How does the 4S handle in bad weather?

Assuming wet roads or light snow on summer tired - Michelin Pilots. My DD is a rear wheel drive with all weather tires.

Although I have not tried it on my C4S, summer tires are bad in cold and especially in snow. It is like being on ice. The 4 wheel drive helps but proper tires make more difference than how many wheels are driven.

Yes I know its bad and all that, but I drove our C4 all winter last year while my P!g was in the shop. It had 6 year old PS2s on at the time. No issues on ice and snow or in the wet. Even on dry days where I would toss it around my favorite areas I never had an issue with the cold roads. Braking wasn't any worse than I would expect in similar summer conditions either. I have no doubt all seasons or winters would be better in a sudden avoidance maneuver at speed, but I never felt the car wasn't in control or more squirrelly than it should be. This year we have fresh PS2s and I'll still take it in once a week and won't be worried if another situation arises that forces it back into DD service again.

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While you can get by -- many do me included -- driving in the winter even in snow on high performance summer tires there's a real risk of having an accident. Most of the time in marginal traction conditions -- snow -- I left the Boxster at home and drove my other car when the roads were covered in snow. A few times I took the Boxster I did ok but in one case really regretted. I spent hours driving through one of the worst snow storms in memory in northern Arkansas and southern Missouri. Depending upon where you live you may be able to leave the car in the garage and when the roads clear you can take the car out and enjoy it. You need to remember the summer tires won't grip very well cold so cool your jets on the turns. But my advice is if in winter you are going to rely upon the car every day, rain or shine or snow, especially snow, you really should have a set of proper winter tires fitted. Real snow tires, not all season. The AWD won't help you. The front axle is driven by a viscous fluid coupling and at low speeds even with the rear tires spinning like crazy the front tires do not receive any real amount of torque. If the spinning of the rears is kept up for too long the viscous coupling could be at risk. If you don't believe me google videos of C4's and C4S's in the snow and watch the rear tires just spin like crazy while the front tires are in some cases stationary. Also, I have had the Turbo in the snow and upon giving the engine some throttle the rear tires break loose but I didm't feel any pulling by the front tires. Essentially the AWD system is a RWD system in marginal traction conditions. Only when you view videos of 997 C4's/C4S's in the snow do you see the front tires spin along with rears. The 997 AWD system uses a different method of transferring torque to the front axle. Even so it is often not the going that is a problem in marginal traction conditions, but the turning and stopping. In the case of that snow storm while the car was moving forward ok -- at around 15 to 20mph -- upon a steeply banked turn on a highway the car just slide down the banking to the shoulder of the road. After some rocking I managed to get the car moving again and up the banking and joined back in the single file procession of vehicles moving north but it was touch and go for a few minutes.

The tires are round... They will roll in any conditions. Your stopping distances will be significantly increased and your cornering ability will be significantly decreased. If you are careful and drive slowly, everything will probably be fine. If you have to make a panic stop or evasive maneuver, then you'll have trouble.

Soit seems the AWD has really nothing to add for bad weather. Macster, if the fronts don't kick in when the rears are spinning then when do they start working?

I was under the impression the design was to help at speed when the yaw reached a certain point combined with rear wheel spin the front received additional horsepower to give the front end a bit of traction and 'tug forward' to help correct an oversteer condition instead of leaving the front end to be light and just getting pushed haphazardly by the mass behind it that was starting to slide out from behind the front. And during deceleration it would likewise keep things stable by modulating some torque to the front if the proper trajectory of the car could be helped by that force.

What macster said. summer tires are not only useless in the snow, they are downright dangerous. And yeah the AWD system in the 996 is for performance input at the high end of the car's ability ON DRY PAVEMENT. It is not 4wd like in a truck, not by a long shot....... not even remotely close to quattro......... the PSM system does more to "save your ***".

The 996 utilizes a viscous coupling to drive the front wheels. It sends 5% of the power to the front wheels and it can send up to 40% of the power to the front under the right circumstances. If you google viscous couplings then you'll find some good videos that show how they work. Essentially, the viscous coupling is filled with a series of clutch discs and a fluid called dilitant fluid. This fluid is very sensitive to temperature. At low temperatures, it has a low viscosity. At higher temeratures, it becomes very viscous - almost to the point of being solid. This change in viscosity allows the clutch packs to engage and drive the front wheels. So, what changes the viscosity of the fluid? It heats up due to friction caused by the wheels on the rear axle spinning at a different rate of speed than the front axle. The fluid reacts very quickly - it doesn't take a long time to change. There are a couple of primary reasons where you might see this speed differential between the two axles. The first is in cornering like you might see on a race track. This condition will allow the front wheels to get some power (up to 40%) and help you power out of a corner. The second is on a slippery road in the winter or in wet weather where you are trying to get traction. The problem with the second situation is that PSM will sense the wheel slip in the rear of the car and it will intervene before the front wheels ever get any power. The system really doesn't do much for you in winter driving conditions. The 997.1 AWD normally aspirated cars utilize the same system, while the 997 TT cars and all of the 997.2 AWD cars utilize a computer controlled AWD system that is more effective in winter conditions. All that being said, I drove my 996 C4S year round when it was my DD. With proper winter rubber it handled very well in virtually all conditions. Deep snow was my only real problem, and that was primarily due to ground clearance.

