911Time
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Hi all,
I know a lot of you will already be familiar with PDK and its operation but for those who are new to Porsche, or would like to know more about its background, I thought it might be interesting to explore the engineering and history behind this gearbox, in more detail.
I've put together the following from a variety of sources, including my own but if there are any errors or omissions, just let me know and I'll blame it on 'brain-fog' :?
My advice is to grab a cuppa and a comfortable chair, 'coz this one's a doozy
Porsche Doppelkupplungsgetriebe (Porsche Double-Clutch Transmission) or PDK
Call it what you will, or whatever you're comfortable pronouncing but PDK isn't as complex as it sounds, it doesn't use any black magic (just well-conceived design/programming) and contrary to popular belief, it's repairable - even if Porsche don't really want you to.
Principles
Although it's often referred to as an 'automatic' or 'semi-automatic' transmission, it's not what most people have come to think of as an 'automatic' (as I'll explain below). It doesn't feature an epicyclic gear-train (planetary gearset) or fluid coupling/torque converter, instead it's more of an automated manual, with some very clever control systems.
The basic design of the PDK gearbox, is the same as any other 'dual-clutch transmission' (DCT): Power is sent to the gearbox directly from the engine and as there is no torque converter, the gearbox is coupled/de-coupled from the engine using two multi-plate clutch units. These dual clutches feed two sets of gears, one for the odd numbered gears and one for the even numbered gears and the two clutches are engaged or disengaged, in order to move between gear ratios.
Most importantly, this design enables the next gear up/down to be pre-selected, providing rapid transition from one gear ratio to another, without loss of propulsive power and leverages the benefits of 'automatic' transmission convenience, with the mechanical efficiency of a manual gearbox.
Basic DCT transmission (illustrating a gearbox with 6 gear ratios)
The History of PDK – A VERY Long & Winding Road
It's a common misconception, that the dual-clutch transmission (DCT) is a modern invention and whilst it's true that the effective and reliable implementation of the technology, wasn't possible until fairly recently, the actual idea goes a long way back in time.
The original concept for a dual-clutch transmission, was patented in 1935 by the French engineer, Adolphe Kégresse (who also invented the half-track propulsion system). You have to remember that in those days, manual gearboxes were pretty agricultural in operation and required the driver to develop a skilled approach to changing gear, timing their actions very precisely. Crunched gears and swearing, were often the order of the day.
He called his creation the AutoServe gearbox-transmission system and it was, to all intents and purposes, what we now consider to be a DCT. Unfortunately, due to various factors (including a lack of financial backing), Kégresse was unable to develop his idea further and so it remained just a concept.
At roughly the same point in history, a German Professor called Rudolph Franke of Darmstadt University of Applied Sciences, was working on his own DCT system and in 1940 applied for a patent for that design but again the idea wasn't progressed.
In the years that followed, various hybrid transmissions were developed but the hydraulically controlled automatic gearbox, based on the principles of the epicyclic gear-train (planetary gear-set), established itself as the most popular alternative to a manual gearbox. Companies like General Motors popularised the option, with their Hydramatic four-speed and this type of transmission, became what most people thought of as an 'automatic'.
You might be surprised to learn, that Porsche was therefore somewhat late to the party, as they began working on their own DCT(PDK) in 1964, before developing a 4-speed version in 1968 but due to the complexities of controlling gear-changes, the 'box was still a long way off being ready for series production – as you'll read below.
Porsche Sportomatic – Almost But Not Quite
Forever innovating technological evolution, Porsche engineers continued to experiment with different forms of transmission and for the roadgoing 911, offered an alternative to the conventional manual gearbox, in the form of Sportomatic.
Produced between 1968 and 1980, Sportomatic combined a conventional design of 4-speed manual gearbox (later reduced to 3 speeds), with a vacuum operated dry clutch – in order to provide the driver with a means of manually changing gears, without having to operate a foot clutch. Whenever the driver moved the 'gear-lever', the vacuum powered clutch would disengage to change gear, before re-engaging once again, as soon as the driver released the lever.
A torque converter enabled smoother transitions and for the car to come to a halt with the clutch still engaged however, the system had its flaws and the Sportomatic gearbox required its own particular approach to driving. Most notable was that, like a conventional manual 'box/clutch arrangement, the driver must remember to lift off the throttle during gearshifts, in order to prevent overrevving – unlike the later PDK.
Porsche Type 995 – Looking Forwards to PDK
In 1979, prompted by the Oil Crisis, the German Federal Ministry of Research & Technology, challenged the motor industry to create a vehicle that would optimise fuel efficiency, as a vision of the future.
Porsche responded with a design study for a future four-seat sports car – type 995, that included various technology to help make the car safer and more fuel efficient – concepts that included ABS braking and a dual-clutch transmission, which proved to be one of the most advanced transmission types within the study.
Automotive Products
The next part of the DCT story takes place in England, at the well-known brake and clutch manufacturer AP (Automotive Products) where in 1980, under the leadership of Harry Webster, many prototypes of the dual-clutch transmission were developed and tested, in a range of vehicles.
Whilst the realisation of a working gearbox moved closer to reality, the control circuitry for the DCT was so complex and bulky, that the concept was shelved once again. Get used to that, because it's going to become a theme.
Porsche's PDK Transmission is Officially Born
At much the same time in Germany, a relatively young (33 year old) Porsche Gearbox Test Engineer, called Rainer Wüst, was working under Helmut Flegl, Head of Advanced Development at Weissach - their aim, to develop a functional dual-clutch transmission, from prototype to reality.
Wüst was an experienced engineer and led a widely skilled team but faced a common DCT problem, that had foiled many before: The greatest challenge, being to overcome the difficulty of designing a suitable means of controlling the clutch units and twin gear-sets, precisely and in unison.
