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
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
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.
1998 Porsche 911 Carrera 4
The major difference between the 993 and the 996 is that the viscous coupling is now part of the front
axle, rather than the rear. This concentrates most of the slightly greater weight of the four-wheel-drive 996
— about 120 pounds in total — onto the front wheels, where it can do the most incremental good. More
important, it separates the coupling from the transmission, which allows the Carrera 4 to come equipped
with Tiptronic automatic transmission for the first time. This will vastly increase the market potential for
This design does extract some penalty, however, in luggage-carrying capacity, since the front trunk is
reduced to 3.5 cu. ft. from about 6 cu. ft.130. You can still fold the rear seat backs down, of course, and
you might as well — you’d be stupid to let anybody you like sit back there anyway.
The safety aspect of this system is further emphasized by the specification of Porsche's first directional
stability-control system, exclusively — so far — on the Carrera 4. Called Porsche Stability Management,
or PSM (and please, do not get those last two letters mixed up), it's essentially the same Bosch
hardware used on other high-end German makes. Sensors measuring yaw — how much the car's body is
actually turning — plus both lateral and longitudinal acceleration and steering wheel angle, determine if
the car is turning as much as the driver is demanding. If not, it automatically applies individual brakes and
invokes the traction-control system as required to bring the car back into line.
Specifically, if the car is understeering — i.e., the driver panics, meaning the car is turning less than he
wishes or is plowing straight off the road — the inside rear wheel is braked. If the car is oversteering,
meaning the passenger is panicking because the car is turning more than is requested or starting to
fishtail, brake pressure is applied to the outside front wheel. Doesn't sound like it should work, but it
1998 Porsche 911 Carrera 4
What's different between the Carrera 4 and other users of the Bosch system is that this one is calibrated
specifically for the sporting driver and is invoked only under pretty severe conditions. Since Porsche
drivers are more likely than most to want to drive their cars sideways, most often (we hope) on race
tracks, the system can be shut off by a console-mounted switch. Should the driver hit the brakes,
however, PSM is reactivated automatically.
On the twisty one-and-a-half-lane mountain roads that wind their way from this gorgeous Austrian resort
town up to the castles of Mad King Ludwig in southern Bavaria, I had little opportunity to put this system
to trial. Naturally, if the system DIDN'T work — and Porsche was ultracareful to warn us that no system
can suspend the laws of physics — the few guardrails that did exist wouldn't have done me much good.
And as Les Bidrawn of European Car magazine from California put it, you'd have starved to death before
you hit the ground.
So Porsche laid on a special demonstration by wetting down the parking lot of a nearby ski resort and
asking former world rally champion Walter Rohrl to pilot a slightly modified 996 Carrera 4. First, he
switched the system off, then gunned the car to about 40 mph and threw it into a simulated left/right lane
change. As you might expect, in the tight confines of this parking lot, this was a violent maneuver. The
car would initiate the left-hand turn, but when he tried to bring it to the right again, the car spun
uncontrollably. Believe me, if Walter Rohrl can't catch it, you or I wouldn't have a hope in Hades.
PSM to the rescue
With the system functioning — one of the modifications to this demo car was a little beeper to provide
audible notice of the system's activation — the car made the lane change with little drama. It's just like
they teach us in advanced driver training — look in the direction you want to go, and steer the car that
way. Thanks, Uncle Bob (Bondurant). If you do countersteer — as former 911 drivers have learned to do,
since Darwin's theory would otherwise by now have taken its course — the system compensates by
reducing the amount of assist.
The Rohrl demonstration was actually more enlightening when viewed from outside the car. First, Rohrl
appears to be the world's most boring driver. He is so calm and controlled behind the wheel that nothing
seems to be going on, excep