For rally use, the transmission of the rally cars was rather rougher than the refined setup used in the Quattro road cars – indeed it was rather closer to the first A1 prototype. Much of the drivetrain would be more familiar to a military service engineer who worked on litis jeeps than to a suburban Audi dealership’s workshop.
The front and rear differentials were connected directly to one another to give only 50/50 power distribution, which reduced the number of parts exposed to potential failure significantly. The front end had no limited slip differential, and the rear end only had a 75% slip, resulting in fairly agricultural behaviour at low speeds, just like the very first time the system was tried out in the little red Audi 80.
The team could pick from three homologated differential ratios in the car’s first iteration – 4.87, 4.55 and 4.11, mated to ratios of 3.00 for first gear, 2.00 for second, 1.50 for third, 1.217 for fourth and 1.040 for fifth. This gave the team sufficient flexibility to be competitive on all stages and surfaces (with the notable exception of Corsican asphalt), and was used through the 1981 season.
For 1982, the team did insert a central differential with 100% slip to refine the Quattro’s handling. This would remain fundamentally the same system operated throughout the long-wheelbase Group B era with the Quattro A1 and A2, albeit with updated differential ratios of 4.625, 4.571 or 4.375 to choose from.
The other main addition of note came in late 1982 with an electro-hydraulic lever on the gearstick to engage and release the clutch in order to allow the driver’s left foot to remain fully occupied with braking. FISA promptly banned it, but later relented. Sometimes.
The introduction of the higher-revving and more powerful 20-valve engine forced some change upon the transmission. Because of the curious problem of facing weight gains by making a smaller car it was harder to stay within the optimum rev band for peak torque, so a new 6-speed gearbox was required to help drivers to exploit the engine’s full potential.
This unit was designed and built in a hurry in late 1984 and made its competitive debut on the 1985 Portugal Rally. It was discovered that the unit was running at alarmingly high temperatures so some vents were cut into the casing to try and prevent failures.
The next step to try and harmonise the car’s mass, engine’s power and available traction came with the addition of a Ferguson viscous coupling on the centre differential to be able to adjust the front/rear distribution. This could be locked on or off, but Audi has always kept quiet on how much power was distributed where using this system.
The 6-speed with the Ferguson differential was used to greatest effect by Walter Rohrl when he took Audi’s last and hardest-won world championship victory, the 1985 Sanremo Rallye. This setup would remain the primary transmission layout used for the remainder of the car’s competition history – although it had one last groundbreaking card up its sleeve.
In the 1960s, Ferdinand Piech was sifting through ideas that his uncle and grandfather had come up with, looking for inspiration, when he unearthed the doppelkupplungsgetriebe –that had never been brought to fruition due to technological limitations of the time. The system effectively created two separate transmissions for the odd-numbered gears and the even-numbered ones. The result would be seamless gear changes, as the switch from one gearset to the other would be instantaneous.
Piech wanted to put this arrangement into the Porsche 917 but it was still too prone to failure because it was impossible to harmonise the components hydraulically. When electronic engine management systems appeared, however, there was clear potential to adapt them for managing the intricacies of such an advanced transmission.
The Porsche doppelkupplungsgetriebe, or PDK for short, was never far from the top of Piech’s wish list. Its development was an integral part of Porsche’s works team in the World Sportscar Championship, and Piech encouraged greater collaboration between Audi, Bosch and Porsche to perfect the system.
When it was operating properly, PDK would allow the driver to pre-select the gear that he wanted and then concentrate on balancing the car with the throttle and/or brakes without worrying about shifting up or down. For Porsche’s sports car team, the single-minded determination to develop PDK was often a frustration because its additional weight sapped performance and its unreliability cost them victories and championship points. In 1984-85, however, Audi Sport had a weapons-grade test driver who was thrilled at the chance to spend time developing the system rather than flogging from one rally to the next: Walter Rohrl.
Inside the cockpit, the driver had a simple lever to move backwards and forwards to selecl the desired gear that they wanted to reach. An LCD display on the instrument panel told them which gear they were currently in and which they had selected. Hammering down a flat-out section towards a hairpin would therefore mean that the screen read 511, and as soon as the driver got on the brakes the downshift would begin, with the ECUs of the engine and transmission operating in unison.
A traditional clutch pedal was used to engage first gear and launching the car from standstill, after which electro-hydraulic actuators handled the entire process as the driver commanded until the car was once again stationary. In time, the up/down lever used to select the desired gear was superseded by buttons on the steering wheel like those in modern semi-automatic racing transmissions.
The PDK was given its competition debut by Porsche at the end of the 1983 World Sportscar Championship season. Throughout
1984 it would run one car with a conventional gearbox and one with the PDK – often resulting in mechanical failure. It pressed on regardless in
1985 and endurance racing fans were amazed at the progress being made – although they were blissfully unaware that by now Audi was also sharing the load.
Walter Rohrl gave the PDK its rally debut in an Audi Sport quattro S1 on the 1985 Semperit Rally, a national level event which he won by 19 minutes. A Torsen Gleason centre differential was used instead of the Ferguson viscous coupling and video footage from the stages attests to the barely discernible blips in the engine note as the actuators did their thing.
Its performance was shattering and Rohrl’s co-driver Christian Geistdorfer later claimed that he had struggled to maintain his rhythm on the notes without the familiar tempo of shifting going on beside him. Rohrl himself enjoys regaling the story of how Audi Sport engineer Dieter Basche, a regular alongside him during their many test sessions, lasted one trip in the PDK car before climbing out and swearing never to do such a thing again. In the end, the PDK transmission only had one world championship outing, in Rohrl’s hands, on the 1985 RAC Rally.
The German detested the event as it was run ‘blind’ with no prior recce of the stages permitted, handing an advantage to drivers with local knowledge. Yet Audi insisted that he should give the PDK its debut outing and a local co-driver was found for him in the form of Phil Short.
Both Rohrl and teammate Hannu Mikkola (in a standard 6-speed manual S1) were delayed by mechanical issues and were running in eighth and ninth positions on the first night. When pushing to regain some time, with the PDK quattro now flying, Rohrl pushed too hard and the car tumbled a long way off the road.
It was always envisioned that PDK would become a standard transmission for Porsche road cars but then the idea was again mothballed. Not until 2003 would the transmission be made available to the public… in the Volkswagen Golf R32 and Audi TT, both of which were at the time under a Volkswagen group that was overseen by… Ferdinand Piech.
No road-going Porsche went on sale with the doppelkupplungsgetriebe until 2009, after the company had been bought by… Volkswagen. Rightly or wrongly, one senses that there would have been considerable satisfaction among the ‘non-nameholders’ of the Porsche family from this turn of events.