strathconaman

You beat me to it. The actual fluid clutch (part number 99633902102) is the same, but the front final drive seems to be 3.33 vs the rear 3.44.

It made me wonder if you could swap a 997.1 front differential into a 996, or run a bigger rear final drive (3.77?) and get the same effect. Either way, it seems that front differentials are about $5K and a new ring and pinion is $3K; a very expensive experiment.

There are a few used ones from Germany on EBay...I tell you what. You guys all chip in to buy me one and I will let you know if it works?

Dock

A 35℅ difference in traction is not at all insignificant.

The front is not going to provide better traction (given the same surface) than the rear. They will break traction at a lower power level than the rears.

Or maybe Berra could just have two of his friends who weigh +180 lb sit on the trunk of his MB and he can attempt to go up his driveway in slippery conditions...

996tnz

I suppose the marketers aren't keen to overly demystify an icon? And the engineers keen to hold some secrets close? But yes, it is very frustrating for those of us who are trying to understand our cars' behaviors to have to scour the internet for unicorn's tears a.k.a. full technical and performance info on some of our cars' systems.

For instance, Porsche has provided lift/downforce figures for the 996T but only for a few particular speeds as opposed to a graph of lift/downforce versus the full range of speeds. And the figures they have supplied are just net figures at each axle for the car as a whole. I would like to know how much downforce the stock wing provides in the raised position. Not the 22 odd kg at 300kph quoted as the net aero downforce on the rear axle, but just the wing's contribution to that. For instance that -22k kg total rear lift could be made up of 40kg of body lift, plus -62kg of wing lift. Or it could be made up of 120kg of body lift plus -142kg of wing lift.

Why do I want to know? The added downforce from adding a gurney/wickerbill to the wing can be estimated if the naked wing's downforce is known. But it isn't, so it's hard to know whether it might be worth a few hundred dollars to add one or not, and to know whether it would be enough to allow some added front downforce. OK, I am aware that it has a dual function as a wing and as a spoiler, so the effect isn't exactly that of a pure wing (nevermind the integrated swelling for the stop light), but a lift figure for the wing alone would be close enough for my purposes.

Same applies to the functioning of the viscous coupling and how it interacts with wheel sizes. Some detailed factory graphs would be nice.

996tnz

But that's not important, as drive is also still going to the rears at the same time - the car never transfers 100% of torque forwards. The point is that the fronts and rears turning together in synchrony will provide more total traction than just the rears turning. If a dog's back paws were a bit bigger than his fronts, that still wouldn't mean a dog goes up a greasy ramp just as well on two legs as on four.

Absolutely, but while they may break traction at a lower axle power level, even at the max transfer limit it still only happens at the same engine power level - due to the 40% limit on the maximum torque split. As stated, that's why the fronts only get up to 40% of total engine torque. Which marries up with them having 40% of the total available traction when on a level uniform surface. So, again as stated, Porsche has already balanced that nicely for you out of the factory. At full VC lockup, the fronts and rears will break loose together. As they should, to ensure that maximum useable traction is available up to that point. And at lower torque splits, the fronts will have more unused traction left available to them than the rears, because Porsche thankfully engineered a very rear-biased AWD car.

Again, engaging the front drive does not mean disengaging rear drive. Under slippery carpark/driveway conditions - with PSM off as advised - there is no shortage of available engine torque. On ice the rears will start to slip at small throttle openings already, and if the rears are spinning up then there is obviously already spare available torque. So directing some torque forwards at those times just gives the engine 4 contact points to act through, rather than just 2. So you are not really robbing the rears of torque to feed the fronts, you're just adding up to two-thirds as much traction again to help move the car. In other words, if a 2WD modded Turbo achieves, say, 100 units worth of traction from its rear axle on a given surface, then an AWD Turbo during maximum torque transfer forwards has 166 units of traction to use.

Dock, you are right that extra weight on an axle helps grip and can be useful, but between a choice of getting 66% more grip via the use of a 50 odd kg AWD system, versus getting 66% more grip by adding 660kgs onto the Turbo's ~1000kg rear axle weighting, I know which I'm picking. Even more so, given that a lot of the extra grip from that added 660kg is just going to be devoted to the very task of moving that very same 660kg up that icy sloped driveway...

This post is a bit shorter, and hopefully clearer. But hey, if you still deeply believe that 2WD is the best way to get about on snow and ice, I'm not here to stop you trading in your AWD for a 2WD and loading up the rear seats with a few potato sacks worth of fishing sinkers to show the rest of us up.

"02996ttx50

geez, i marvel at the technical wisdom imparted here during the holiday season!

but does this mean i shouldn't proceed with my xmas gift to myself of toth dive planes and splitter?! lol

TheKane

Here is a quick video from my trip to Canada. Of note:
As previously stated - Nokian Hakka 7 studded, PSM off (always), guard lsd.
-15 degrees Fahrenheit (not that it matters).
Video has sound.

Dock

But it IS important.

The bigger rear paws would in fact be more helpful on a two legged dog if the dog had a weight distribution of 38/62.

I never said that was the case.

It is more than useful; it is the key.

996tnz

OK, just enjoy your car.

Dock

I do...in the snow when I can.

996tnz

Carlo_Carrera

Anyone who knows anything about driving in the snow will tell you skinnier tires work best.

Carlo_Carrera

Sure you do.

jumper5836

According to Adrian Streather's "Porsche 996: The Essential Companion" (pg. 257)

"One major disadvantage of any viscous coupling system is that the vehicle actually has to be moving or both rear wheels spinning before drive can be transferred forward. The advantage that the 964 Carrera 4 still has over its later rivals is that there is always a minimum of 31% front wheel drive available. This was excellent if stuck in heavy snow or in sand at the beach. A viscous coupling cannot provide this.".

According to Streather (Pg. 259)

"In the standard (read original 993) viscous coupling the fluid hardens at 160C. When both sets of plates are spinning at the same speed and the friction level insufficient to bring the the fluid to hardening temperature, it will remain in a liquid state and 100% of the drive is directed to the rear wheels and nothing to the front wheels....

The ZF design (read new for 996) ensured that the friction generated within the coupling under normal driving conditions is sufficient to exceed 160C, causing the silicon fluid to harden around both sets of plates to provide a small percentage (5%) of permanent front drive. In reality this takes approximately 45 seconds to develop after driving off. "

996tnz

Thanks for that info Jumper, and have looked over those pages too now. Streather doesn't point it out, but spinning up the rears rather than 'normal driving' would heat up the VC fluid from dead cold much quicker of course. I may do some experiments, but if I was forced to guess already, I'd pick maybe a few seconds at most.

Carlo_Carrera

Thanks for the info Jumper.

As I wrote in an earlier post. When I put my car up on a lift with the four wheels hang freely I can spin anyone of them and the other three bigine to rotate almost instantaneously. So there is some power redirection even with the silicone at room temperature.