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Here are a few thoughts:
wwest: Regarding the Porsche 911 AWD. I find it surprising as an owner of this fine vehicle that you wouldn't agree that an awd system incorporating LSDs, center VC combined with traction control is superior to a system of traction control and 3 open diffs such as in the Sequoia. Do you think that your Porsche's performance would be superior if they eliminated the center VC and LSDs?
Here are two good sites on the subject:
The first <http://www.austinmonthly.com/articles/default.asp?ArticleID=54&mode=detail> shares the following views on the advantage of AWD in your Porsche, which I'm sure you agree with.
"With the AWD, you can actually feel the changing amounts of grip on the front tires as the car glides through an expanding corner.
The AWD offers exceptional traction on slippery surfaces, even though it was designed for performance on mainly dry surfaces, not as an all-weather traction control system. From a stop, you can crank the wheel 90-degrees and stand on it without making any steering correction. The rear end won’t slide (power oversteer) and the front end won’t wash (understeer). The Carerra 4 just accelerates away—and quite quickly at that."
For road performance this is the exact type of traction sensation (albeit at much slower speeds) that I have experienced in a 2.5 ton SUV with AWD, a center VC with 38/62 torque distribution and a rear LSD. In poor road conditions it is unflappable and superior to traditional 4wd systems, IMO. The question is whether its superior to a 4wd system utilizing 3 open diffs and traction control.
Theoretically, I believe it is, although I have little experience behind the wheel of a 4wd system like the Sequoia's other than a short test drive on dry pavement. I would think if it were superior that Porsche (and all the other world-wide manufacturers of awd systems) would have dropped the expense and weight of LSDs and VCs in favor of open diffs and trac control. The reason they don't, IMO, is the diminished traction of an open diff system, even though the trac control will reduce slippage it will not force a minimum amount of torque to a wheel.
The second site further explains the Porsche system of AWD, which is very similar to the GMC setup without the electronic traction control.
The following is an exerpt "To make the viscous-coupling always engaged the front wheels, the rear tyres were made marginally smaller in diameter, enhance established a small speed difference between the drive shafts to front and rear. With the speed difference, the viscous liquid normally transferred 5-15% torque to the front axle, which was much less than the 964’s system. In abnormal conditions, that is, whenever one axle lost grip, the viscous-coupling LSD may send up to almost 100% torque to the other axle. Both the center LSD and rear LSD were now pure mechanical, but clever electronics was used in the newly-added ABD (Automatic Brake Differential). Again, ABD was simple yet effective. It was just a program, sharing all the hardware with ABS. Whenever rear wheels spin, it braked the spinning wheel thus the rear differential would send more torque to the other wheel. It was particularly useful for extreme conditions such as on snow, while LSD covered most normal conditions."
The Porsche's have now increased the front bias range to 5-40%, instead of 5-15%.
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