Autoblog

If you don't want AWD... Get a Carrera or Carrera S, rather than a 4 or 4S.

That essentially IS optional.

And with Porsche 911 Turbo's amount of horsepower, AWD is pretty necessary to keep that power getting to the ground, otherwise there is the GT2 for people with an adrenaline addiction problem.

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Somebody is missing the point of AWD (not 4x4) in a sports car... let alone a sports car with 500+ hp.

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I agree with you. Everyone assumes I get 4wd in my trucks here in Michigan. I don't and have never really needed it. I keep the money, save gas and get a truck that rides more comfortably.

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With all due respect most people who buy Porsche Carreras don't give a rats ass about MPG, if they can afford a car, they can surely afford the gasoline for it, and if they care about the environment, they don't buy sport cars in the first place.

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Haldex is not viscous coupling, neither faux AWD. The initial system was a simple/cheap clutch based differential, compared to the clutch based differentials available at that time (much cheaper than Porsche PSK for instance).

For manufacturers today, Haldex-like differentials are any clutch based differentials. Haldex can be used for any axle differential, as well as for the center differential.

Clutch based differential are better than any of the alternatives, mechanical (Torsen) or viscous coupling, because they can be controlled better and in most cases are more efficient as well.

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well but haldex simply does not work as well as old fashioned mechanical diff. at least versions used in vw group fwd cars. it goes like that: you lose traction, you try to correct with steering/throttle and _then_ rear end gets it's part of torque and ruins your efforts. to avoid this you have to delay your reaction to the moment when haldex kicks in. higher audi models get quattro system based on torsen and they are much more nice to drive.

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Not sure where you get your info...

But most haldex systems are viscous couplings, some of the newest ones with a computer controlled clutch associated with it.

That is not a differential at all. Viscous-bias and/or clutch-engagement elements can be used INSIDE a differential as a limited slip device, or torque-bias adjustment, but those components are not the differential themselves. A differential uses gears to multiply motion outputs but not lock those outputs together, and is constantly engaged, and can't be disengaged.

Haldex does not use a center differential at all. It uses a rear differential which is driven by a driveshaft that takes power off of the front differential, by way of a viscous coupling, and maybe a clutch, as I mentioned.

The Bugatti Veyron's Haldex system is, of course, more advanced, and with a mid-engine arrangement drives the rear wheels natively, with power taken off for front assist, which is more natural to a performance car. It probably also uses a host of computer controlled clutch packs to manage torque distribution, considering how much the car costs, and how much power it has.

Chances are, if Porsche 911 were to get haldex, it would be similar to that, rather than the simpler, less capable FWD-rear-assist Haldex systems, but a center differential is still better.

A car with a real center differential, like Audi's longitudinal-layout Quattro, and Subaru's AWD systems are much more capable, and are constantly engaged, they don't "automatically engage", they never disengage, and have a more even torque bias split, and they prevent more slip than a reactive system like haldex possibly can.

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@mk

You are totally confused, I am sorry to say that. But it is not surprising because I heard this confusion repeated a thousand of times, even in auto press.

Viscous coupling does NOT use clutches and it transfers torque only when the driving axle looses grip. Haldex uses clutches (always did) and can have a permanent split ratio between axles (like 50:50, 40:60 etc).

Here's how the viscous coupling looks like:

http://www.we-todd-did-racing.com/wetoddimage.wtdr?i=wNTU5NDk2czQxM2RmZDMxeTU0MQ%3D%3D

and this is a cutaway through the real thing:

viscous coupling:
http://www.technolab.org/Bilder/pic-hako/Pic-cutaway/Hako1139.JPG

mechanical Torsen diff:
http://www.technolab.org/Bilder/pic-hako/Pic-cutaway/Hako1272.JPG

Haldex active diff:
http://www.technolab.org/Bilder/pic-hako/Pic-cutaway/Hako1241.JPG
Note the clutch pack in the Haldex diff.

And yes, the Haldex clutches are actuated electro-hydraulically, but to say that Haldex is viscous coupling because of that, is as far off as saying that DSG transmission is a hydraulic transmission, because the wet clutches are actuated electro-hydraulically.

