Carrera 4 Power Distribution

I've heard plenty of conflicting information about this in the past, but how exactly is the power bias (front and rear) managed in the C4? I assume that the same system is used in the Turbo. To my understanding, under non-slipping conditions, 90% of the power is routed to the rear wheels, making it feel as RWD as possible (after all, it is a sports car). When there is slippage in the rear, then power is routed to the front, up to 40%. However, is power constantly managed? i.e. When you enter a turn, there is more rear bias to aid turn-in, as you accelerate out of it power shifts to the front to balance the onset of oversteer tendencies. I know some AWD systems behave in this way. Or is it purely 90% to the rear and transfer ONLY occurs when there is slippage? Does anyone know exactly how it works? Question 2: I've only driven the C4, but have been wanting a more RWD feel. For those that have driven both the C4/C4S/Turbo hard and then gotten into the C2 for the same thing, is there any noticeable difference without approaching the limit? i.e. When you aren't approaching the limits of the tyres, sliding/oversteering/etc. Any other comments/solid info regarding those questions are much appereciated! Thanks a bunch. Sam EDIT: This is from Porsche's website: "At least 5% of engine power is always applied to the front axle, increasing to roughly 35% in normal driving conditions. In extreme situations, as much as 40% of drive torque can be delivered to the front wheels." So that answers part of it...

5% always front wheels. Blends to 40% front as needed via viscous clutch arrangemaent. Stick with the 4wd IMHO

You are right in the first paragraph. Its a center viscous-coupling differential system that distributes the torque between the rear and front wheels from 5% to 40%. So its not a permanent AWD system, like the Audi Quattro's Torsen differentials, in which the torque is permanently split and fixed. (except A3 platform based models who's Quattro is a Haldex AWD system). The whay Porsche's viscous diff works is that normally only 5% of the torque is sent to the front wheels but when a certain amount of rear slippage occurs, i.e. they spin faster, the viscous plates friction progressively transfer more torque from the rear faster wheels to the front up to 40% of the total torque to compensate. But the diff only comes into play when ones start to spin faster than the others, if you are coming out of a corner and there is no slippage, the torque should remain the same, it doesn't act before slippage happens to "prevent" oversteer for example, as neither does PSM. Thats how all viscous coupling differentials behave. However I have heard somewere that at high speeds, the C4 AWD transffers some more torque than normal to the front wheels for greater directionability, if this is true, I don't know how its done. As to the RWD feel, I sugest you get the RoWM030 factory sport suspensions, besides the much better performance, it also changes the handling characteristics and dials out the factory induced understeer of the standard suspensions. The greater oversteer will gives you more of a RWD feel while still benefiting from the AWD.

I test drove a C2 with USA M030 and a C4S before I purchased my car. My impression was that the C4S felt more stable making turns at 80 mph and above and that the C2 with M030 felt like it had quicker turn in and much more feedback through the steering wheel in 30 - 60 mph turns. The C4S also felt slower in straight line accelleration. Since the majority of my fun driving will be in 30 - 60 mph turns, the choice was obvious for me. (Before anyone flames be because they bought a C4S and think I am full of it, please note that I said "felt like" not "is" in the description above.)

No need to get defensive, you're right on. However, let's hit those 60MPH turns in the snow or rain

The viscous fluid in the front driveline "coupling's" base, native, viscosity is formulated specifically to only couple 5% of the engine torque to the front "natively". It's ONLY when the rear driveline and the front driveline have differing rotational rates that things begin to happen VC wise. When the two sets of interspersed clutch disks within the VC "stir" the fluid it heats up and it's volume increases. Since the fluid is in a hermetically sealed case it cannot expand. So the result is a very high pressure buildup inside the case. It's that pressure that increases the coupling coefficient between the clutch disks, not the actual viscosity. Since PSM incorporates traction control, which will apply the brakes if the rear wheels begin to slip in comparison to the front, with PSM engaged it is very unlikely the VC coupling will ever increase. I'm sure that the "virtual" LSD functionality of "traction" control is not disabled with PSM "off", but does anyone know about front/rear traction control status with PSM "off".