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That's a good question. The short answer is as best I understand it the fronts kick in (receive torque) shortly after one drives off in the car. However, in marginal traction conditions in cold temperatures with the rears spinning and the fronts stationary who knows? The long answer, well, not so much an answer but some background to the above short answer, follows. The torque is transferred by a viscous fluid coupling consisting of discs connected to the cardan shaft, which is driven by the transmission, and discs connected to the front diff and which are driven by the front diff, with the discs enclosed in a housing filled with a special (silicon based) fluid. This fluid receives its special classification because it becomes viscous (some sources say it hardens) when it gets to its working temperature (320F). The collection of discs is kind of like a multi-plate clutch pack except in this case the discs never make contact with each other. Under normal driving conditions, as the car moves down the road and the rear and front tires rotate at their slightly different speeds -- courtesy of different tire diameters (the diffs have the same gear ratio) -- and the discs connected to the rear diff and the discs connected to the front diff spin they do so at different speeds. Just so it is clear the discs driven by the rear diff spin slightly faster than the discs driven by the front diff. This difference in speed between the discs creates friction in the fluid in the tight spaces between the discs. This friction heats the special fluid which as it gets hotter becomes more viscous. As I mentioned above, but it bears repeating, the fluid's working temperature is quite high: 320F. This now hot and thus much more viscous fluid transmits the torque from the discs connected to the rear diff to the discs connected to the front diff. Porsche states the front axle gets anywhere from 5% to 40% of the engine's torque. My info is it takes some time -- less than a minute IIRC -- of driving before the difference in rotational speed between the driving discs and the driven discs has generated sufficient heat from fluid friction to cause the fluid to harden to the point torque is transferred. The official stated minimum is 5% torque, but this almost certainly varies from car to car due to allowable manufacturing tolerances. As an aside, this might explain some of the while rare early or premature front diff failures. The diff left the factory in such a condition the fluid ran hotter than normal even under normal driving conditions and this hotter running fluid resulted in higher than normal operating temperature which ruined the coupling, or the diff. So say you get in your Turbo after it has completely cooled down. You start the engine and drive the car. After maybe close to a minute of driving there could be 5% torque directed to the front diff. 'course, it depends upon the type of driving. If you immediately enter a highway and accelerate up to speed the time could be less. OTOH, if instead you head off on city streets with stop sign and stop light controlled intersections it could longer for the torque to appear at the front axle. Also, I do not know what role ambient temperature plays. In Porsche 996 Essential Companion in the chapter on AWD systems in the section on the 996 AWD system under a Note it says "Hot is good. Cold is bad." At some speed -- I have read it is over 150mph -- the maximum amount of torque -- 40% -- is directed to the front diff. As another aside, I think this is amazing. The engine produces 415 ftlbs of torque between 2700 and 4600 RPMs. So the fluid transmits 166 ftlbs of torque with the car traveling at 150+ mph yet also permits enough slippage so the slower rotational speed of the front tires doesn't act as brake to the faster rotating rear tires. Amazing. Simply amazing. What about marginal traction conditions? Now one would think -- at least I did initially -- that with the rear wheels spinning this would transmit torque to the front axle and front wheels/tires. It probably does. What happens though if the front tires are stationary (or not turning within some narrow range of speed as the rear tires) while the fluid does get hot enough and viscous enough to transmit torque the fluid sheers. It then gets even hotter which ruins the fluid, or this overheats the discs and these are compromised, or the fluid at high temperature abrades or in some mechanical way damages the discs, or the viscous fluid coupling bearings overheat and fail, or the viscous fluid coupling housing gets too hot and overheats the diff fluid and this causes the front diff hardware (bearings/seals) to fail. Now perhaps in cold weather starting out with a cold car the viscous fluid doesn't get hot enough and thus no torque is transmitted. However, I have been told is is verboten to if in a situation that the front tires are on dry pavement and the rear tires are on snow/ice to spin the rear wheels. Thus suggests to me that too great a difference in rotational speed damages the viscous fluid coupling. Maybe the 996 Turbo factory manual can be of some help? In the Turbo factory manual there is a test to check the function of the viscous fluid coupling. Some high points: Test only with coupling at room temperature. Test speed of front wheels between 3kph and 5kph (wheels driven rollers of roller brake tester). Test duration 3 minutes; record the traction every 30 seconds. Repeat measurements after viscous clutch has cooled for one hour. Briefly, the test consists of putting the front wheels on a driven roller (the rear wheels are *not* on rollers) and driving this roller and measuring the traction 6 times during the 3 minutes of testing. The viscous clutch functions properly if the traction measurement of the front wheels lies within the limit curves, shown in a graph, depending upon the drive speed of the roller brake tester rollers. I'd give the ranges for 4kph and 3kph but the graphs are confusing. I refer you to the two graphs below. Given the test conditions above, I would think the traction reading at the outset with the fluid cold should be less and over time as the roller turned the front wheels the traction reading would go up. But that does not appear to be the case. The other possibility is the graphs are wrong, there is a mistake of some kind. Understand the measurements of traction are in Newtons, not Newton-meters. Newtons are units of linear force. Newton-meters measure torque. So I do not know what the readings on the charts mean regarding the amount of torque that can be transmitted from the diff connected to the rear diff to the discs connected to the front diff. Thus the factory manual really sheds no light on the question and in fact confuses the issue some.