An electrohydraulic design was settled upon however, the control electronics and much of the electrohydraulic valve system, had to be completely developed from scratch.
Early PDK Prototype
Various testbeds were created but in a Porsche 924 Turbo, the team were finally able to develop the basic functions of the DCT to a workable level and in 1981 PDK was officially born. Unfortunately, despite making great strides in its implementation, the electrohydraulic control system was still very bulky & complex and the difficulties of providing sensitive operation of the clutch units, meant it was not yet suitable for the road.
Testing in a 924 mule
Rainer Wüst continued at Porsche as Technical Director, specialising in transmission and chassis development – he officially retired in 2009, after 38 years with the company.
Rainer Wüst
Motorsport - The Ultimate Proving Ground
At the same time as DCT/PDK was being investigated for road cars, Porsche motorsport engineers were searching for ways to further improve the competitiveness of their turbo-charged 956 Group C race cars.
Ferdinand Piëch, who was head of Porsche/Audi motorsport in those days, had driven the PDK test cars, he understood the potential advantages that a dual-clutch transmission could offer and that it might deliver valuable gains on the track: Without any loss of propulsive power, a DCT's rapid upshifts could help to maintain momentum and the almost immediate transfer of drive (via the dual clutch system), could prevent the engine from coming off boost between changes – all the more important at that time, as turbochargers were large and therefore took more time to 'spool up' between shifts. Mechanised gear changes, would also help reduce the potential for a missed gear because, even the best drivers were only human and could still make mistakes, costing valuable seconds – or the entire gearbox and race.
The DCT/PDK principle was a good one however, the realisation of its full benefits in the heat of motorsport, presented its own set of issues.
In 1983, Porsche undertook a test run of PDK, in its 956.003 race car and the following year, went on to test the system again at Nürburgring. In 1986, it ran a similarly equipped 962 C in the World Sportscar Championship and at Monza's 360km race, the PDK car scored its first victory.
Early iterations used a lever to change between gears but this was replaced by steering wheel mounted buttons. The new 'Ground Effect' aerodynamics, meant the 956/962 was capable of pulling high lateral g but the steering was very heavy, so anything that helped keep the driver's hands on the wheel, was a real bonus.
Living only a couple of hours from Weissach, racing legend Hans-Joachim Stuck was heavily involved in the development of PDK and as a works driver logged many hours testing the transmission. Here he is seen with Rainer Wüst (right), the man who was effectively the 'Father of PDK'.
Stuck was quite enthusiastic about its potential. His team-mate however, the usually affable Derek Bell, wasn't quite so effusive. PDK added considerable weight and although the technology held promise in principle, it could and often did, turn out to be a different story come race-day.
The final race of the '86 season, was at Japan's Fuji Speedway and Porsche were keen to use PDK but not so Derek Bell, who feared it wouldn't last the 1,000km race and would cost him the overall championship. Despite complaining to HQ, he was effectively told to 'get on with it'!
Unlike the modern PDK, the system in these cars, used two dry clutches to engage the gear-sets. Wear of the friction plates was high but unlike a road car, at least the clutches could be changed after every race. Unfortunately, the rapid transfer of power that the system was capable of, was hard to tame and full-throttle upshifts of 640 horsepower, could result in broken half-shafts.
On the day of the race, both Bell and Stuck campaigned hard. A thousand kilometres later, despite several issues and more half-shaft problems, their Rothmans 962C crossed the line and Derek Bell secured his second Group C Driver's World Championship.
Audi Sport Quattro E2
Because Ferdinand Piëch had control of both Porsche & Audi motorsport programs and was in essence, the man behind the Ur-Quattro, at the same time Porsche were developing PDK for the 956, similar testing was also taking place at Audi, for the 5 cylinder 'beast of the forest' – the Sport Quattro E2.
Whilst the Evolution 2 engine, had been further improved over the S1, both to reduce turbo-lag and provide more mid-range torque, the single high-boost turbocharger meant it was still 'peaky' and its c500hp at 8,000rpm, dealt the transmission a sledgehammer blow with every upshift.
For the 1985 Semperit rally in Austria (a national not championship race), Audi decided to test the PDK unit in the heat of competition and two of the cars had their close-ratio 6-speed manual transmissions swapped out and replaced by a 5-speed PDK unit – one vehicle for pre-race testing and one for the actual event. Rapid upshifts delivered equally rapid progress (0-100kmh in around 2.5 secs) and driven by the legendary Walter Röhrl, the PDK equipped car romped home in first place.
Despite these early successes for Porsche and Audi, the complexity of controlling the PDK transmission, particularly in respect of shift timing & quality, was still proving difficult and wasn't of sufficient quality or reliability to consider for series production. This and Audi's decision to pull out of rallying in '86 meant that yet again, the system was shelved. We've been here before...
Tiptronic – A Manually Selectable Automatic
It might be an oxymoron but seems like the best way to describe the thinking.
Through the late 1970's and early 1980's the Porsche brand became more and more widely recognised as an aspirational vehicle, no longer just the preserve of sportscar enthusiasts but an object of desire for the nouveau riche – many of whom associated a luxury car brand with convenience and would rather eschew the manual changing of gears, in favour of an automatic transmission. This was especially so in the USA, where automatic cars had traditionally outsold manuals for many years.
There was no doubt about it, to fill all those potential sales, Porsche needed an 'automatic' but as Sportomatic had its issues and the technology to deliver a road-going PDK wasn't available, it would have to be an epicyclic auto and for Porsche, any old 'slush-box' just wouldn't do! Porsche was synonymous with performance and that meant active driver involvement, something that was the antithesis of driving a normal automatic.
