From UCP:
With two Le Mans wins in their bag and the 917 no longer eligible, Porsche decided to turn their attention to highly popular Can-Am racing series in 1972. Having to make up for a considerable displacement deficit, Porsche fielded heavily modified, Turbocharged 917 Spyders. These all conquering 917/30s are still the most powerful racing cars ever constructed and maybe more importantly taught Porsche valuable forced-induction lessons. Back in Europe sportscar racing was far more regulated with a displacement limit of the top 'Group 6' class of just three litres. After Porsche's European withdrawal, privateers continued to campaign the three litre 908/3s, but their flat-eight could not match the performance of the high revving Matra, Ferrari and Alfa Romeo 12 cylinder 'F1' engines. Interested to see how far they could push the Turbo envelope, the engineers at Weissach developed a new forced induction flat-6.
Following the FIA's equivalency formula of 1.4 for supercharged engines, Porsche's new engine could not displace over 2.143 litre to remain eligible for the three-litre class. With 2.142 litre Porsche came extremely close, which they initially feared would not be enough to match the 450+ bhp of the competition as the 917/30 engine produced around 200 bhp / litre. That engine was not initially designed to take the additional stresses of Turbocharging, so with a bespoke design the new flat six could be made strong enough to survive the 225 bhp/litre required. Another problem that the engineers faced was the extreme heat produced by the single Turbocharger, which greatly hampered the performance. Taking a cue from WWII airplane design, they fitted an intercooler that would cool the compressed air going into the engine. This was the first ever application of an intercooler in a (racing) car.
Fitted in the back of a 911 RSR body, the Turbocharged engine made its debut early in 1974. Using a road going chassis, this meant the Porsche Turbo could be homologated as a Group 5 car, which did not allow for one-off racers like Group 6, but also did not have the very stringent homologation requirements of the Group 4 GT class. The new car was raced throughout the season, but the many reliability niggles showed that the engine was still very much under development. Porsche's racing philosophy of using the track to develop new technology for road cars became apparent when the 911 Turbo (Type 930) debuted in 1975. This car would form the basis of a series of highly successful Group 4 (934) and Group 5 (935) racing cars that would be used to great effect by the Works and privateers well into the 1980s. It was by no means the end of the 2.1 litre engine though and a first sign of things to come was the delivery of the powerplant to 908 racers to replace the old three-litre flat-eight for the 1975 season.
Even though Porsche never officially supported the likes of Joest and Kremer with their six cylinder 908s, the Works engineers became ever more involved as the season progressed. Before the year was over, Porsche assisted not only with the engines, but also with the aerodynamics and some of these 908s were even tested at Weissach. Porsche's involvement was by no means a secret; it was nevertheless quite a big surprise when Porsche announced they would enter the 1976 World Championship with a brand new Group 6 car. Perhaps 'brand new' was not the best way to describe the 936 as it was a combination of the 2.1 litre Turbocharged engine, a 908/3 tubular spaceframe, 917 running gear and a scaled down version of the 917/30 spyder body. Of course there was a little more to the Group 6 racer and especially the chassis was modified extensively to cope with the power increase over the 908's flat eight.
Less than a half year after the first design was penned, the 936 hit the track early in April of 1976. The use of 'off the shelf' parts certainly contributed to the quick to development. In its latest guise, the six cylinder engine now produced 520 bhp, which looked sufficient to take on the latest Alfa Romeos and Renaults. Painted flat-black, the 936 made its competition debut in the Nürburgring 1000 km race, where a stuck throttle cable slowed the car down. Porsche's day was saved as Joest's Turbocharged 908/3 took the win. Throughout the season the 936 was developed and most notably grew a large central air scoop that fed air to the fan cooling the engine block; the heads were water cooled through a front mounted radiator. Although it was by no means a perfect race, Jacky Ickx and Gijs van Lennep piloted one of the two 936s to victory at Le Mans that year. The 936 would continue to dominate the season and won every one of the remaining six rounds of the World Sports Car Championship.
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