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Not all bad weather is snow. Here in Fla sometimes heavy rains slick roads with many leaves on my country drive are no match for my C4S. The AWD may not make your 996 a snow plow but in all but those very extreme conditions its a significantly more stable and capable all weather car than a C2.

Macster: Thanks for the response. Is the fluid in the center diff just gear lube rated for a LSD or some super special Porsche fluid? If you had a LSD in the transmission, you would just want to make sure you gear lube is rated for a LSD. I have a C4, but am too lazy to look up the fluid specifications. Wonder if it would be worth changing it next year. Thanks!

Thanks Macster for the very comprehensive explanation. Next time in snows here in DC, I'll grab the keys to my 328 with all seasons and leave the P-car parked.

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There is no center diff. There is the rear diff and the front diff. What I think you are referring to is the viscous fluid coupling. This is part of the front diff. Below is at line drawing of the unit. It appears to be in a sealed housing contained entirely in the front diff housing, albeit a quite odd shaped front diff housing, due to having to accommodate the viscous clutch. While there is a fluid change schedule for the diff fluids the viscous clutch unit is sealed and AFAIK the fluid is not a serviceable item. You should change the front/rear diff fluids on schedule (or more often if you want) and use the proper fluid, but there is no servicing of the viscous clutch required. AFAIK, Porsche didn't offer a LSD except (and I haven't confirmed this just working from memory (or imagination...)) in ROW cars and only in the S model and only one year. I do not know what fluid Porsche specs for these diffs fitted with an LSD. If you have an aftermarket LSD unit installed you probably need to use whatever fluid the manufacturer of the LSD recommends.

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I have not had my Turbo in much rain, but when I encounter rain in the Turbo I slow down and I use the throttle judiciously. The biggest problem is not the capability of the AWD -- I just do not get any feel the AWD is doing anything though I know it is "working" -- in the rain but the tires' inability to deal with water. With new tires the car is not bad but it still doesn't live up to my memory of other cars that were fitted with tires that just handled water better. Again the problem in marginal or potentially marginal traction conditions is *not* the moving the car forward. It is the turning and the stopping. In the case of rain with reasonable application of the throttle the car will move off and gain speed at a pretty impressive clip with no drama like tires spinning or any flashes from the nanny systems' warning lights. While I haven't had much time in the Turbo in the rain -- it hasn't rained much where I live and drive -- it has been raining recently and I did drive the Turbo in the rain, pretty heavy rain, too, the other day. I was able to accelerate the car quite hard and get on the freeway and get up to speed with no signs of tire slippage. However, I made sure I allowed for plenty of room to stop should the traffic require it. That is what gives me ******* (well, that and the concern about hydroplaning) when driving in the rain, the having to slow down quickly, even stop, in an emergency situation. And out here in the rain on a freeway often one is required to slow down even stop. Case in point I got on the freeway in Pleasanton CA heading east to Livermore CA driving a bit below the the limit of 65mph and getting passed right and left by faster moving vehicles and just coming into Livermore encountering several just happened accidents, cars hydroplaning and careening around traffic. As usual the traffic around me is moving at the limit or above and then without any warning other than the brake lights of vehicles ahead the traffic slows to nearly a stop and in one or two lanes in which the vehicles involved in the accident ended up in the traffic comes to a stop. In situations like this I don't think AWD plays a role. It is up to the tires mainly how well they grip. But I will grant you that maybe that is the beauty of the AWD system that its contribution is real but transparent. I guess maybe I was expecting the car to feel like some old 4-wheel drive truck, or what I imagined one to feel like, with all 4 tires clawing at the pavement or at the snow -- I've had it in the snow a time or two -- and instead it really doesn't feel like anything. I'm referring specifically to the AWD system now.