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@dondonel,

You may be right. Haldex may use clutches, rather than viscous couplings. Some that I have heard of, were claimed to have a viscous coupling after the power-take-off for the rear axle, and were primarily FWD, with rear-wheel assistance when slip occurs, and heats the viscous fluid, making it "thicker" and transmitting more torque to it's output, to the rear driveline. Perhaps some do use fixed-bias clutch plates, or electro-hydraulic variable clutch packs.

But my main point is that is STILL not a center differential, it is still a power-take-off of the main differential, which is the front differential, in most FWD cases, the Rear differential in mid or rear-engine cases.

The Viscous coupling and torsen unit (which is NOT a differential itself, but a limited slip device INSIDE a differential) cutaways you showed each have a linear rotational motion. in one side, and out the other, the output being varied by the device itself, by it's design parameters.

A differential is what you showed in your last link. One input, and TWO somewhat independent outputs, with your example having a clutch pack on the input side. The design of the cutaway appears to be a REAR differential for a FWD-based system, or a Front differential in a rear-based system. You can see the driveshaft flange input, and the flanges for lateral outputs to wheel hubs, presumeably.

That however, does not address the main split in the driveline, BETWEEN THE AXLES, which with Haldex, is still likely a power-take-off shaft, feeding into either a viscous coupling, or a helical gear arrangement (torsen), or a clutch pack, ALL of which allow different methods of the same basic variance between the input and the output, to account for speed differential between the axles, or between the wheels on each axle.

That Haldex Active Differential you showed has clutch packs in it, due to the fact that there is NO central differential between the axles, and that clutch pack is the only variable link (the clutch can slip somewhat) between the front differential and the integral rear differential that you show. Such a variable link is required, otherwise the front and rear differentials would be disengaged, or locked together, like a transfer case 4x4, or a locked-hub front axle on a 4x4. I've had one of those, too, and they don't like to turn corners.

If you look at a good Quattro system, or Subaru's Symmetrical or VDC systems, you will see three, not just two, differentials. The transmission output is mechanically multiplied by the center differential between front and rear. The Front differential gets input from that, and multiplies again, for the front wheels. The central and front differentials are contained in the transmission case, making it a transaxle. The rear differential ALSO multiplies it's input from the center differential, for it's two tires.

With a viscous coupling, or a clutch pack, there is the potential for the input and output to be essentially disconnected (clutch disengaged, or viscous fluid absorbing the input, and not transmitting any output). With a differential, a mechanical disconnect doesn't happen, especially with a viscous, helical, or clutch-pack limited slip element inside it, which are the same things that regulate speed differential between the outputs, after the multiplication.

Getting viscous couplings, helical gear couplings, or clutch pack couplings confused with a differential is where the issue is. They are not the same sort of machine, but they can work together or independently.

And btw, some Haldex systems have reported that a permanent split ratio is as much as 95:5 front-bias standard, with increasing capacity under slip to 50:50, but not higher than that.

On the other hand, variable 3-diff systems, like Subaru's VDC or DCCD system, can vary torque bias with a clutch pack in the center differential (not in place of the center differential, but inside it) from a standard 65:30 rear bias to much more front OR rear bias, with a Torsen Limited Slip element in the rear, and some installations the front differential, as well. Their symmetrical AWD has a standard 50:50 bias absolutely with no loss in driveability, and turning a corner doesn't require wear-causing clutch-pack slippage, but standard differential behavior. They also have a viscous Limited slip element in the rear differential.

I drove to work this morning through a snow event on symmetrical AWD, when there were a lot of 4x4s in the ditches, mired in the recently thawed mud under the new snow cover. I'd say the system is pretty darn capable, without requiring wheel-spin or slip to be active, like many VW, Ford, Volvo, Mazda, and other haldex systems do seem to require for AWD to really transfer torque.

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Okay, due to manufacturing limitations they might be limited to 30/70 +-10%
20/80 on front overrun
40/60 on rear overrun

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