1998 Porsche 911 Carrera 4 Photo
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SEEFELD, Austria — When you think four-wheel drive, the first name that jumps to mind probably isn't Porsche.

Yet the original Dr. Porsche developed a four-wheel-drive prototype — electric powered, at that — way back in 1900. There was also a Porsche-designed four-wheel-drive Volkswagen prototype in 1934. Then Dr. Porsche's son Ferry engineered the stillborn four-wheel-drive Cisitalia Formula One race car in the late 1940s.
But it was the Porsche 959 supercar that brought four-wheel drive into admittedly limited   production chez
Porsche in 1985, followed by the series-production 964 in 1989, the 993 Carrera 4 in 1993, and the twin-turbo 911 in 1995.

In early 1999, the newest four-wheel-drive Porsche, based on last year's all-new 996, hits North American
Porsche showrooms.

All-wheel-drive safety

Unlike most carmakers' four-wheel-drive systems, the Carrera 4 isn't so much about traction as it is
about safety. Thomas Herold, project leader on the new car, says that straight-line stability at high speed
and more secure cornering are the main motivations behind driving all the wheels most of the time on the
newest 911.

Unlike the 959 and 964, which used computer-controlled multiplate clutches to direct variable amounts of
traction to the front wheels, both the 993 and the 996 Carrera 4 use the relatively simple viscous coupling
system. This method employs a housing filled with special silicone fluid containing two series of circular
plates, which are alternatively connected to the front and rear axles.

Under normal driving, the rear wheels push the car, which in turn pushes the front wheels, and there is
almost no relative motion between the two sets of plates; the fluid stays thin enough that only about 5
percent of the engine's torque is transmitted to the fronts. Should the rear wheels start to spin, the fluid
immediately thickens, and as much as 40 percent of the torque goes forward. Once wheelspin is
checked, the rear-dominant aspect of the car is regained.

Viscous coupling stays

Why not a more sophisticated system, like the 959/964, or the variable torque split differentials used by
the newest Audis and Jeep Grand Cherokee? Herold says the viscous coupling does everything Porsche
wants it to do, and it's both lighter and more compact than competitive mechanisms. He didn't mention
"cheaper'' — but it is.

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