Porsche’s PDK trans­mis­sion is noth­ing new – it’s been with us since 1983

Classic Porsche - - Contents - Words: Keith Seume Pho­tos: Porsche Archiv and au­thorʼs col­lec­tion

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


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.


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


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

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