Working in conjunction with engineers at well-known transmission experts ZF Friedrichshafen, the two companies developed a 4-speed automatic gearbox, based on the traditional epicyclic geartrain but with clever electronic 'management', that not only afforded the driver a greater degree of control but monitored inputs from the vehicles sensors, to deliver a more adaptive and sporty experience – something that created a point of difference and would in later years, form the founding principles, for controlling PDK.
Processing information on the vehicles road speed, engine revs, throttle position, fuel flow and lateral/longitudinal acceleration, with newly developed computer algorithms, the gearbox's electronic control unit could choose from 5 different 'shift maps'. These 'maps' controlled a variety of parameters, including the timing of the gear change, whether a gear was held, for how long etc. In this way, the transmission was able to be far more reactive to driver inputs and could consequently deliver a more 'sporty' driving experience. Porsche referred to this ground-breaking electronic control as the 'Intelligent Shift programme'.
In addition to the conventional floor mounted gear-lever, with traditional automatic 'P,R,N,D' selection choices, the driver could move the lever into a 'Manual' gate and choose gears themselves.
Internally designated as the ZF 4HP22, the 'Tiptronic' gearbox was introduced as an option, for the 1989 version of the 911 (964) and remained in production from 1989 to 2008, becoming a 5-speed along the way.
The Golf R32 DSG
In 2003, Volkswagen introduced a dual-clutch 6-speed gearbox, as an option for the Golf R32 road-car and later the Audi TT. The DSG (Direkt-Schalt-Getriebe or Direct Shift Gearbox), was the first dual-clutch transmission to be sold in a series production road car.
Manufactured by Borg Warner, DSG moved the game on substantially and offered lightning-fast changes however, unlike Porsche's later PDK, it was designed for transverse FWD installation, using a unified lubrication system and the software lacked a number of key features that would make the later unit more suited to a high-performance Sportscar. At launch, DSG was also only capable of handling 350Nm maximum torque.
Audi followed soon after, with their longitudinally mounted S Tronic dual-clutch gearbox, with separate clutch & gear oil lubrication. Both DSG and S Tronic have remained in production ever since.
Porsche PDK by ZF Group - Launched for the Road (finally)
In 2005, bringing together all they had learned from the past and combining it with the power of modern electronics, Porsche decided the time was now right to press ahead with development of a PDK for the road and contracted development & manufacture of the unit, to their long-term partners at ZF.
ZF drafted some 300 engineers from throughout its group, to productionise PDK. In a process that took 3 years, the team created the 7DT (7 speed) transmission, with the bulk of its major components manufactured within ZF group of companies, including the dual clutch units, supplied by ZF Sachs.
This production version of the PDK gearbox was then launched in 2008, as an option for the second generation 997.
Design Principles of the 7DT Transmission
The original 7DT/PDK, followed the basic ethos of a dual-clutch gearbox: Two parallel, switchable, constant mesh gear-sets (featuring full synchromesh), providing 7 forward gears and one reverse, with power transferred by two wet clutches.
The operational elements being contained within two distinct areas*:
1) RED: The electro-hydraulic control elements, running in a specially developed transmission fluid, including the hydraulic pump, dual wet clutches and valve-body (similar to that used in a conventional 'automatic' gearbox), with solenoid engaged, hydraulic control of the dual-clutch units, as well as gear shift operations.
2) BLUE: Two constant-mesh geartrains, running in hypoid gear oil, with gear-shifting operated by electro-hydraulically controlled selector forks and with full synchromesh.
*NB: Later versions for other Porsche models, used a common oil system. Additional oil-cooling was also added to the original 7DT/PDK, to increase its longevity.
The gearbox was designed to be highly efficient mechanically, with fluid levels kept low enough to reduce churning losses (power lost through churning oil) but sufficient for both high performance and long fluid drain intervals - 56,000 miles (6yrs) for the hydraulic oil and 112,000 miles (12yrs) for the gear wheel oil, although some owners (including myself), may choose to change the fluids earlier.
NB: The officially recommended drain interval for the hydraulic oil was later extended, after MY2014 to 12 years. Thanks deMort for that info.
It's important to note, that whilst the hydraulically controlled implementation of the gear changes, are undertaken by the transmission, the entire operational control of the gearbox and the gear-shifting strategy, are all managed from outside of the 'box, by the electronic Transmission Control Unit (TCU).
The engineering integration of these systems, combined all that Porsche had learned from its previous Sportomatic and Tiptronic programs and enabled PDK to finally deliver the full benefits of the dual-clutch design, with seamless full-power shifts, undertaken in around 400-500 milliseconds and a true sportscar-focussed driving experience.
Careful design and material specification for the clutch units by ZF Sachs, provides for a wide range of operating parameters, whilst ensuring efficiency and longevity. In cars optioned with the Sport Chrono pack, selection of 'Launch Control' implements additional transmission strategies: A control valve in the hydraulic control circuit, increases the flow of fluid from the hydraulic pump, in order to supply additional cooling to the clutch units, to reduce the effects of frictional heat. Additionally, the Launch Control option is disabled for a distance of 1.5 miles after use, to allow the clutch packs to cool down.
Due to its increased performance envelope, an upgraded clutch specification is used for PDK, when fitted to the Turbo/Turbo S models.
The Following video from ZF, provides a good overview of the construction of the 7DT/PDK gearbox – in this instance a 911 Carrera 4/S model (with front power take-off).
Driver Options and Adaptations
The 7DT/PDK transmission provides many advantages but for a sportscar driver, chief amongst these are its controllability and adaptability. This was one of the key breakthrough areas, where Porsche & ZF engineers were able to leverage the computational power of modern electronics and combine it with lessons learned from previous research programs, as well as live data, gathered from sensors within the vehicle.
