Porsche’s PDK transmission is nothing new – it’s been with us since 1983
If thereʼs one subject that arouses passion among owners of modern Porsches, itʼs the choice between PDK or manual transmission. However, Porscheʼs amazing semi-automatic PDK gearbox is far from being a new concept, its origins being traced back to 1939 – even Porsche tested it in its Group C race cars as far back as 1983. You see, there really is nothing new under the sun…
When Porsche broke the news half a decade ago that its flagship models would only be available with the companyʼs revolutionary PDK transmission, it caused something of a stir among those who feel that real race cars – or should that be, real cars that can be raced – havemanual gearboxes. Surely the skilled driver wants to take total control of the car, right down to synching every push of the clutch pedal with every shift of the gear lever and every rev of the engine? But Porsche knows better than to trust wemeremortals with such a task…
The PDK transmission (thatʼs short for ʻ Porsche Doppelkupplung ʼ, by the way – or, to put it in Queenʼs English, ʻPorsche double clutchʼ) has now been available for ten years, having first been introduced in 2008 on the second generation 997-series 911. This slick system replaced the ageing Tiptronic semi-automatic gearbox, Porsche claiming that not only was the PDK capable of shifting some 60 per cent faster than the Tiptronic, but it also helped to reduce fuel consumption thanks to its more efficient design.
But, as impressive as it was on paper, the PDK system didnʼt meet with universal praise. Well, to be truthful, it wasnʼt so much the transmission itself that came in for a pasting in the press so much as the way in which it was operated. For some reason best known to the gents at Zuffenhausen, the Pdk-equipped 997 featured rocker switches mounted on the steering wheel, rather than the more familiar paddles located behind the wheel. The ergonomics of this early design left much to be desired, the result of which was that the PDK gearbox itself came in for rather unfair criticism.
To fully appreciate the benefits (or otherwise, as some might argue) of PDK, letʼs take a step back into history, for the DNA of this amazing design can be traced back far further than you might imagine. Forget for a moment the Tiptronics of the 1990s and Sportomatics of the 1970s, even though they were both important steps in the evolution of the modern 911. Instead, cast your mind back (or do a quick ʻGoogleʼ) to the days of pre-war cars like Armstrong-siddeleys and Daimlers. Or double-decker buses and lumbering lorries… Seriously. They have more in common with the modern Porsche than you might at first realise.
The thread which connects the past with the present is the concept of the preselector gearbox. Itʼs almost unheard of now outside the commercial vehicle and vintage car scenes but is a very clever idea. Vehicles with preselector gearboxes allowed the driver to select the next gear (either a higher or lower ratio) which was engaged only when the left-hand pedal was depressed. Note we donʼt call it the ʻclutch pedalʼ as cars with preselector gearboxes use centrifugal clutches, which engage with rising engine speed.
In a similar fashion, with a Pdk-equipped Porsche the transmission is always in a state of readiness to almost instantly engage the next ratio, awaiting only for a signal from the driver (using the paddle shift) or the engine electronics. There is no clutch pedal. But weʼll come back to that in a while.
Driving a car equipped with a preselector gearbox is a unique experience. To start the car, first you must make sure the ʻgear leverʼ – usually nothing more than a spindly lever on the dashboard or steering column – is in the neutral position. Start the engine and then move the lever to ʻfirstʼ. Nothing will happen until you depress the left-hand pedal and release it – that engages first gear. Now, using the throttle, you bring the engine revs up to the point where the centrifugal clutch bites and the car moves off.
As soon as the car is underway, move the gear lever into second and, when youʼre ready, simply lift off the throttle and depress/release the left-hand pedal again. And thatʼs it. Continue the process until youʼre in top gear, at which point you move the gear lever into the next lower ratio ready for when you need to change down a gear. If this sounds all very ponderous – for example, when struggling up a steep hill – never fear: you could effect the change of ratios without lifting off the throttle. Oh, and somewhat scarily, you can also move the lever into the reverse gear position when travelling forwards in anticipation of backing into a parking space…
Although never intended as a sporting option, the concept of ʻhaving the next gear readyʼ, so to speak, would clearly be of
“THE TRANSMISSION IS ALWAYS IN A STATE OF READINESS…”
advantage to a driver who wished to press on without having to worry about grabbing a gear lever midway through a series of twist and turns.