Thanks for the comments!! I'm glad to know that the power isn't constantly shifting. I've wondered about that for a while, and the official Porsche website has a somewhat vague description. I think I would consider a C2 mostly for the "feel" of things, as in tstanding's example of greater steering feel, as opposed to dialing out the bit of "safety" understeer that's in the chassis. I do very little tracking, so there isn't any instance where I could have fun with more oversteer. Also, the chassis is so grippy that you would need to be going a very healthy click to horse around. Things happen too fast and only the track has enough room for you to sort things out if it gets hairy. Of course, this pertains to my abilities. Also, I would not go with 030 simply because the standard ride as as stiff as I will tolerate in my road conditions (decent for the most part, just not as smooth as, say, German roads). Maybe I'll wait for a 997 C2.

i read lots on here about people going with suspension upgrades and mentioning they get rid of factory dialed in understeer that way. i just had my tech dial in some negative camber (minimal) in the front for my c4s and and waalaa, no more understeer. didn't cost a dime since that was just part of the alignment.

The camber adjustments will help minimize the understand but its still there. Better.... just not neutralized. I've also had the camber adjustments made on my C2 and though it handles better..... I'm told with aftermarket upgrades such as the PSS9 / various coilovers combined with GT3 sway bars, the car does handle altogether different. I really need to save more $$$ !!

I don't really see the concern about the understeer, there's more than enough power to kick the tail out if you so desired... :-)

Sam- while that may be true of low speed corners, power application shifts weight to the rear and increases understeer once you're at a speed when you can no longer break the tires loose. Lifting off the gas gives you oversteer in that scenario. AS

exactly Alexander. trying to throw a 996 around like an older vintage 911 or 356 can get real ugly quickly. especially considering the amount and better quality of rubber on the ground these days

You know, I was reading the Porsche's website, which says "At least 5% of engine power is always applied to the front axle, increasing to roughly 35% in normal driving conditions. In extreme situations, as much as 40% of drive torque can be delivered to the front wheels." That seems to contradict with what you guys have been saying. Porsche website claims that roughly 35% of power is routed to the front during "normal driving conditions". According to Carlos' explanation, this should not be happening unless there is slippage in the rear. And I think it's safe to assume that "normal driving conditions" does not mean you're spinning the rear tyres. Any comments regarding the discrepancy?

Sam, the above portion was not in the original description of the Porsche AWD system. Like Carlos, I too assumed that 5%/95% was the distribution under normal conditions, and that anything over the 5% up to 40% to the front was only under conditions where there was discrepancy between front and rear wheel rotation. This is new to me, but is probably the accurate description since it comes straight out of the horses mouth. The viscous coupling and heat generated by differing wheel rotation speeds controls the distribution of torque. I didn't realize there was only a 5% diff between normal and extreme. Regardless, in the real world, or at least in the real world while driving on the track, I find that the AWD allows for earlier gas application and modulation as you hit the apex and go for the exit. The front wheels do pull the car around and help rotate it, so long as you modulate and not mash the gas pedal. I always assumed that I was up to 40% to the front wheels during those times. It's a different and exhilarating form of throttle steer from the C2, or any other 2WD car. At the extremes, the C4S behaves like any other 911; mash the pedal and it heads straight for the barrier, lift and the rear tries to overtake the front. On wwest question. PSM and VC are two separate systems. The VC controls torque distribution front-rear at all times, the 5% to 35% under normal driving conditions, up to 40% if extreme. PSM only comes in if certain parameters that the designers deem to mean unsafe car beahvior are met. The "virtual LSD" is called ABD or Automatic Brake Differential, which is a part of PSM that can not be turned "off". I drive with PSM off at the track. More fun that way. I guess I'm an unsafe driver .

Back to Sam's Q: The Viscous coupling always has a 5% level of lock - thus at low speeds, with good traction, you'll have that amount of power delivered to the front wheels. As vehicle speed increases, the difference in tire diameter Front to Rear creates a different rotation speed on the shafts, thus more pressure in the Viscous coupling, up to a max of 35%. This can be achieved at very high speeds on a good traction surface. If there is additional rotational difference (due to loss of traction, thus faster wheel spin), the Viscous Coupling can acheive an additional 5% of lock for a maximum of 40%. Turning the front wheels can (not always) change the rotational differences, thus the Viscous Coupling's % of lock can change during steering inputs. Hope this helps, J PS - this is why all *4 models REQUIRE different tire sizes front to rear (see owner's manual)

Thanks Sam P and Jeff ES, that explains perfectly what I have always puzzled about with the C4 which is why the wheels don't slip (not even a little bit) when you stab the throttle on wet pavement. They way the viscous compling works it had always seemed to me that the the laws of physics dictate that the back wheels must spin a little in order to get the fronts to engage. So now I get it!!! If you think about it this really is the only logical explanation (which pisses me off that it never occurred to me before reading it here).

