A Tale of Two Porsche Seven-Speeds: Manual and PDK

Rummaging through the parts bin to build several cars out of the same basic bits is nothing new and, in rare cases, can even result in cars with distinct personalities. Porsche’s Boxster, Cayman, and 911 don’t suffer from sharing a multitude of body, chassis, and powertrain components. Now Porsche’s transmission engineers have moved parts-sharing down to the micro level to reap two transaxles from one blueprint. The world’s first seven-speed manual gearbox is an offshoot of the seven-speed PDK dual-clutch automatic introduced three years ago for the 997-series 911. Both are available in the 2012 991-series 911 Carrera and Carrera S.

Porsche and ZF have been collaborating on the design and development of these transaxles—code-named the DT11 (PDK) and the MT11 (manual)—since 2003. From the start, the two companies planned on sharing the maximum number of parts and manufacturing tools. Credit ZF engineer Dr. Michael Ebenhoch with inventing the twofer shift kinematics.

Each gearbox is comprised of three aluminum castings—a front section, a rear section, and one removable cover. While those castings are different to suit each transaxle’s functional distinctions, roughly one-third of the internal parts are common [see above]. The main shaft, output shaft, differential, some of the gears, and the ­synchronizer mechanisms are shared. The PDK ’box has two input shafts versus the manual transaxle’s single shaft. A drive flange added to the output shaft sends torque to the front axle in four-wheel-drive models.

While the manual transaxle is lubricated with 3.5 quarts of hypoid-gear oil, the PDK requires two types of lubricant. Gears and shafts run in 3.7 quarts of hypoid-gear oil. Another 5.7 quarts of  hydraulic oil keep the PDK’s wet clutches and control circuit happy. Naturally, the PDK trans is the heavier unit. Including its lubricants and oil cooler, it weighs 256 pounds. The manual transaxle weighs 189 pounds, not counting its clutch components.

Breaking a long-standing tradition, the new 911’s top speed (ranging between 178 and 189 mph) occurs in sixth, not top, gear. The overdrive seventh-gear ratios—0.617:1 (PDK), 0.711:1 (manual)—help maximize highway mileage while minimizing driveline commotion. Third- and seventh-gear ratios are specific to each gearbox; the rest are shared between the two transaxles.

One minor hitch is the manual’s five-gate shift pattern. According to Porsche, this is a necessity. First and reverse gears had to be in close proximity for convenience, and moving first from its traditional 911 upper-left location was not an option. That leaves seventh all by its lonesome in the extra gate.

Our hats are off to Porsche for both ­saving a manual and for improving the 911’s functionality by adding a seventh gear ratio.