In 1939, when preselector gearboxes were popular, a German engineer by the name of Adolphe Kégresse tested a new transmission in a Citroên ʻTractionʼ, a design which he felt would make driving easier, dispensing with the need for manual gear changes. Kegresse was born in 1879 in France but moved to Russia in 1905 to work for Tsar Nicholas II. There he developed the ʻKegresse trackʼ, a half-track conversion for conventional cars, allowing them to be driven in mud and snow.
On his return to France in 1919, Kégresse began work with Citroên but left after a brief few years to concentrate on developing his own gearbox: the twin-clutch Autoserve transmission. Pre-war manual gearboxes tended to be rather agricultural in operation, requiring drivers to carefully synchronise road- and engine speed to prevent clashes between gear teeth. Kégresseʼs patented design was ingenious, compact and efficient, and proved satisfactory in operation in his Citroên ʻguinea pigʼ. However, the onset of hostilities brought a premature halt to his work, and Kégresse sadly passed away in 1943 at the age of 64.
He had already filed a patent in 1939 and further patents were submitted posthumously in 1946, and granted in 1951: ʻThe search for automatic operation of change-speed transmissions as applied to motor cars has led, in some systems, to connecting the engine to the gear trains by means of two independent clutches forming a unit and mounted on the same axis by means of two concentric shafts, as in French Patent No. 861,394 of 28th July 1939 in particular. An arrangement is thus obtained in which some of the shifts, the even numbered shifts for example, are taken on one of the clutches, and the odd numbered shifts on the other.ʼ
The essence of Porscheʼs current PDK design, however, is contained in the following paragraph: ʻThe use of two clutches on concentric shafts enables a more compact transmission to be obtained with shorter shafts and having less parts than in the usual construction.ʼ
Over the next decades, several related patents were applied for by other companies, many of which made direct reference to Kégresseʼs Autoserve design. Dodge in the USA (1950) and Panhard & Levassor in France (in 1957) both tipped their hats to the Frenchman, as did Zanhradfabrik Friedrichshafen AG, also in 1957. If that name is not immediately familiar, the initial letters will be: ZF. In fact, the list of patent applicants who made reference to Kégresseʼs design reads like a whoʼs who of the motor industry – and right up until as recently as five years ago, ZF still acknowledged his work. He is truly the father of the PDK gearbox, yet his name rarely appears in connection with Porscheʼs fast-shifting transmission.
It wasnʼt until the 1980s that PDK development really moved into top gear (sorry, couldnʼt resist it…), but the Porsche connection actually has its roots in the late 1960s with the arrival of a Hungarian engineer, Imre Szodfridt, who had taken a keen interest in Kégresseʼs design. Szodfridt worked under Helmut Fleigl at Weissach who, according to Karl Ludvigsen, considered him ʻa very innovative character, (but) very difficult to controlʼ. Kégresseʼs double-clutch system was championed by Szodfridt and shown to Ferdinand Piëch, who saw it as a possible option on the 911. However, the design was shelved due to a lack of refinement, only to be resurrected a few years later when Porsche collaborated in a programme to design a fuel-efficient car of the future: Type 995, based on the aluminium structure of the 928 and equipped with an early form of doubleclutch transmission. In the early 1980s, the PDK idea was initially seen as only being relevant to road cars, but all that was about to change as the race department came under pressure to take a closer look at this innovative design.
“FERDINAND PIËCH SAW IT AS A POSSIBLE OPTION FOR THE 911…”
The advantages for racers were obvious, for the driver would be able to concentrate on keeping his eyes on the road, his hands on the wheel, while making full-power gear changes. Not only that but, as with the preselector design of old, the next gear ratio was always lined up ready to go as soon as the driver hit the ʻupʼ or ʻdownʼ shift buttons. But there was another advantage…
Porscheʼs racing programme almost entirely centred around turbocharged machinery, the problem here being that conventional gear changing allowed the engine to drop off boost if the driver couldnʼt change gear quickly enough. Race transmissions tended to be heavy and relatively slow, rugged so as to withstand hours of abuse. If only the driver could make fullpower shifts there would be no loss of turbo boost, no momentary lag in performance. And thatʼs where the PDK came into its own.
