Behavior modification: All-wheel drive tames the 911

These days, you can’t walk through a Porsche dealership without bumping into an all-wheel-drive-equipped vehicle. Customers for Porsche’s SUVs, the brand’s best-selling vehicles, expect the all-weather traction of four driven wheels.

On the other hand, 911 loyalists remain split over rear-wheel drive vs. AWD, as they have been since 1990, when the company first offered the latter in volume production for its immortal sports car.

Porsche says that a third of 911 buyers choose a Carrera 4, as the AWD models have been called since the system was introduced. On its website, the company acknowledges the divide: “For 911 fans it’s a question of faith, for tech lovers it’s an issue of performance: rear or all-wheel drive? The answer may turn out differently on either side, but neither would be wrong.”

Even with the older models, buyers generally have a preference for one over the other, said Paul McDonald of Performance Automotive, an independent service shop in Malvern, Penn., specializing in Porsche and BMW.

“For most, it’s a collector car, a fourth or fifth car. They know exactly what they want,” he said.

Reliability of the AWD, he points out, is generally not an issue.

“They’re robust systems,” he said. “We have plenty of those cars come in that have no problems with all-wheel drive.”

The choice, McDonald explained, comes down to handling and performance preferences. Carmakers that offer AWD have long promoted it as a traction aid for slippery road conditions. Porsche, however, gave the 911 AWD in 1990 to expunge any last vestige of the rear-engine car’s penchant for snap oversteer – once notorious, but mostly defanged by that point – when the driver abruptly lifts off the throttle in a corner.

Porsche proved the idea with the 959, a 195-mph supercar that was not certified for sale in the U.S. The company then introduced all-wheel drive for the redesigned 911 introduced for 1990 that is known by its model code of 964. Although that Carrera 4 weighed about 200 pounds more than its rear-drive sibling, acceleration barely suffered.

Considered simpler than the 959’s AWD system, which employed computer-controlled clutches to continuously vary torque distribution, the Carrera 4’s setup was nevertheless quite sophisticated. A planetary-type center differential provided a 39:61 torque split. Multiplate electrohydraulic clutches in the center and rear differentials could vary the split when the wheel-speed sensors of the 911’s first antilock brake system detected slippage.

In its August 1989 road test of a 911 Carrera 4, Car and Driver credited the system with “…bulletlike stability when you bury the throttle and reassurance when you reluctantly lift at the last instant.”

All-wheel drive would add confidence in inclement weather, although tire choice would also be a factor. In the first-generation Carrera 4 AWD system, the driver could push a switch inside the car to lock the center and rear differentials for driving in slippery conditions at speeds below 25 mph.

Porsche used a simpler, lighter AWD system for the final air-cooled generation 911 that debuted in 1995, known as the 993. Instead of a center differential, a viscous coupling housed in the transaxle could shift up to 40 percent of available torque to the front. In 1999, a similar system carried over to the first water-cooled 911, the 996 series, but with the viscous unit mounted behind the front differential. That change enabled Porsche to offer AWD in conjunction with an optional automatic transmission.

Today’s 911 Carrera 4 uses an entirely different AWD system. Instead of a viscous coupling, which responds to slip mechanically, a computer-controlled multiplate clutch actively engages the front differential to transfer torque forward. According to Porsche, the system, adapted from the 911 Turbo, for the first time gives the Carrera 4 quicker acceleration than the rear-drive model, albeit just one-tenth of a second difference in the run to 60 mph.

Buying an older Carrera 4 comes with a few minor cautionary notes, McDonald said. A pre-purchase inspection should reveal any potential problems; he advises looking for leaks near the front differential, listening for differential chatter when driving and paying attention to front-wheel vibration.

“Sometimes, the front seal leaks where the axle meets the diff,” he said.

Any of those issues might indicate a maintenance need, not necessarily a costly component failure, he explained. In his experience, front differential failures have been more common in the water-cooled models. For maintenance, McDonald advises keeping up with factory-prescribed differential oil changes.

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