At the most basic level, the TCU offers the driver a choice of either automatic or manual control, the driver can then choose from Normal, Sport or *Sport Plus settings, providing a choice of gear-shift strategies (or maps), that are oriented towards comfort, performance or racetrack. In Normal mode for instance, the comfort of the upshift is optimised, by interrupting engine torque (via a gap in ignition timing), during the shift operation, whereas the interruption is lessened when Sport mode is engaged and removed completely during Sport Plus – which optimises performance & shift speed, at the expense of comfort.
*Cars optioned with Sport Chrono.
Beyond its basic shift maps, the TCU also monitors inputs from the vehicles other control systems to effectively read the road and the driver's mind and deliver suitable gear-shift strategies – these affect not only the engine speed at which gears are changed, or how the gear-shift is delivered (soft/hard) but most crucially, the major benefit of a dual-clutch system – which gear the TCU 'predicts' the driver will need next.
Live vehicle data, including engine revs, throttle position, throttle change speed, gear-selector position, vehicle speed, steering angle, lateral & axial acceleration, engine and transmission temperature and altitude, provide for a continuous flow of information to the TCU.
Learning and adapting to its driver, driving style and route, gears can be pre-selected, or the strategy changed at a moment's notice, in reaction to evolving circumstances, so as to deliver the most responsive, efficient and safe means of progress.
For instance, should the driver decide to overtake and adopt a more dynamic driving style, then even without the transmission set to 'Sport', the TCU will utilise the 'Sport' map for a brief period, in order to make the vehicle more responsive.
Unlike some other iterations of the dual-clutch gearbox, PDK is also capable of working beyond a sequential change pattern (1,2,3,4 etc) and can 'skip' gears up or down – saving time and delivering the right gear as quickly as possible.
Driver adaptations occur, in both automatic and manual operation, of the Normal and Sport maps, but not in *Sport Plus.
Rapid Warm-Up
In a bid to aid efficiency and reduce emissions, the 7DT/PDK, uses a warm-up map when the vehicle is first started. Should the coolant temperature be lower than 20 Deg C, then the TCU implements higher shift points, which enables the catalytic converters to reach their optimal temperature as soon as possible – this strategy also ensures that the engine and transmission reach operating temperature more quickly.
Cornering – Gear Retention
Enthusiastic drivers know it's a bad idea to change gear mid-corner, as it upsets the delicate balance of the vehicle, therefore in a PDK equipped car the TCU is programmed to prevent loss of traction during cornering. In response to input from the vehicles lateral acceleration sensor (which is integrated with its stability management systems), the TCU avoids changing up/down a gear when lateral g force exceeds a set value – helping to maintain stability and aid driver control.
'Virtual' Gears
The PDK gearbox in the 991 Turbo and 991.2 were further improved in a number of key areas, including the ability to provide what Porsche refer to as 'Virtual' gears. If the vehicle is travelling along in a steady state, on a light throttle, in one of the lower 3 gears (which have the widest spacing of gear ratios) and the TCU determines that none of the current ratios offers the best efficiency, it can effectively run two gears at once.
By engaging both clutches lightly and slipping them, it's possible to provide an intermediate ratio, that better suits the state of the vehicle and thereby maximises efficiency. As soon as the conditions change, such as the driver accelerates or brakes, the gearbox reverts to the most appropriate single ratio. Clever stuff.
Fuel Efficiency
By using an overdrive ratio for 7th gear, PDK helps to deliver exceptional fuel economy. This is added to by the gearbox's 'Sailing' Function – where, when the vehicle is being driven under light load, at a steady state and under part throttle, the TCU can open the clutch units, to disconnect the engine from the gearbox and drop the engine's revs to tickover.
All of these clever features and many more, make PDK one of the most highly developed, effective and efficient alternatives to a manual gearbox and a worthy testament, to the many engineers and years of development that went into its creation.
Repair and Re-Use
PDK repair – oh what a thorny issue that is? Some think PDK can't be repaired, that it's too complex but that's just an urban myth – other DCT gearboxes are able to be repaired, including Volkswagen's DSG/Audi's S Tronic. The reality is, that for some reason (un-known and un-fathomable to me), Porsche don't seem to want to repair your PDK when it goes wrong (or allow anyone else to do it for you) and that seems at odds with the very principles the company have stood by for decades i.e., to maintain vehicles, repair them and keep them on the road – after all, it's why over 70% of all 911s are still on the road today.
Generally, it looks like PDK's manual gear-train is pretty robust and the issues that affect the 'box are mostly to do with its electrohydraulic system (shift position sensors or speed sensors esp) or the TCU. In some cases, parts that are fairly easy to access physically and cost next to nothing (in comparison to a new gearbox) could be replaced but Porsche, or maybe ZF, don't want to supply the individual parts.
We live in a world where repairing and re-using items had gone out of fashion but now we realise, such rampant consumption of new resources doesn't make any sense and those old attitudes are no-longer socially acceptable – so, despite being a life-long fan of Porsche, I have to ask, why condemn an otherwise healthy transmission, or require its costly replacement, when it could be repaired? Come on Porsche, do the decent thing – you know it makes sense!
In the Meantime
This very informative 2019 post on Rennlist, by thread starter PV997 (an Aerospace Engineer), outlines some of the common PDK faults. Whilst it focuses mainly on the first iteration of PDK in the 997.2, the principles are the same for the 991.1 and it goes to show just how easily some of the problems can be rectified, with access to the suitable parts:
https://rennlist.com/forums/997-for...airing-a-pdk-transmission.html?ispreloading=1
For comparison of the disassembly of other DCT transmissions, this tear-down of a VAG DSG gearbox by a transmission expert at Getriebe Doc ( https://www.getriebe-doc.de ), shows how dual-clutch transmissions, aren't anymore complex than an epicyclic auto and in many ways, far simpler:
For all those keen to know the inner workings of their 911, I hope that's been enlightening and an enjoyable read.