I liked the old defintion better, the one based on some form of tire slippage. This new one doesn't make sense to me. Help me figure it out. So, the differing diameters between the front and rear tires cause the differing rotations on the respective shafts causing the pressure build up in the viscous coupling. The more the difference in rotation, the more the coupling, up to 35% in normal driving. But, in that instance, the rotational difference between the front and rear tires is fixed by difference in their sizes. Neither tire gets bigger or smaller under normal conditions. For example, on the C4S, the OEM front tire makes 826 revolutions per mile, while the rear makes 836 revolutions per mile, a fixed difference of 10 rpm. Therefore, the rotational differences between the respective shafts is also fixed, as would be the pressure in the viscous coupling. That means the power delivery to the front is also a fixed number, not a number varying from 5% to 35%. That would contradict Porsche's claim. There has to be another explanation. It has to be based on something that causes varying the rotational differences between front and rear, and not the fixed difference in size between the front and rear tires. Sllipage is one answer, which is what happens when you mash the gas pedal and more power is transferred to the front to compensate for some rear tire slippage. The other is corner turning, which causes a change in the rotational difference between front and rear tires. Both make more sense to me, particularly since the coupling is based on demands for performance. This is all in theory, and I may be completely wrong. Anyone else have a different theory that explains the varying coupling?

Well... In 2001 Lexus added VSC/TRAC, BA, etc, etc. to the RX300. The idea behind Trac was to provide a "virtual" LSD at the rear, and adverse roadbed dynamic traction via automatic braking to any slipping wheel/tire(s). Plus engine dethrottling soon after if the idiot at the wheel didn't have the good sense to lift the throttle on their own. As some of you have likely noticed it didn't take very long (well, too long, really) for them, Lexus, to figure out that the VC, viscous clutch/coupling had been made useless. For the new 04 RX330 the VC has been discarded but with the center open diff'l still in place and the brakes are now used exclusively for engine torque apportionment. If you happen to drive one of those 300HP high torque NorthStar engined FWD Cadilacs you will discover that you cannot "spin" the drive wheels because the engine ECU is too quick to dethrottle the engine on even the tiniest of "slips". Now I cannot tell you that Porsche has done the same thing but I think you would agree that it is certainly within the realm of possibilities. Porsches have NEVER been dragsters, or even known as. And what good do you get out of the tires breaking loose and spinning? Faster off the line? Is the Porsche front driveline VC useless as a traction enhancing device? Since the Porsche has no center diff'l the VC might be there only to provide the required F/R driveline slack while turning. Only time will tell. Oh, the problem with disparate tire circumferences front vs rear so as to heat and thus "stiffen" the VC is that stiffening the VS doesn't bring the rotational rate back into line so the heat just builds and Builds and BUILDS. My daughter just paid Chrysler $2000 for a new PTO, "transfer case", for her AWD T&C as a result of 235/60/16s on the rear vs 225/60/16s on the front. Ate the PTO ring and pinion alive. 84.195" front circumferance vs 85.702" rear, about 1.7% Do you know that some VC cases have an air/gas bubble of a controlled size so the onset of VC activity is delayed until the air bubble is completely compressed by the rising pressure?

Has it yet occurred to anyone else that if the VC becomes stiff enough to route 45% of the engine torque to the front as a result of different tire circumferences F/R then the tire tread would either overheat or get "rubbed" off very quickly? Anyone with a C4(S) getting extremely poor tire wear?

Methinks palting is correct. The VC only does it's thing during turns. Disregarding the few dragsters that might be out there, other than during turning what is the need for engine torque to the front wheels? Even with AWD this is by no means a wintertime adverse roadbed, snow and ice, vehicle. Mine stays in its nice warm garage all winter while the RX shoulders the burden.