The early 1980s saw a lot of behind the scenes work at Porsche, with the water-cooled 944 programme well under way. One omission from the range was an automatic version of the 944S. The non-s models were offered with a three-speed Audibuilt auto ʼbox, but this wasnʼt considered strong enough to cope with the added torque of the 944Sʼs engine. Volkswagen had been developing an electrohydraulic four-speed unit, which was tested in a 944 and found to be just what Porsche was after. However, VW then dropped the bombshell that the unit wasnʼt yet ready for production and they would be unable to meet the proposed 1989 launch of an automatic 944S.
ZF had also been developing its own four-speed automatic, although in prototype form it was only available with hydraulic control. ZF pointed out that it could be updated to the more advanced electronic control if Porsche was prepared to make further investment. This proved too much for Porsche and the idea was dropped.
So, where did this leave Porsche? By now, the PDK idea had been bubbling away in the background for some time, with particular emphasis on the race programme. However, it is said that expenditure on the PDK concept could only be justified if there was a spin-off for road cars. Once Imre Szodfridt had presented his (or, more correctly, Adolphe Kégresseʼs) ideas to his superiors, a 924S was built equipped with an early version of the PDK gearbox. It was massively over-engineered for the road car but allowed the system to be evaluated.
The control system fitted to the 924S test car was nothing more than a single lever, which could be pushed in any one of four different directions: forwards to shift up a ratio, back to change down, to the left to select reverse and to the right to select ʻparkʼ. This was the same system as used in the race cars, with one exception: it lacked the facility to pre-select ratios, which could then be engaged by pushing a button on the steering wheel. This was a deliberate omission as Flegl and his team were concerned that an inexperienced driver on the road might accidentally select an inappropriate ratio which could then be engaged at the wrong time.
Paul Frère tested the Pdk-equipped 924S for Road & Track magazine and was impressed, although doubtful that it would go into production much before the end of 1987. Helmut Flegl had his own 944 Turbo fitted with a PDK unit, finding it both quicker and more economical than the regular 944 Turbo. Tests on a variety of roads around Weissach showed the Pdk-equipped car to be some 12 per cent more fuel-efficient than a similar model fitted with a conventional automatic and even fractionally more frugal than a 944 Turbo with a manual transmission. Hans-joachim Stuck also enjoyed driving a Pdk-equipped 928, the torque of which was a far better test of the new transmission than any four-cylinder-engined car.
The problem, however, was that these first PDK gearboxes were not very smooth in operation. That, of course, was not a problem in a race application, but was unacceptable as far as a road car was concerned. Twin dry-plate clutches were responsible for the PDKʼS rather brutal character, the only option being to develop a wet-clutch design familiar to motorcycle engineers. However, production costs proved prohibitive – it is estimated that Porsche would have needed to sell 40,000 Pdk-equipped 944s to make the concept viable – so the PDK programme was quietly dropped, at last as far as road cars were concerned.
The PDK programme was perfect for the race department, though, and this is where the most interest lay. The double-
depress the clutch, but it added a lot of extra weight – it felt like you had a trailer on the back. We tried to persevere with it but the system just wouldnʼt last a 1000 kilometre race. It nearly cost me the World Championship in 1986…ʼ
However, Bell did appreciate that the new gearbox was ultimately a good idea, as it removed the worry of the driver messing up gear changes to the detriment of the transmission. The old dog-clutch boxes were brutal and needed a firm hand, and wear on components was higher than an engineer would wish for.
Extensive testing ahead of the 1986 season reaped rewards, despite Bellʼs initial scepticism. Installing the PDK transmission in the 962 added a not-insignificant 44 kilos to the overall weight, all at the rear of the car. Despite this apparent handicap, Stuck lapped the full 3.6-mile Paul Ricard circuit some 0.77 seconds quicker than his best time in the manual transmission car, while Bob Wollek lapped the shorter 2.03-mile circuit 0.81 seconds quicker.
By the end of the 1986 season, Pdk-equipped 962s had competed in no fewer than 11 major races, notching up three wins and two seconds. There were three retirements, two of which were directly attributable to the PDK system, the third when both team cars collided in poor visibility at the Nürburgring!
