PORSCHE 911 G4RRERA 4

ble to all kinds of drivers under all kinds of conditions. We had to love it.

The 1989 911 Carrera 4 is no 959— as Porsche representatives are quick to point out—but we suspect it deserves a similar judgment. The stabilizing influence of its effective all-wheel-drive system does dull the sharp reactivity of its rear-wheel-drive senior sibling. But on balance, for all but the rarest over-the- road missions, what’s been lost from the traditional 911 formula is precisely what had to go.

An automatic, “transparent” four- wheel-drive system is the key to the Carrera 4’s capability, just as it is to the 959’s. But the systems themselves are quite different. The 959 is a cost-no- object technological test bed, and its complex all-wheel-drive system does everything by electronic computation:

Multiplate clutch units in the transaxie and in the front drivelme handle differential action as well as front/rear torque split, calculating what kind of traction to assume based on how accelerating and turning are affecting the car’s static

weight distribution. It works well for research and experimentation because software changes alone can produce huge variations in its operating characteristics.

With the Carrera 4, however, thanks to lessons learned on the 959, Porsche knew what it wanted. So torque split is normally fixed at 3 1/69 percent (biased to the rear because the weight distnbution still gives more traction there) and is handled mechanically by a planetary gearset on the nose of the transaxie. There are still two electronically managed multiplate clutches, but they operate only as limited-slip devices for the rear and center differentials. And they function not by calculating where traction ought to be, but by actual observation: Using the ABS wheel-speed sensors, the central processor recognizes a loss of traction by the beginning of a change in rotational speed of one or more wheels. In one-tenth of a second (a third of the time the 959 took to react), hydraulic pressure increases the “lock” of one or both clutch units, reducing slip and more tightly tying the speed of the wayward wheel to that of its mates.

In practice, this system accommodates front-rear and right-left differential requirements (wheels travel in different arcs through a turn, remember), while providing limited slip to keep tires with good traction working even if one or more have poor grip. But more important, it prevents the car from acting like a 911 under certain critical circumstances. The two classic rear-engine heart stoppers are off-gas oversteer (that is, an inattentive driver tries to slow suddenly as he’s pulling the car into a curve, and that pendulum of a tail passes the overtaxed outside rear tire) and power oversteer (too much throttle exiting a turn when the outside rear tire is already pushed to capacity by cornering force). In both cases, the rear wheel most critical to carrying cornering loads starts rotating at a speed out of sync with the car’s velocity over the road (too slow in the first case, too fast in the second). Its rolling friction is replaced by much less effective sliding friction, and that end of the car duly obeys the laws of physics and lets go.

No Carrera 4 wheel is allowed to get

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