12.9in 25.9in 81.2in 780 1.8%
295/30-18 3.5in 12.5in 25.0in 78.4in 808 -1.7%
295/35-18 4.1in 13.1in 26.1in 82.1in 772 2.9%
305/30-18 3.6in 12.6in 25.2in 79.2in 800 -0.8%
305/35-18 4.2in 13.2in 26.4in 83.0in 764 4.0%
315/30-18 3.7in 12.7in 25.4in 79.9in 793 0.2%

Specification Sidewall Radius Diameter Circumference Revs/Mile Difference
245/40-18 3.9in 12.9in 25.7in 80.8in 784 0.0%
265/35-18 3.7in 12.7in 25.3in 79.5in 797 -1.6%
275/35-18 3.8in 12.8in 25.6in 80.4in 788 -0.5%
285/30-18 3.4in 12.4in 24.7in 77.7in 815 -3.8%
285/35-18 3.9in 12.9in 25.9in 81.2in 780 0.5%
295/30-18 3.5in 12.5in 25.0in 78.4in 808 -2.9%
295/35-18 4.1in 13.1in 26.1in 82.1in 772 1.6%
305/30-18 3.6in 12.6in 25.2in 79.2in 800 -2.0%
305/35-18 4.2in 13.2in 26.4in 83.0in 764 2.7%
315/30-18 3.7in 12.7in 25.4in 79.9in 793 -1.1%

Dock

In a 996 Turbo, ther maximum that can be sent to the front is 40%.

jumper5836

which is why I thought I blew the engine and it had no power

strathconaman

I accept that the larger tires on the rear will cause braking on the front axle. It has caused me much googling over the past few days. Then I thought. How much braking? Well, the diff transfers between 5-40%...then, not a lot, at anything less than supersonic speeds.

If I thought I could live without my Pilot Super Sport tires, I would change them, again. Yet, there is a diminishing set of tires that fit these cars and my posterior sensors haven't measured any adverse effects. I actually prefer the handling in every situation I am generally presented with. Probably because the car slightly oversteers now that there is more grip on the front and the AWD system has been neutered.

I think I am with Macster on this one. The 996 AWD system is next to useless.

Please, please Michelin, make a 295/30r18 Super Sport.

Road King

Amen to that, it's the perfect all around street tire for our cars.

This is all I could find on their site as a place to send feedback/requests.

http://www.michelinman.com/US/en/help/contact.html

I think between Rennlist and Sixspeed we should organize a campaign and send email/feedback requesting this size.

jumper5836

I'd rather see a 305/30/18, though I am not sure why you guys would just run, it's within 3% tolerance
235/40-18
295/35-18

"02996ttx50

... or for that matter, simply stick with gt2 sizes for the matched rolling diameters.. 235/40/18 x 315/30/18.

or re-invent the wheel lol

jumper5836

^Super Sports are not made in that size, as well 315 require a 12" wide wheel.

Third-Reef

My local Porsche dealer says i can have 225/40/18 and 295/30/18 N rated PSS tires in two days for my car.

jumper5836

not in 18" maybe 19"

strathconaman

Sayeth Michelin:

"We appreciate your request for Pilot Super Sport in a 295/30ZR18. Unfortunately, we do not manufacture this tire in the size specified. Your request will be forwarded to the appropriate Marketing Group for future consideration."

Well, at least they got back to me.

strathconaman

Fixed that for you.