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Unfortunately repetition of internet misconceptions does not make them right. There is a lot of truth in Macster's text above but some of the basic assumptions are wrong: Viscous coupling fluids are dilitant and thermally stable, which means that they thicken when sheared but the viscosity is reasonably stable with temperature change. The coupling transmits torque due shear thickening of the fluid and temperature rise is a consequence of the friction in the fluid not a mechanism for thickening it. Temperature stability is an important factor in the design of the fluid. Under normal operation conditions the viscosity of the fluid will drop slightly with increase in temperature and the passage of time. This is also shown clearly in the Porsche awd tests referenced by Macster. Dependent on the particular fluid in use there were generally two modes of failure which occurred if the fluid was heated beyond its design limits: the fluid would either gel due permanent entanglement of the chain molecules which provide the shear thickening characteristic of the fluid, the result being a viscous coupling which is very stiff and provides too much torque transfer at all times. ...or the the temperature chemically breaks down the chain molecules into simpler units and renders it incapable of transmitting torque which seems to be the case with the 996 viscous couplings. The viscous coupling is a very simple device, the only function of which is to passively transmit torque from one side of the coupling to the other; if the input shaft is moving at a different speed to the output shaft torque is transmitted. There are no sensors or controls on the device so it is irrelevant whether the vehicle is standing still or doing 150mph. Spinning rear wheels on a static vehicle will produce a bigger speed differential across the coupling that that produced by the front to rear speed differential of a car travelling at 150mph with correctly sized tyres. The viscous coupling is a wear item, the clutch plates wear, the fluid deteriorates and may fail completely if abused. Anecdotally the lifespan of this type of coupling is about 70k miles to 80k miles so inevitably a lot of 996 C4s, C4Ss and turbo owners will driving around oblivious to the fact that their viscous coupling has failed. There are no symptoms other than of a lack of awd. I don't know if you guys have annual safety testing out your way but one the tests over here is a brake test on a rolling road tester. I have noted the following when testing all of the awd Porsches which I have owned. When doing the brake test, on completion of each test, when the tester releases the brakes with the rollers still driving the car tries to jump off the rollers but drops back as the tyres lift of the rollers and then tries to jump out again as soon as the wheels receive drive from the moving rollers. The other set of wheels are of course static during this test. Unfortunately we have no snow here with which try the other test.

Not sure how the "clutch" plates can wear as there is no contact with the adjacent plates. If they wear then it must be from some kind of abrasion from the shearing of the fluid. The behavior you report arising from that safety test is due using the wrong setup to "test" whatever the test is trying to test. Porsche clearly states if the car is used on a roller dyno the front rollers must be driven at the same speed as the rear rollers. What is being done in the test you described is one axle is being spun while the other remains stationary. This over stresses the coupling and one is lucky of the unit doesn't fail. Ignoring the real risk of damage it subjects the Porsche AWD viscous coupling too I'm not sure the test has any legitimacy anyhow for even non-AWD vehicles.

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The only reason to chose the BMW 328 over the Porsche is the tires on the BMW although all season tires are not all that suited for snow. Provided the Porsche was fitted with proper snow tires and the snow wasn't too deep I'd take the Porsche over the BMW with its all season tires. Even though the Porsche AWD system doesn't give the Porsche the off road capabilities of a Jeep with the right tires fitted and on paved roads the Porsche should get around just fine. My only caveat is if you find the car "stuck" and the rear tires spinning to avoid this and rock the car without spinning the rear tires until the car gets free or just arrange to have the car towed out of where it is. You have a tow eye in the tool kit. When I drove in snowy weather I carried a heavy duty u-bolt that I could put through towing bolt eye and then connect a heavy duty towing strap to the u-bolt and attach the other end to another vehicle that could then tow my car out of where it had become stuck. The strap wants to be long enough so the tow vehicle doesn't have to get right on your car's bumper for it to reach your car. I might have carried another u-bolt to perhaps make it easier to connect the strap to a tow vehicle but I can't recall now. I never needed this setup even though the few times I ventured into snow with my Boxster I had summer tires on the thing.

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How does the 4S handle in bad weather? Well the short answer & maybe the most important fact to keep in mind regardless of conditions. It reacts predictably to driver imput!

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Have you ever noticed that the propellers on ships wear over time when they are in contact with nothing more than water. Where there is friction heat is generated and there is wear. If your referring to the test from the 996t WSM, it is designed to test that torque is being correctly transmitted by the VC and there is nothing wrong with the procedure. It also verifies the documented fact that VC fluids lose viscosity slightly with increased temperature. That is correct for a dynamometer. Porsche state that brake tests can be carried out on single roller brake test stands provided that the test is carried out at no more than 7.5kph for no more than 20 secs. A brake test typically takes no more than 5 to 10 seconds. Note this is essentially the same apparatus as Porsche mention in the VC test procedure.