Words by: Glen Smale
Images by: Corporate Archives Porsche AG
The Porsche 956, and its later sibling the 962, became the most successful sports prototype in racing history during its domination of Group C between 1982 and 1992. Once the Porsche factory released the race car to private teams from 1983, it became the weapon of choice. Winning around the globe, this phenomenal racing machine racked up no less than 232 major international victories during its reign.
The full development and racing history, plus race results in the World Endurance Championship, DRM, IMSA, Interserie, including some non-championship races, can be found in Glen Smale’s 500-page book: Porsche 956/962.
Buy It Here: Porsche 956/962: A Photographic History by Glen Smale
Under the watchful eye of Norbert Singer, the new Group C racer, called the Type 956, began to take shape. The step up from the 936 to the new 956, besides the obvious differences in upper body form, lay also in the under car developments and chassis construction, which was no longer a spaceframe, as in the 936, but an aluminium chassis fitted to a monocoque. With this new breed of racer, Porsche entered the realms of a super sports racing car with a full ground effects body with chassis design carried out by Horst Reitter.
It had become clear that a tube frame chassis would not meet the then current safety standards specified for the new Group C racers, and this prompted Porsche to explore the fabrication of a monocoque chassis. Porsche racing driver and engineer, Jürgen Barth, explains, “It was clear that a tube frame chassis was not as strong as a monocoque in a crash, but it was the first time that the factory had made a monocoque, and so it was quite a challenge for Norbert Singer, but it worked out quite well I think.”
Every new racing car must have a starting point, and for the 956 the windscreen from the Porsche 917 was that starting point, at least as far as the initial discussion of the Group C rules was concerned. “To find the measurement of the car was quite interesting. I remember going into our museum in Zuffenhausen and measuring the inside of the windscreen of the 917, which was 1metre 20centimetres,” Barth recalled. “But actually the windscreen was completely different on the Group C cars because we had to have a maximum width of 95cm, so it was much smaller,” Norbert Singer pointed out.
Ground effect, already seen in action in Formula One, was to enter the world of sports car racing, but there was little common ground between these two very different racing classes as far as under-body airflow was concerned, as Norbert Singer was to find out.
An official proposal was drawn up at the beginning of 1981, according to Singer, that roughly laid out the dimensions of the car, and which looked into the feasibility of constructing a car based on a fuel consumption ruling. “For a manufacturer like Porsche, this was the most challenging thing because this was not just to make a race car and to go racing, but it was also a technical challenge,” Singer explained.
Although the world of Formula One was awash with ground effect know-how in 1981, fabricating a sports car with this new found technology was not a straightforward task. Working with a one-fifthscale model made in the composites shop at Weissach, Singer began his research on the aerodynamics of the new 956 at the wind tunnel at the Stuttgart University. “This was a very old facility, but it was the only model wind tunnel close to Porsche, so we didn’t have much choice. The results were pretty good, though,despite the age of the wind tunnel,” Singer laughed.
Norbert Singer had already tried a ‘kind of ground effect’ on the 935 in 1979, but work on this project had not progressed beyond the study phase. As a result Porsche had to start from scratch in mid-81 using proven Formula One ground effect technology on the 956, but after just two to three days Singer had to abandon this approach as it was clear that the system that worked on open-wheel racing cars was not suitable for sports cars. Whereas in open-wheeled Formula One cars, the air for the ground effect was drawn in under the car from the front, the body shape of the sports car dictated that the air had to be pulled in under the car between the front and rear wheels, and not the front of the car. Whenever skirts were added to the one-fifthscale model of 956, the ground effect was negated, and in this way Group C would write its own aerodynamic requirements.
Given that the body length was set at 4800mm, Singer and his team had to come up with the ideal wheelbase taking into account the front and rear overhangs, which were in turn a factor of the wheelbase dimension. “We were clear it should have been bigger than the 917, and so we said okay lets try it a little longer and, together with the overhang regulations, we agreed it would be 2650mm,” Singer recalled. This was the longest wheelbase of any Porsche racer to date, and could be compared with the 917 which featured a 2300mm wheelbase and the 917/30, which until then had been the longest at 2500mm.
The design concept then evolved from this basic idea, taking into account that the car needed to be a mid-engined layout with the gearbox behind the engine, while the fuel tank should be located between the engine and the cockpit, unlike the 917. Singer again, “In the automobile industry we worked with 80%-90% [size] people, and this measurement of people is what we put into the car, so the smaller drivers had no problem, but the taller drivers did.”
Once the driver size was agreed, the designers could position the seat and the pedals, but controversially the engineers wanted the driver to sit as far forward as possible which placed the driver’s feet 12cm ahead of the centre of the front axle. Norbert Singer explains the rationale behind this design parameter, “We wanted to move the driver as far forward as possible so the feet were at least 12cm ahead of the centre line of the front axle. This was then changed later with the 962.”
As described above, the 956 was designed for the ‘average’ driver, but this posed difficulties for taller drivers. One in particular who found this to be a problem was none other than the car’s first test driver, Jürgen Barth. “This was fine for drivers like Jacky Ickx who is about 1.68m, but I am 1.83 m and Stuck was even taller. Driving the first car (chassis No. 001) for first time in Weissach, our knees were hitting the dashboard as the car had such hard suspension, and at a certain stage this became a big problem for the drivers, because the drivers could not keep their foot on the pedal because it was always moving owing to the vibrations.”