The one event where PDK wasnʼt able (or perhaps ʻallowedʼ is the right word) to prove itself was Le Mans, Singer not trusting the double-clutch transmission to last the full 24 hours. A PDKequipped 962 did run at the ʼ86 Le Mans test day and proved to be the fastest car on track, but at the main event only one car was thus equipped. The drivers, Vern Schuppan and Drake Olson, were informed that if the unit broke in the warm-up ahead of the race, their entry would be withdrawn. The ʼbox survived the warm-up session, only to break 41 laps into the race.
PDK transmissions continued to be used in the 962 throughout the 1987 season, but had still yet to make an impact on the road-going range. In 1985, there was talk of resurrecting the technology for use in the 964-series 911s, following the decision to end further development of the old ʻclutchlessmanualʼ Sportomatic system. The PDK programme was again to be a joint venture with Audi, who wanted an automatic transmission for the new V8-powered front-wheel-drive saloon. But then Audi backed out, and Porsche once again considered abandoning PDK for its road cars. In 1990, PDK was cited for use in the proposed Type 969, a 3.5-litre water-cooled, twinturbocharged supercar built on the 964 Carrera 4 chassis but styled along the lines of the mighty 959. It was to be fitted as standard with the PDK transmission, with manual as an option. This range-topping, 185mph coupé was to enter production in 1991 but the project was cancelled in favour of the 964 Turbo.
Another 17 years would pass before the Porsche Doppelkupplung system would finally be adopted for road use, despite having proved itself time and again on the race track. In the interim years, Porscheʼs successful Tiptronic semi-automatic transmission had found favour, but its days were numbered almost from the outset. PDK was clearly the future for, as Volkswagen proved with its similar DSG transmissions, no other technology could match the double-clutch design for speed and ease of use. Like it or not, PDK was here to stay.
Something tells us that, right now, Adolphe Kégresse is looking down from on high, smiling at Porscheʼs seven-speed, lightning-fast gearboxes that rely on his 1939 patents for inspiration. Porsche may have claimed it took 25 years of development to bring PDK to the table, but in reality, it was more like 70 years… CP
“INSTALLING THE PDK ADDED 44KG TO THE OVERALL WEIGHT…”
Porsche first began to investigate the full potential of the PDK system in the former Le Mans-winning 956, chassis number 956-003, as far back as 1983
Above left: Frenchman Adolphe Kégresse was born in 1879 and worked for Tsar Nicholas II before returning to France to concentrate on the development of his own Autoserve double-clutch transmission. He is without question the father of the modern day PDK gearboxAbove right: Sectional drawing of Kégresseʼs double-clutch transmission formed part of a patent application filed posthumously in 1946. The layout, with its concentric shafts, is virtually identical to the PDK of today
Above left: Artwork from a 1930s Armstrong-siddeley brochure demonstrates use of the ʻself-changing gear ʼ – otherwise known as the preselector gearbox. Slogan proclaims ʻEyes on the road, hands on the steeringʼAbove right: In cars fitted with preselctor gearboxes, the left-hand pedal is referred to as the ʻgear changing pedalʼ, rather than a clutch pedal. Centrifugal design means no ordinary clutch pedal is needed
Top left: Interest in the double-clutch system was aroused in the early 1980s when Porsche sought an alternative to the conventional automatic transmission. Both 924s and 944s were adapted to use the PDK gearbox, this 944 Turbo being Helmut Fleglʼs personal carTop right: If the PDK could stand up to the torque of a 944 Turbo, how would it survive the grunt of a V8? Hans-joachim Stuck put the double-clutch system to work in his own 928 and loved itAbove: Type 995 was a design for a 928-based car of the future which, among other things, relied on a double-clutch transmission. The project never got off the ground but kept interest in the PDK alive
Top left: Previously unseen photo of disassembled 956/962 PDK transmission was recently given to the Porsche archives by a former Weissach engineerTop right: Lever on the far right allowed the driver to preselect a particular gear, while punching the buttons on the steering wheel effected an up or down shift. Digital display informed the driver which ratios had been selected in the 956/962Above left: The Type 969 was close to becoming the first production road-going Porsche to be equipped with PDK technology, but the project was cancelled in favour of the 964 TurboAbove right: Cutaway PDK transmission from the 956