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Discussion Starter #1
Cars like the 1986 Porsche 959, current Techological Tour De Force Porsche 911 Twin Turbo, and Lamborghini Diablo VT were not given All Wheel drive so that they can go in the snow or go rally racing. That system is used in those cars to enhance their dry weather handling. How exactly does that help? In a comparison between the BMW 328i and Audi A4, Car & Driver said that the Audi's all wheel drive did not help it in the dry weather road course. http://www.caranddriver.com/FrameSet/0,1350,_sl_NewArticle_sl_0_cm_1633_cm_432_5_17_cm_00,00.html
It was the BMW's outstanding chassis balance and suspension set up that led to better handling results than the A4....I know that the S4/upcoming S6/S8 also has all wheel drive to help it in dry weather handling as well (in addition to inclement weather conditions). However, I don't see how it helps. It seems that if you have the proper suspension set up as well as near perfect F/R weight distribution in a rear drive car, you can can handle just as well if not better than an All Wheel Drive Performance car...
Eric....
 

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Discussion Starter #2
So why do the first three cars I mentioned have All-Wheel-Drive...
Eric...
 

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Eric, you're probably right. Weight distribution makes all the difference - although awd helps too. But then many people go for awd because of it's advantage in wet/icy weather. With a rwd car, regardless of weight distribution, you can still get stuck in mud or snow. But with awd your chances of getting out of that mud/snow are much better. And more than actually needing awd, many people just like to know that it's there. It's like having an airbag, sorta.
Personally, I prefer rwd cars over awd. It's much easier to throw them around - their more tail happy!
But if you live up north and get a lot of snow, it doesn't hurt to have awd.

black/ivory
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My take on it is:
AWD systems can be tuned for the track or for the street (rain, snow). Audi Sport's racing team runs S4s that are tuned very differently from the stock A4 or S4 Quattro system. There are big advantages on the track due to AWD, not just when it rains or snows, but on dry, you can power out of turns sooner, you have more grip, and it's harder to spin. But the heavier awd cars sometimes get dusted on the straights.
The reason an A4 doesn't gain an advantage in dry, is that it doesn't have enough power to take advantage of the system - plus Quattro weighs about 330 lbs, AND it is less efficient in power transfer so some power is lost, negating any dry performance advantage, in fact it's slower than the 2wd A4.
Finally, awd cars are easier and safer to drive fast than rwd cars, and since most customers are modestly skilled, it makes sense to add AWD to high performance cars for safety reasons.
 

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I think the Porsche and the Diablo have AWD for a single purpose--to put an enormous amount of HP onto the asphalt without consistenly burning up your tires, peeling out and sliding in less than dry weather. The AWD on most of the Audi's is for security in inclement weather--it is not needed to manage the power that those engines generate. For the Porsche/Diablo, however, by diverting some of the power to the front wheels I think it just helps to make the cars managable. Also, at the speeds that those cars can attain, if you are running at a track you probably want some extra grip from the front wheels to keep the tail from slipping out. Short of using racing slicks, this extra grip helps to keep the cars stable at super high speed. I don't think AWD will do any good at all in terms of maneuvarability in dry weather; in fact, it might even hinder. And, it's a real gas mileage killer!

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There are some basic rules to (acceleration) performance including:

1> More power gives better performance.
2> Less weight gives better performance.
3> Loss of drive wheel traction reduces performance.

When they add AWD systems, they add weight - which is not a good thing - so... they generally have a reason - like they are finding that they have so much HP that they need more traction.

(but you knew all that, didn't you?
)

This is not to say that all high HP supercars need or have AWD. Look at the Dodge Viper for example. It has massive HP/torque and still achieves DiabloVT/PorscheTurbo acceleration.

Also, if you look at Porsche/Lamborghini - they offer various special edition 911's or Diablos with major HP and going back to RWD.

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I would say the main reason to have AWD in a car like that is to maintain acceleration capability in less than ideal traction conditions - like in rain for example.

It isn't just for snow in winter, but any less than ideal driving surface.

There is a reason why Rally cars (ultra performance on dirt roads) all tend to be AWD...
 

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Keep in mind that the alternative to an AWD Audi S4 is a front wheel drive S4 - no thank you. This obviously doesn't apply to the original question, but I think it explains Audi's reasoning - if they had the option (a rear wheel drive platform that size and marketing folks that could look the other way for a while), I bet the S4 would be 2 wheel driven from the rear.

Terry
 

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Discussion Starter #8
Thanks for the info guys...
An interesting thing to ponder
Eric..

[This message has been edited by EricK (edited October 19, 2000).]
 

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To see when AWD would help on dry pavement, it is revealing to find out how much acceleration is put to the ground in each gear. For example of an M5 or a WRX, the peak longitudinal acceleration are ~1 g (1st gear, upto ~40 mph), ~0.6 g (2nd gear, upto ~65 mph), ~0.4 g (3rd gear, upto ~100 mph), and so on. To maintain longitudinal traction, the M5 would need to have a coefficient of friction at the rear tires of ~2 (1st gear), ~1.2 (2nd gear), ~0.8 (3rd gear), etc. Pretty much in the lowest 2 gears, this is stretching the longitudinal traction limits with street tires and conditions. On the other hand, the longitudinal traction is spread over all four tires on the WRX, essentially reducing the requirements of the coefficient of friction by almost a factor of 2 and hence, AWD helps at the launch of a drag race. The advantage of distributing longitudinal traction works similarly in high-speed cornering of a road race, when some of the tire friction is borrowed to hold lateral traction.

As compared to an A4 or a 328, the peak longitudinal acceleration is only ~0.55 g in 1st gear and even less in higher gears. The longitudinal traction on dry pavement in these lower-torque cars is already maintained without the help of AWD.
 
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