Til next time, safe travels.
I know a lot of you will already be familiar with PDK and its operation but for those who are new to Porsche, or would like to know more about its background, I thought it might be interesting to explore the engineering and history behind this gearbox, in more detail.
I've put together the following from a variety of sources, including my own but if there are any errors or omissions, just let me know and I'll blame it on 'brain-fog' :?
My advice is to grab a cuppa and a comfortable chair, 'coz this one's a doozy
Porsche Doppelkupplungsgetriebe (Porsche Double-Clutch Transmission) or PDK
Call it what you will, or whatever you're comfortable pronouncing but PDK isn't as complex as it sounds, it doesn't use any black magic (just well-conceived design/programming) and contrary to popular belief, it's repairable - even if Porsche don't really want you to.
Principles
Although it's often referred to as an 'automatic' or 'semi-automatic' transmission, it's not what most people have come to think of as an 'automatic' (as I'll explain below). It doesn't feature an epicyclic gear-train (planetary gearset) or fluid coupling/torque converter, instead it's more of an automated manual, with some very clever control systems.
The basic design of the PDK gearbox, is the same as any other 'dual-clutch transmission' (DCT): Power is sent to the gearbox directly from the engine and as there is no torque converter, the gearbox is coupled/de-coupled from the engine using two multi-plate clutch units. These dual clutches feed two sets of gears, one for the odd numbered gears and one for the even numbered gears and the two clutches are engaged or disengaged, in order to move between gear ratios.
Most importantly, this design enables the next gear up/down to be pre-selected, providing rapid transition from one gear ratio to another, without loss of propulsive power and leverages the benefits of 'automatic' transmission convenience, with the mechanical efficiency of a manual gearbox.
Basic DCT transmission (illustrating a gearbox with 6 gear ratios)
The History of PDK – A VERY Long & Winding Road
It's a common misconception, that the dual-clutch transmission (DCT) is a modern invention and whilst it's true that the effective and reliable implementation of the technology, wasn't possible until fairly recently, the actual idea goes a long way back in time.
The original concept for a dual-clutch transmission, was patented in 1935 by the French engineer, Adolphe Kégresse (who also invented the half-track propulsion system). You have to remember that in those days, manual gearboxes were pretty agricultural in operation and required the driver to develop a skilled approach to changing gear, timing their actions very precisely. Crunched gears and swearing, were often the order of the day.
He called his creation the AutoServe gearbox-transmission system and it was, to all intents and purposes, what we now consider to be a DCT. Unfortunately, due to various factors (including a lack of financial backing), Kégresse was unable to develop his idea further and so it remained just a concept.
At roughly the same point in history, a German Professor called Rudolph Franke of Darmstadt University of Applied Sciences, was working on his own DCT system and in 1940 applied for a patent for that design but again the idea wasn't progressed.
In the years that followed, various hybrid transmissions were developed but the hydraulically controlled automatic gearbox, based on the principles of the epicyclic gear-train (planetary gear-set), established itself as the most popular alternative to a manual gearbox. Companies like General Motors popularised the option, with their Hydramatic four-speed and this type of transmission, became what most people thought of as an 'automatic'.
You might be surprised to learn, that Porsche was therefore somewhat late to the party, as they began working on their own DCT(PDK) in 1964, before developing a 4-speed version in 1968 but due to the complexities of controlling gear-changes, the 'box was still a long way off being ready for series production – as you'll read below.
Porsche Sportomatic – Almost But Not Quite
Forever innovating technological evolution, Porsche engineers continued to experiment with different forms of transmission and for the roadgoing 911, offered an alternative to the conventional manual gearbox, in the form of Sportomatic.
Produced between 1968 and 1980, Sportomatic combined a conventional design of 4-speed manual gearbox (later reduced to 3 speeds), with a vacuum operated dry clutch – in order to provide the driver with a means of manually changing gears, without having to operate a foot clutch. Whenever the driver moved the 'gear-lever', the vacuum powered clutch would disengage to change gear, before re-engaging once again, as soon as the driver released the lever.
A torque converter enabled smoother transitions and for the car to come to a halt with the clutch still engaged however, the system had its flaws and the Sportomatic gearbox required its own particular approach to driving. Most notable was that, like a conventional manual 'box/clutch arrangement, the driver must remember to lift off the throttle during gearshifts, in order to prevent overrevving – unlike the later PDK.
Porsche Type 995 – Looking Forwards to PDK
In 1979, prompted by the Oil Crisis, the German Federal Ministry of Research & Technology, challenged the motor industry to create a vehicle that would optimise fuel efficiency, as a vision of the future.
Porsche responded with a design study for a future four-seat sports car – type 995, that included various technology to help make the car safer and more fuel efficient – concepts that included ABS braking and a dual-clutch transmission, which proved to be one of the most advanced transmission types within the study.
Automotive Products
The next part of the DCT story takes place in England, at the well-known brake and clutch manufacturer AP (Automotive Products) where in 1980, under the leadership of Harry Webster, many prototypes of the dual-clutch transmission were developed and tested, in a range of vehicles.
Whilst the realisation of a working gearbox moved closer to reality, the control circuitry for the DCT was so complex and bulky, that the concept was shelved once again. Get used to that, because it's going to become a theme.
Porsche's PDK Transmission is Officially Born
At much the same time in Germany, a relatively young (33 year old) Porsche Gearbox Test Engineer, called Rainer Wüst, was working under Helmut Flegl, Head of Advanced Development at Weissach - their aim, to develop a functional dual-clutch transmission, from prototype to reality.
Wüst was an experienced engineer and led a widely skilled team but faced a common DCT problem, that had foiled many before: The greatest challenge, being to overcome the difficulty of designing a suitable means of controlling the clutch units and twin gear-sets, precisely and in unison.
