With its powerful public image and its impeccable reputation for engineering excellence, Porsche is not a company which seizes on a technical innovation purely for the sake of it, or because it is what everyone else is doing. The company has only just adopted direct gasoline injection, for instance, an engine technology that has been found on mainstream family cars for almost a decade.

So, though Porsche can claim credit as the company which in the 1980s pioneered the dual-clutch transmission on long-distance Le Mans racing sports cars, it has waited until now to apply the technique to its road-car models.

Racing cars are very different, argues Porsche. With the Porsche Doppelkupplung (PDK) equipped 962 Group C endurance racers, outright performance was the overwhelming priority, with durability (beyond the length of the race) and cost only counting as secondary considerations. Right at the bottom of the list were two of the qualities most important for road-car users: ease of operation and the refinement of the gearshifts themselves. 

And, says Porsche, it is only now, with the recent advances in electronic transmission control systems, that the shifts can be controlled accurately enough to give the smoothness and consistency required to satisfy choosy premium customers.

Porsche’s partner in developing the roadgoing dual clutch transmission has been ZF, with work being carried out at the Kressbronn site in Brandenburg, where the 7DT units are also manufactured. Among the challenges facing the engineers were the need to accommodate high power and torque throughputs as well as engine speeds up to 8,000 rev/min – considerably higher than in existing DCT applications. Crucial, too, was a wide range of shifting programs: for its Porsche application the 7DT needed to offer everything from the seamlessly smooth automatic shifting of a luxury transmission to the rapid, direct-feel manual gearshifting demanded by highly focused sports car drivers.

First application: the iconic 911
The first Porsche to receive the dual clutch transmission – which Porsche designates PDK, just like the original racing system – is the iconic 911 Carrera. In this rear-engined application it is configured as a transaxle unit, with the differential and final drive housed within the same external casing. As the seven-speed PDK offers fully automatic operation it is able to completely replace Porsche’s earlier torque converter-based Tiptronic S planetary automatics, though the conventional six-speed manual remains available.

The PDK presents numerous advantages over the outgoing Tiptronic S. Not only is it 10 kg lighter but its shifts are up to 60 percent faster, enabling the already-rapid 911 to trim almost a full second off its 0-100 km/h (62 mph) acceleration time.

Thanks to the seven speeds and the quicker shifts, the PDK car can even out-accelerate the manual version, too, shaving around three tenths of a second off the 0-160 km/h time. However, the most dramatic improvement comes in efficiency, as shown by fuel consumption and the directly proportional CO2 emissions in the official EU combined test cycle. The PDK-equipped 911 Carrera emits just 230 grams per kilometer of CO2, compared with the outgoing car’s 262 g/km – and the 242 g/km of the six-speed manual version.

Design and construction
With its twin 202 mm and 153 mm clutches mounted concentrically ahead of the single layshaft gearbox, the PDK transmission is familiar in its basic configuration. However, it its execution it incorporates many notable details aimed at eliminating some of the compromises and performance trade-offs that can afflict this type of transmission.

In particular, the conflicting lubrication requirements of the wet clutches, the bearings and the gears themselves are resolved by the provision of separate lubrication systems for the clutch pack and the gearbox itself.

ZF says it has a special type of hypoid oil for the gear set and the differential and hypoid drive area. The oil specificaltion is that recommended by Porsche. For the clutch and control chamber, says ZF, a special ATF lubricant is specified. The hydraulically controlled clutches are both lubricated and cooled by the oil supply in the clutch chamber, which in the 911 is between the engine and the rear axle line. With their compact dimensions wet clutches offer both high performance and a long service life, says Porsche. A new speed-governing concept, used for the first time on a production transmission, contributes to the benchmark-setting speed of the gearshifts, claims ZF, and precise control of the flow of cooling fluid keeps drag losses low.

This, explains ZF, adjusts the flow of cooling oil according to the activities the transmission has just performed. After a race start, for instance, the clutches will get extra cooling to deal with the high loads they have just sustained. This, along with the intrinsic mechanical efficiency of the seven-speed gearbox, is a key enabler in allowing the 120 kg transmission to offer auto-mode fuel consumption values better than those of the equivalent six-speed manual car.

A new speed-governing concept is claimed to further speed up gearshifts. Depending on the exact driving situation, many differerent functuions combine to achieve what ZF describes as a high synchronisation of the gearchange sequence. Details of this process are, says ZF, company knowhow.

ZF has also developed an inline version of this transmission, the 7DT52, for installation in front-engined vehicles.

Comprehensive control
The raison d’être of any dual clutch transmission is of course the immense spectrum of shifting strategies opened up by the precision electronic control of gear selection and clutch application and release. Integrating these with engine functions and dynamic data from chassis sensors allows a level of control of the powertrain never before seen on a sports car: even the most basic automatic transmission program, for instance, knows not to allow upshifts when the driver suddenly lifts his foot from the gas, such as when entering a bend. Likewise, lateral acceleration sensors inhibit upshifts in mid corner, and the vehicle recognizes uphill and downhill travel and adjusts its shift points automatically.

By signing up for the optional Sports Chrono Plus package the 911 driver gets a further range of functions: Sports and Sports Plus buttons, giving even quicker gearchanges and faster reactions, and a Launch Control mode to allow racing starts from 6500 rev/min.
Control of the PDK takes place via the central gear selector or shift buttons on the steering wheel spokes: in contrast to conventional industry practice, both of these buttons have the same function, a forward push triggering an upshift and a rearward pull prompting a downchange.

Premium position
Sitting at or near the top of the premium sports car segment, Porsche is one of the few automakers who can confidently expect a significant uptake for a €3,000 ($4,500) transmission option. One of the company’s principal competitors, Ferrari, was however first to announce a standard dual clutch transmission, on its new California coupé-convertible; this model is likely to cost twice as much as the Porsche and deliveries will not begin until the first quarter of 2009.

Competition from below comes in the shape of Nissan’s GT-R, boasting a highly advanced all wheel drive chassis to complement its six-speed BorgWarner dual clutch transmission. Other transmission suppliers and premium sportscar makers are poised to follow suit. One way or another, it is clear that DCT is already becoming the transmission of choice for the world’s finest and fastest.