Barth and his colleagues had known from the beginning that the seat position would pose some problems for taller drivers, and so from the next chassis, No. 002, they changed the dashboard slightly, but this was still not enough. Barth continues, “For guys like Stuck we had to open the seat in the bottom so that in the end he was sitting on the floor.”
Broadly, the coupé body style of the 956 followed that of the 917, with a large rear wing similar to the 917 LH, with the total length of the two cars being very close. The amount of downforce created by the rear wing and the ground effect under-body of the 956 required a much stiffer chassis and body. As a result, the monocoque, which was fabricated from aluminium sheet with each panel being bonded and riveted together – not only was the monocoque construction new to Porsche, but the bonding process was also a first for them – providing a much stiffer core. The body panels, too, were load bearing and formed from composite materials, which made the whole body and chassis an extremely stiff structure.
In an effort to aid the passage of air through the under-body channels, the angle of the engine and gearbox was inclined towards the rear, a move that allowed the venturis that exited to the rear to be larger. The rear suspension components were mounted higher and the exhausts were routed sideways, exiting the bodywork just ahead of the rear wheels, thus reducing the amount of ‘kit’ blocking up the air channels under the car. Large water and oil radiators were located in the sidepods just aft of the doors, being fed via deep channelling in the bodywork. Close attention was also given to ensuring that the rest of the car was sufficiently rigid to cope with the additional stresses placed on the body and chassis by the increased downforce and lateral forces created by the higher cornering speeds.
Porsche race mechanic, Klaus Bischof, outlines a key step for the construction of the 956, “Firstly, Professor Bott called for a completely new team to be created for this project. This was headed up byPeter Falk, and for the engine department we had at first it was Mr Weber and later on Mr Schaeffer, Mr Singer and myself. The engine design department was under Hans Mezger, and Eugen Kolb was in charge of bodywork while Horst Reitter and his teamwere responsible for all suspension and chassis engineering.”
Importantly, no parts or construction methodology was carried over from the 917 era and the first thing that the team built was a wooden box that showed the dimensions of the 956’s monocoque. This was the first time the factory had stepped away from its long heritage in tube frame construction, which had started with the 550 Spyder back in 1956 – so monocoque race car construction marked a new way forward.
The move into a purpose-built facility at Weissach heralded a new start for a new racing department, but initially this relocation presented its own problems, as trying to build a sophisticated new race car and move an entire department wasn’t without its challenges. Jürgen Barth explains, “It was during the end of 1981 and the beginning of 1982. We took everything from Zuffenhausen over to Weissach, more than 15,000 pieces with just my secretary and two other guys working with me. We worked each evening and weekends, it was quite nice. They built a new storage place for us because we had been sitting in what we called the Old Barracks in Zuffenhausen, where they designed the tanks in the past.”
Previously, it had been quite common practice for Porsche to use outside contractors to make the chassis (KWW made the 908 chassis and Bauer the 917 chassis) or body panels (Heinkel made the 904 fibreglass body panels). However, as Klaus Bischof explains, “[With the 956] the whole of Weissach was involved, the mechanical department, the fibreglass or bodywork department, monocoque, and of course we had some outside companies making some housings, but it was brought all together in Weissach.”
Bischof continued, “A few customer cars later on were made at Thompson in England and the IMSA cars were made in America because we didn’t have the capacity to build 148 [race] cars. But all factory cars, and we had ten 956 and ten 962 factory cars, all of those monocoques were made in Weissach with aluminium. We had the engine department and workshop very close to the construction department, and so we made the monocoque and bodywork ourselves. Also, we had a wooden model department where the measurements for the forms of the very first body panels were taken from a full 1:1 scale wooden model. It was all made in-house, in Weissach.”
As had become Porsche’s custom, the race department made use of existing, proven engine technology, and in this case, the 956 utilised the shelved Indy Interscope engine, a six-cylinder four-valve water-cooled head unit, but that is where the ‘hand-me-downs’ ended. Up to and including 1980, the 936 had used a 2.1-litre turbocharged unit which, with the turbo factor (1.4) applied, gave a displacement of 2988cc.But a change in the rules for Le Mans in 1981 allowed the 2.65-litre Indy engine to be fitted to the 936/81. Singer explains, “We changed this [Indy] engine from methanol to a fuel engine which meant it could run at Le Mans in ’81 because Le Mans had changed the regulation. Until then you had to use a 2.1-litre engine giving a 3.0-litre engine maximum capacity [with turbo multiplication factor]. We knew that the 2.65-litre engine was a more reliable engine because it was made for high speed, and in testing it was much better.”
A major factor in the Group C regulations was the introduction of a fuel consumption rule which stipulated that the fuel tank was restricted to 100 litres with a limit of a total usage of 2500 litres, and this meant only 25 refuellings were permitted during the race which included the initial full tank at the start of the race. In order to maximise the team’s chances of lasting the distance, an adjustable turbo boost adjustor was fitted inside the cockpit giving the driver a degree of control over the car’s fuel consumption while still delivering a useful 620 horsepower in race trim. Smaller twin turbos were fitted to the 956 to improve fuel consumption.