An electrohydraulic design was settled upon however, the control electronics and much of the electrohydraulic valve system, had to be completely developed from scratch.
Early PDK Prototype
Various testbeds were created but in a Porsche 924 Turbo, the team were finally able to develop the basic functions of the DCT to a workable level and in 1981 PDK was officially born. Unfortunately, despite making great strides in its implementation, the electrohydraulic control system was still very bulky & complex and the difficulties of providing sensitive operation of the clutch units, meant it was not yet suitable for the road.
Testing in a 924 mule
Rainer Wüst continued at Porsche as Technical Director, specialising in transmission and chassis development – he officially retired in 2009, after 38 years with the company.
Rainer Wüst
Motorsport - The Ultimate Proving Ground
At the same time as DCT/PDK was being investigated for road cars, Porsche motorsport engineers were searching for ways to further improve the competitiveness of their turbo-charged 956 Group C race cars.
Ferdinand Piëch, who was head of Porsche/Audi motorsport in those days, had driven the PDK test cars, he understood the potential advantages that a dual-clutch transmission could offer and that it might deliver valuable gains on the track: Without any loss of propulsive power, a DCT's rapid upshifts could help to maintain momentum and the almost immediate transfer of drive (via the dual clutch system), could prevent the engine from coming off boost between changes – all the more important at that time, as turbochargers were large and therefore took more time to 'spool up' between shifts. Mechanised gear changes, would also help reduce the potential for a missed gear because, even the best drivers were only human and could still make mistakes, costing valuable seconds – or the entire gearbox and race.
The DCT/PDK principle was a good one however, the realisation of its full benefits in the heat of motorsport, presented its own set of issues.
In 1983, Porsche undertook a test run of PDK, in its 956.003 race car and the following year, went on to test the system again at Nürburgring. In 1986, it ran a similarly equipped 962 C in the World Sportscar Championship and at Monza's 360km race, the PDK car scored its first victory.
Early iterations used a lever to change between gears but this was replaced by steering wheel mounted buttons. The new 'Ground Effect' aerodynamics, meant the 956/962 was capable of pulling high lateral g but the steering was very heavy, so anything that helped keep the driver's hands on the wheel, was a real bonus.
Living only a couple of hours from Weissach, racing legend Hans-Joachim Stuck was heavily involved in the development of PDK and as a works driver logged many hours testing the transmission. Here he is seen with Rainer Wüst (right), the man who was effectively the 'Father of PDK'.
Stuck was quite enthusiastic about its potential. His team-mate however, the usually affable Derek Bell, wasn't quite so effusive. PDK added considerable weight and although the technology held promise in principle, it could and often did, turn out to be a different story come race-day.
The final race of the '86 season, was at Japan's Fuji Speedway and Porsche were keen to use PDK but not so Derek Bell, who feared it wouldn't last the 1,000km race and would cost him the overall championship. Despite complaining to HQ, he was effectively told to 'get on with it'!
Unlike the modern PDK, the system in these cars, used two dry clutches to engage the gear-sets. Wear of the friction plates was high but unlike a road car, at least the clutches could be changed after every race. Unfortunately, the rapid transfer of power that the system was capable of, was hard to tame and full-throttle upshifts of 640 horsepower, could result in broken half-shafts.
On the day of the race, both Bell and Stuck campaigned hard. A thousand kilometres later, despite several issues and more half-shaft problems, their Rothmans 962C crossed the line and Derek Bell secured his second Group C Driver's World Championship.
Audi Sport Quattro E2
Because Ferdinand Piëch had control of both Porsche & Audi motorsport programs and was in essence, the man behind the Ur-Quattro, at the same time Porsche were developing PDK for the 956, similar testing was also taking place at Audi, for the 5 cylinder 'beast of the forest' – the Sport Quattro E2.
Whilst the Evolution 2 engine, had been further improved over the S1, both to reduce turbo-lag and provide more mid-range torque, the single high-boost turbocharger meant it was still 'peaky' and its c500hp at 8,000rpm, dealt the transmission a sledgehammer blow with every upshift.
For the 1985 Semperit rally in Austria (a national not championship race), Audi decided to test the PDK unit in the heat of competition and two of the cars had their close-ratio 6-speed manual transmissions swapped out and replaced by a 5-speed PDK unit – one vehicle for pre-race testing and one for the actual event. Rapid upshifts delivered equally rapid progress (0-100kmh in around 2.5 secs) and driven by the legendary Walter Röhrl, the PDK equipped car romped home in first place.
Despite these early successes for Porsche and Audi, the complexity of controlling the PDK transmission, particularly in respect of shift timing & quality, was still proving difficult and wasn't of sufficient quality or reliability to consider for series production. This and Audi's decision to pull out of rallying in '86 meant that yet again, the system was shelved. We've been here before...
Tiptronic – A Manually Selectable Automatic
It might be an oxymoron but seems like the best way to describe the thinking.
Through the late 1970's and early 1980's the Porsche brand became more and more widely recognised as an aspirational vehicle, no longer just the preserve of sportscar enthusiasts but an object of desire for the nouveau riche – many of whom associated a luxury car brand with convenience and would rather eschew the manual changing of gears, in favour of an automatic transmission. This was especially so in the USA, where automatic cars had traditionally outsold manuals for many years.
There was no doubt about it, to fill all those potential sales, Porsche needed an 'automatic' but as Sportomatic had its issues and the technology to deliver a road-going PDK wasn't available, it would have to be an epicyclic auto and for Porsche, any old 'slush-box' just wouldn't do! Porsche was synonymous with performance and that meant active driver involvement, something that was the antithesis of driving a normal automatic.
