The viscous clutch slips quite a bit. There's some charts in the shop manual (sect. 39-8)which show front wheel driving force at various speed differences between the front and rear wheels. With a 3kph difference the front wheels exert 145 pounds of forward force (total). At a 5 kph (3mph) differential the force is only 208 pounds. As the viscous oil heats up the force decreases.

The PSM system has no way to transfer additional torque to the front, it's a function or the front and rear axle speed differential and oil temperature. The chart only goes to 5kph. I don't know what happens if you are spinning the rear tires 25 mph.

 

Remember, the 141 lb-ft of torque is at WOT and on boost, so with regard to your snow encounter, I don't think you had very much power going to the front wheels.

The PSM would have to be removed too, because it is programed for the AWD.

BTW, the power going to the front wheels is not totally wasted, it is pulling the car.

 

Ebaker, that is very interesting info. I would imagine Porsche would allow more front torque transfer depending on accelerator depression upon launch to get the car going, and maybe tranferring most of the torque to the rears once up to speed.

Weird thing is...am I the only person who noticed the front wheels do not spin while being stuck in the snow. Maybe if I had snow tires, I would not be asking this, but on Monday, most of the snow melted here in MD, but I only had I street to go on to get to nice dry road. Baltimore City sucks for snow removal...where I live in Howard County, there was absolutley no snow on the roads at all.

 

Is there a difference of how the PSM and the 4WD behaves when you have Tiptronic on your TT? I had my TT in the snow a couple of times and I never really had any serious problems (even with summer tires). My TT has a Tiptronic.

 

Those force numbers are qualified in terms of throttle position and rpm I hope.

 

Hey Marx, my car is a 6speed, but never thought there would any difference in how PSM works. I am a new (7 months total) Porsche owner and absolutely love the car, but not in any way an expert on Porsches. I have had 2 BMW's, one I still have, the 2002 M3 Cab, and that car is horrendous in the snow.

Funny thing is that I am a finance manager at an Acura dealer, and was offered an MDX for the weekend, but knew we would only have 2 days of bad weather, and thought on Monday the roads would be ok to drive out on. I really only had about 200 feet to go to hit a dry road, but only made it about 3 feet forward slipping and sliding....and about 1/2 mile in reverse to a nice clean street.

The TT stays in once we have a sign on snow....and I just need to grab a demo for a few days once it snows.

 

I don't think so. The viscous clutch doesn't have any input from the computer or throttle position. It just provides torque to the front axle based on the difference in the front and rear axle rpm.

 

I understand that, it's just that for torque to be provided it first has to be made at the engine, and that engine torque is a function of throttle position and rpm. At 2000 rpm and steady state throttle the engine output is pretty low, and 5% of that low number is very low. Even 40% would be a low power number going to the front wheels.

 

M3CAB, what you're describing is definitely NOT normal! I drove my C4 in 3 Boston winters, and while it's not a Turbo, they share the exact same AWD and PSM systems. With PSM on, you should be able to spin all four tires in the snow without any problem and still have the car go forward, even with summer tires. If the car starts to go sideways, PSM will cut the power to certain wheels. But as long as you're going straight, you can spin all four tires until the sun goes down! With PSM off, the rear wheels will spin and you won't get much power to the front wheels. In my experience, with PSM off the car only had the minimum 5% power at the front wheels, no matter what the traction was like.

If you can replicate this problem, I'd take it to the dealer and show them. There's definitely something not right here! Even with summer tires on a hill, you should be able to drive it, unless it was truly solid ice or extremely packed snow. The only other thing I can think of is that you're not used to driving in snow and always gassed it way too much. You can get any car stuck if you do that!

As far as not driving a Turbo in the snow, why not?!?!?!?!??! These cars are meant to be driven in ALL weather!

 

Hey teflon_jones, I have lived all of my life here in MD, and have driven all kinds of cars in the snow...even though we only get 5-10 days of it here compared to where you live in Boston. I tried with the PSM on, and had no front wheel spin at all. I will take your advise and take my car to my local dealer and have it checked out. I did notice with PSM on though that a little yellow light with an "!" would flash. I did try light throttle and kinda heavy to make the car move, I even tried to rock the car, but no luck.

 

For some reason (which is totally beyond my understanding) BMWs are the absolute worst cars to drive in the snow. I had various RWD cars, but the BMWs were just incredibly bad (from the 316i coupe up to the 850i). E.g. Mercedes, although also RWD, appears to be much better in the snow.

P.S.: As everyone knows, BMW stands for "Bayrische Motorenwerke" (= Bavarian Motorworks). Funny thing is, that BAVARIA is the place with the most snow and the most mountains in Germany. It's really strange, why BMWs cannot be driven properly in the snow (no matter with what tires on).