The PDK Transmission

How Porsche’s quick-shifting PDK works.

The PDK Transmission 1
January 16, 2015

Throughout most of its history, Porsche has been synonymous with sports cars that involve the driver. The majority of these cars have been equipped with manual transmissions, and many hardcore Porsche enthusiasts would not have it any other way.

Porsche, however, has always been an engineering firm at heart that has continually worked to develop new technologies to enhance performance, driver comfort and safety. It was to this end that the Sportomatic transmission of the late 1960s was developed. This semi-automatic transmission concept was based on a conventional manual transmission, but it lacked a clutch pedal; the clutch was vacuum actuated via a microswitch that responded to movement of the gearshift lever.

The Sportomatic also featured a torque converter to allow the engine to idle without stalling while the vehicle was at rest. The “Sporto” was offered as an option in the 911 from 1968 and was initially well received, but consumer interest waned through the 1970s and it was eventually discontinued.

Porsche’s next attempt to expand its market share via the proffering of a two-pedal transmission option was much more successful. The Tiptronic transmission of 1990 was based on contemporary automatic transmission design, with planetary gearsets and a torque converter. Its innovation was electronic controls that allowed the driver to manually shift gears at will. This feature eventually became common in more conventional vehicles, but the more important implication of the Tiptronic was that its take rate greatly increased among buyers of Porsche sports cars throughout the 1990s and 2000s, especially in the American market.

The PDK

While the torque converter-equipped Tiptronic was vastly improved over the years, Porsche was well aware that it was no substitute for a proper manual gearbox in the view of performance-oriented drivers. The engineers at Zuffenhausen had been intermittently tinkering with the idea of a fast-shifting dual-clutch automated manual transmission since the 1970s in the form of the Porsche DoppelKupplung, or Porsche dual-clutch transmission.

Porsche’s main vessel of PDK development was the 956/962 endurance race car program of the 1980s. Despite its considerable weight penalty of 88 pounds (which is significant in a sub-2,000-pound race car), the original PDK transmission offered the promise of much quicker gearshifts and lap times. Porsche product planners also realized the PDK’s viability as the gearbox of the future in its road cars, so race development went full steam ahead.

The PDK Transmission 2

Porsche had limited success on the racetrack with the PDK, as it was very bulky and complicated, with many reliability issues stemming from both the electronic and hydraulic systems. Weissach engineers also installed versions of the PDK in several prototype road cars, but the engineers did not consider it ready for primetime given the limited electronics technology of the day and its lack of refinement for road use.

In the last decade, the necessary technical advances in electronics and hydraulics have come to fruition, and Porsche finally released the production version of the PDK for the 2009 model year 911 and Boxster/Cayman models.

Previously available single-clutch automated manuals such as Ferrari’s F1 transmission and BMW’s SMG offer lightning-fast full-throttle upshifts, but part throttle shift smoothness could be compared to that of a 15-year-old learning to drive a manual transmission! The key advantage of the PDK dual-clutch setup is when one gear is selected, the gear ratios above and below it are already preselected, meaning that an up or downshift requires the transmission to merely switch clutches, enabling seamless upshifts that take place in milliseconds with minimal interruption of power flow.

How it Works

The PDK transmission utilizes the same constant-mesh gear set and synchronizer configuration as Porsche’s modern manual gearboxes, but there are two separate transmission input shafts, each with its own multi-plate clutch pack. The two clutch packs are arranged concentrically within a shared drum that resides in the transmission bell housing. The clutch drum is in turn splined to a dual-mass flywheel that is bolted to the engine’s crankshaft.

The outer (#1) clutch pack is splined to an input shaft that incorporates the odd numbered gears 1, 3, 5, and 7, while the even numbered gears 2, 4, and 6 operate via a separate, hollow input shaft that is splined to the inner (#2) clutch pack. The #1 input shaft actually runs inside the hollow #2 shaft to save space and weight, and Porsche was successful in this regard, as the modern PDK’s weight penalty is only 44 lbs. more than a manual gearbox.

The PDK transmission uses pressurized hydraulic fluid for clutch engagement, and the same fluid is used to cool the friction discs of the clutch packs. Hydraulic circuits are also used to actuate the shift rods when preselecting gear ratios, and magnetic distance sensors are present to electronically determine the selected gear ratio. The gear wheels themselves reside in a separate chamber and are lubricated by conventional gear oil.

The PDK Transmission 3

The PDK has a valve body like that of a conventional automatic transmission, which contains electronic shift solenoids that actuate mechanical spool valves for directing hydraulic fluid flow to the clutches and shift rods. An electronic control unit orchestrates PDK operation, and it is in constant communication with the engine control unit and the ABS/PSM control unit to optimize shift strategy and perfectly match engine revs during shifts.

The driver can control shifts via buttons or paddles on the steering wheel or with the shift lever. There is also a fully automatic mode, with the normal shift setting oriented towards smooth shifting and optimum fuel economy. When moving the vehicle from rest, the #1 (outer) clutch pack engagement is programmed to mimic the slippage of a torque converter when moving from rest, and it almost succeeds at this. Porsche considers the PDK good enough in this regard to offer it as the exclusive transmission available in the Panamera luxury/sports sedan (with the exception of exceedingly rare manual 6-cylinder models, and the Tiptronic-equipped Hybrid) and in the Macan SUV.

Sport or Sport Plus modes completely transform the shift strategy, providing firm and lightning-fast shifts at far more precise intervals than most mortals can muster. The latest generation PDK transmissions incorporate new fuel-saving modes, such as “sailing,” or decoupling the transmission from the engine to allow the engine to idle while coasting. The new 991 Turbo introduces a slightly preposterous “virtual gears” mode that slips the clutches while cruising in automatic mode to allow the engine to lug along at idle speed.

Service & Reliability

The PDK seems to be as reliable as its Tiptronic forebears thus far, with only a handful of factory technical service bulletins present. Porsche’s prescribed service intervals are 56,000 miles for the hydraulic fluid and about twice that for the gear oil, though it would be wise to change the gear oil at the same time as the hydraulic fluid.

Hydraulic fluid level check and replacement is beyond the realm of the DIYer as it requires the use of a Porsche Integrated Workshop Information System (PIWIS) or an equivalent scan tool to monitor fluid temperature and to activate the requisite “fill mode.” Clutch pack life is yet to be determined, and time will tell if Porsche’s long-term development of the PDK has paid dividends in long-term reliability.

What is certain at this time is that a two-pedal transmission in a modern Porsche is certainly not a compromise from a performance standpoint, and the PDK, in fact, outperforms most of us. Porsche customers have overwhelmingly endorsed the concept by choosing the two-pedal option 90 percent of the time.

Hopefully, the purity of a true manual transmission will still be available in Porsche cars in years to come for those of us who still yearn for that direct mechanical linkage between man and machine.

Also from Issue 226

  • 991 Carrera GTS
  • Bangkok 911
  • Market Update: 1989-1998 911s
  • A 911 racer’s R Gruppe street car.
  • Porsche’s Sporting Scout Car
  • Grant Wolfkill
  • Profile: Peter Falk
  • Enzmann 506
  • The Keeper 924
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