Working in conjunction with engineers at well-known transmission experts ZF Friedrichshafen, the two companies developed a 4-speed automatic gearbox, based on the traditional epicyclic geartrain but with clever electronic 'management', that not only afforded the driver a greater degree of control but monitored inputs from the vehicles sensors, to deliver a more adaptive and sporty experience – something that created a point of difference and would in later years, form the founding principles, for controlling PDK.
Processing information on the vehicles road speed, engine revs, throttle position, fuel flow and lateral/longitudinal acceleration, with newly developed computer algorithms, the gearbox's electronic control unit could choose from 5 different 'shift maps'. These 'maps' controlled a variety of parameters, including the timing of the gear change, whether a gear was held, for how long etc. In this way, the transmission was able to be far more reactive to driver inputs and could consequently deliver a more 'sporty' driving experience. Porsche referred to this ground-breaking electronic control as the 'Intelligent Shift programme'.
In addition to the conventional floor mounted gear-lever, with traditional automatic 'P,R,N,D' selection choices, the driver could move the lever into a 'Manual' gate and choose gears themselves.
Internally designated as the ZF 4HP22, the 'Tiptronic' gearbox was introduced as an option, for the 1989 version of the 911 (964) and remained in production from 1989 to 2008, becoming a 5-speed along the way.
The Golf R32 DSG
In 2003, Volkswagen introduced a dual-clutch 6-speed gearbox, as an option for the Golf R32 road-car and later the Audi TT. The DSG (Direkt-Schalt-Getriebe or Direct Shift Gearbox), was the first dual-clutch transmission to be sold in a series production road car.
Manufactured by Borg Warner, DSG moved the game on substantially and offered lightning-fast changes however, unlike Porsche's later PDK, it was designed for transverse FWD installation, using a unified lubrication system and the software lacked a number of key features that would make the later unit more suited to a high-performance Sportscar. At launch, DSG was also only capable of handling 350Nm maximum torque.
Audi followed soon after, with their longitudinally mounted S Tronic dual-clutch gearbox, with separate clutch & gear oil lubrication. Both DSG and S Tronic have remained in production ever since.
Porsche PDK by ZF Group - Launched for the Road (finally)
In 2005, bringing together all they had learned from the past and combining it with the power of modern electronics, Porsche decided the time was now right to press ahead with development of a PDK for the road and contracted development & manufacture of the unit, to their long-term partners at ZF.
ZF drafted some 300 engineers from throughout its group, to productionise PDK. In a process that took 3 years, the team created the 7DT (7 speed) transmission, with the bulk of its major components manufactured within ZF group of companies, including the dual clutch units, supplied by ZF Sachs.
This production version of the PDK gearbox was then launched in 2008, as an option for the second generation 997.
Design Principles of the 7DT Transmission
The original 7DT/PDK, followed the basic ethos of a dual-clutch gearbox: Two parallel, switchable, constant mesh gear-sets (featuring full synchromesh), providing 7 forward gears and one reverse, with power transferred by two wet clutches.
The operational elements being contained within two distinct areas*:
1) RED: The electro-hydraulic control elements, running in a specially developed transmission fluid, including the hydraulic pump, dual wet clutches and valve-body (similar to that used in a conventional 'automatic' gearbox), with solenoid engaged, hydraulic control of the dual-clutch units, as well as gear shift operations.
2) BLUE: Two constant-mesh geartrains, running in hypoid gear oil, with gear-shifting operated by electro-hydraulically controlled selector forks and with full synchromesh.
*NB: Later versions for other Porsche models, used a common oil system. Additional oil-cooling was also added to the original 7DT/PDK, to increase its longevity.
The gearbox was designed to be highly efficient mechanically, with fluid levels kept low enough to reduce churning losses (power lost through churning oil) but sufficient for both high performance and long fluid drain intervals - 56,000 miles (6yrs) for the hydraulic oil and 112,000 miles (12yrs) for the gear wheel oil, although some owners (including myself), may choose to change the fluids earlier.
NB: The officially recommended drain interval for the hydraulic oil was later extended, after MY2014 to 12 years. Thanks deMort for that info.
It's important to note, that whilst the hydraulically controlled implementation of the gear changes, are undertaken by the transmission, the entire operational control of the gearbox and the gear-shifting strategy, are all managed from outside of the 'box, by the electronic Transmission Control Unit (TCU).
The engineering integration of these systems, combined all that Porsche had learned from its previous Sportomatic and Tiptronic programs and enabled PDK to finally deliver the full benefits of the dual-clutch design, with seamless full-power shifts, undertaken in around 400-500 milliseconds and a true sportscar-focussed driving experience.
Careful design and material specification for the clutch units by ZF Sachs, provides for a wide range of operating parameters, whilst ensuring efficiency and longevity. In cars optioned with the Sport Chrono pack, selection of 'Launch Control' implements additional transmission strategies: A control valve in the hydraulic control circuit, increases the flow of fluid from the hydraulic pump, in order to supply additional cooling to the clutch units, to reduce the effects of frictional heat. Additionally, the Launch Control option is disabled for a distance of 1.5 miles after use, to allow the clutch packs to cool down.
Due to its increased performance envelope, an upgraded clutch specification is used for PDK, when fitted to the Turbo/Turbo S models.
The Following video from ZF, provides a good overview of the construction of the 7DT/PDK gearbox – in this instance a 911 Carrera 4/S model (with front power take-off).
Driver Options and Adaptations
The 7DT/PDK transmission provides many advantages but for a sportscar driver, chief amongst these are its controllability and adaptability. This was one of the key breakthrough areas, where Porsche & ZF engineers were able to leverage the computational power of modern electronics and combine it with lessons learned from previous research programs, as well as live data, gathered from sensors within the vehicle.