Viscous-coupling differential (and all other mechanical type differentials such as LSD lock or Torsen for that matter) such as the 993/996AWD only acts when there is slippage as I understand it. This is a good explanation from Autozine:

Most of the autozine wording is correct but I take issue with the statement... "...theoretically up to 100% torque can be sent to any axle..." The very best that any VC can do, of itself, is a roughly even split. There are basically two ways a VC is used, the method for the Porsche C4 and Chrysler T&C minivan, and the method Toyota and Lexus uses in the HL and RX respectively. Chrysler and Porsche both have a PTO, power take off, from the main driveline, rear for Porsche and front for Chrysler, with a VC in series with the driveline to be torque proportioned. As one can see immediately, even if the VC were completely solid only 50% of the torque would be delivered to "its" drive wheels. The reality is that in order for the VC to maintain a certain level of coupling it must not be allowed to cool back to normal operating temperature so some level of slippage between the two VC shafts must also remain during dynamic VC coupling activity. And let's not forget that were the VC to become too solid the driveline would be horribly stressed during a tight and/or accelerating turn. And did you know that ABS cannot be functional if the two drivelines are solidly coupled? The RX300 uses a center open diff'l with a VC mounted across the two output drivelines, one for the rear and one for the front. If disparate rotational rates develop the VC starts "locking" the center diff'l. 50/50 at best. Don't know how, but on a 4 wheel dyno the RX300 never gets better than 75/25 F/R with EXTREME slippage, 95/5 normal. l

How does the torque curve plot on the power bias transfer? Is it proportional (is the push as great as the pull)?

It would certainly be interesting to test the c4 handling characteristics with same circumference tires front and rear. In that case in theory there would be no power transfer to the front unless the back wheels lose traction or until you get to a corner. Once in a corner the outside wheels spin faster so you'd have power transfer to the front outside wheel in every turn with the tighter the turn the more traction going to the front outside wheel. It almost sounds like a desireable characteristic.

Palting, You're right, of course, if the world were a big straightaway. "Real world" is quite a bit different. Here's what happens: At any speed, the rotational difference (F/R) creates the 5% lock (base level). At low speeds, with good traction, "driving" (a mix of straights and turns) will cause the shafts to turn at slightly different speeds, thus increasing internal VC pressures slightly (perhaps a point or two). As vehicle speeds increase, the radius of turns increases (a sharp turn at high speed is a spin), thus a greater increase in wheelspeed differential side to side (differential), and front to rear (again, at very high speed). If you were to find a sufficiently long straightaway, you'd find that the VC lock to drop back down to @ 5% - until you turned the steering wheel - unless, of course, you held a perfect radius (again, keeping both F & R turning at the same rate). In real world testing, it's very common for VC's to be at a good percentage of lock during "enthusiastic" driving. Paul Frere quotes a range of 5% to 15% - in his book "Porsche 911 Story", and points out that up to 40% can be achieved on a dry surface. Since higher speeds demand ever larger radius curves, I stand by my "as one goes faster" commentary. Carlos' post is also correct - VC's have a "ramp up" time, so to cover this, some makers "pre load' the VC - but others don't, leading to some very interesting "issues" as the VC locks & unlocks. (Try a Pontiac Vibe on a mixed traction surface). As far as "why" a VC vs a Torsen or Haldex: Both the Torsen & Haldex take up quite a bit of room, while a VC is @ the size of a two liter Coke bottle (actually a bit smaller) - and Porsche claims that size/weight was an issue (a claim to be taken w/ a bit of salt, to be sure) WW, I'm not following your post (perhaps it's bc I was up too late after the games yesterday....). I don't know what Toyota was trying to do with the RX, but Porsche have used VC's since the 993 C4 & derivitives with good reliablilty. J PS - I apologize for the lengthy - and my muddled posts. If you can get your hands on Paul Frere's excellent book, you'll find far better explanations. He's a much better writer than am I. ( and no doubt a bit smarter, too.....)

Jeff25, That would work - except for the VC ramp up time. You'd be out of the curve by the time the system kicked in. J

Torags, Don't know. Anybody out there have a chart???? J