At the most basic level, the TCU offers the driver a choice of either automatic or manual control, the driver can then choose from Normal, Sport or *Sport Plus settings, providing a choice of gear-shift strategies (or maps), that are oriented towards comfort, performance or racetrack. In Normal mode for instance, the comfort of the upshift is optimised, by interrupting engine torque (via a gap in ignition timing), during the shift operation, whereas the interruption is lessened when Sport mode is engaged and removed completely during Sport Plus – which optimises performance & shift speed, at the expense of comfort.
*Cars optioned with Sport Chrono.
Beyond its basic shift maps, the TCU also monitors inputs from the vehicles other control systems to effectively read the road and the driver's mind and deliver suitable gear-shift strategies – these affect not only the engine speed at which gears are changed, or how the gear-shift is delivered (soft/hard) but most crucially, the major benefit of a dual-clutch system – which gear the TCU 'predicts' the driver will need next.
Live vehicle data, including engine revs, throttle position, throttle change speed, gear-selector position, vehicle speed, steering angle, lateral & axial acceleration, engine and transmission temperature and altitude, provide for a continuous flow of information to the TCU.
Learning and adapting to its driver, driving style and route, gears can be pre-selected, or the strategy changed at a moment's notice, in reaction to evolving circumstances, so as to deliver the most responsive, efficient and safe means of progress.
For instance, should the driver decide to overtake and adopt a more dynamic driving style, then even without the transmission set to 'Sport', the TCU will utilise the 'Sport' map for a brief period, in order to make the vehicle more responsive.
Unlike some other iterations of the dual-clutch gearbox, PDK is also capable of working beyond a sequential change pattern (1,2,3,4 etc) and can 'skip' gears up or down – saving time and delivering the right gear as quickly as possible.
Driver adaptations occur, in both automatic and manual operation, of the Normal and Sport maps, but not in *Sport Plus.
Rapid Warm-Up
In a bid to aid efficiency and reduce emissions, the 7DT/PDK, uses a warm-up map when the vehicle is first started. Should the coolant temperature be lower than 20 Deg C, then the TCU implements higher shift points, which enables the catalytic converters to reach their optimal temperature as soon as possible – this strategy also ensures that the engine and transmission reach operating temperature more quickly.
Cornering – Gear Retention
Enthusiastic drivers know it's a bad idea to change gear mid-corner, as it upsets the delicate balance of the vehicle, therefore in a PDK equipped car the TCU is programmed to prevent loss of traction during cornering. In response to input from the vehicles lateral acceleration sensor (which is integrated with its stability management systems), the TCU avoids changing up/down a gear when lateral g force exceeds a set value – helping to maintain stability and aid driver control.
'Virtual' Gears
The PDK gearbox in the 991 Turbo and 991.2 were further improved in a number of key areas, including the ability to provide what Porsche refer to as 'Virtual' gears. If the vehicle is travelling along in a steady state, on a light throttle, in one of the lower 3 gears (which have the widest spacing of gear ratios) and the TCU determines that none of the current ratios offers the best efficiency, it can effectively run two gears at once.
By engaging both clutches lightly and slipping them, it's possible to provide an intermediate ratio, that better suits the state of the vehicle and thereby maximises efficiency. As soon as the conditions change, such as the driver accelerates or brakes, the gearbox reverts to the most appropriate single ratio. Clever stuff.
Fuel Efficiency
By using an overdrive ratio for 7th gear, PDK helps to deliver exceptional fuel economy. This is added to by the gearbox's 'Sailing' Function – where, when the vehicle is being driven under light load, at a steady state and under part throttle, the TCU can open the clutch units, to disconnect the engine from the gearbox and drop the engine's revs to tickover.
All of these clever features and many more, make PDK one of the most highly developed, effective and efficient alternatives to a manual gearbox and a worthy testament, to the many engineers and years of development that went into its creation.
Repair and Re-Use
PDK repair – oh what a thorny issue that is? Some think PDK can't be repaired, that it's too complex but that's just an urban myth – other DCT gearboxes are able to be repaired, including Volkswagen's DSG/Audi's S Tronic. The reality is, that for some reason (un-known and un-fathomable to me), Porsche don't seem to want to repair your PDK when it goes wrong (or allow anyone else to do it for you) and that seems at odds with the very principles the company have stood by for decades i.e., to maintain vehicles, repair them and keep them on the road – after all, it's why over 70% of all 911s are still on the road today.
Generally, it looks like PDK's manual gear-train is pretty robust and the issues that affect the 'box are mostly to do with its electrohydraulic system (shift position sensors or speed sensors esp) or the TCU. In some cases, parts that are fairly easy to access physically and cost next to nothing (in comparison to a new gearbox) could be replaced but Porsche, or maybe ZF, don't want to supply the individual parts.
We live in a world where repairing and re-using items had gone out of fashion but now we realise, such rampant consumption of new resources doesn't make any sense and those old attitudes are no-longer socially acceptable – so, despite being a life-long fan of Porsche, I have to ask, why condemn an otherwise healthy transmission, or require its costly replacement, when it could be repaired? Come on Porsche, do the decent thing – you know it makes sense!
In the Meantime
This very informative 2019 post on Rennlist, by thread starter PV997 (an Aerospace Engineer), outlines some of the common PDK faults. Whilst it focuses mainly on the first iteration of PDK in the 997.2, the principles are the same for the 991.1 and it goes to show just how easily some of the problems can be rectified, with access to the suitable parts:
https://rennlist.com/forums/997-for...airing-a-pdk-transmission.html?ispreloading=1
For comparison of the disassembly of other DCT transmissions, this tear-down of a VAG DSG gearbox by a transmission expert at Getriebe Doc ( https://www.getriebe-doc.de ), shows how dual-clutch transmissions, aren't anymore complex than an epicyclic auto and in many ways, far simpler:
For all those keen to know the inner workings of their 911, I hope that's been enlightening and an enjoyable read.
Til next time, safe travels.
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