The Porsche 917 is a sports prototype race car developed by German manufacturer Porsche to exploit the regulations regarding the construction of 5-litre sports cars. Powered by a Type 912 flat-12 engine which was progressively enlarged from 4.5 to 5.0 litres, the 917 was introduced in 1969 and initially proved unwieldy on the race track but continuous development improved the handling and it went on to dominate sports-car racing in 1970 and 1971. In 1970 it gave Porsche its first overall win at the 24 Hours of Le Mans, a feat it would repeat in 1971. It would be chiefly responsible for Porsche winning the International Championship for Makes in 1970 and 1971. Porsche went on to develop the 917 for Can-Am racing, culminating in the twin-turbocharged 917/30 which was even more dominant in the role. Porsche drivers would win the Can-Am championship in 1972 and 1973. 917 drivers also won the Interserie championship every year from 1969 to 1975.
In an effort to reduce the speeds at Le Mans and other fast circuits of the unlimited capacity Group 6 prototypes (such as the seven-litre Ford GT40 Mk.IV and four-litre V12 Ferrari P) the Commission Sportive Internationale (then the independent competition arm of the FIA) announced that the International Championship of Makes would be run for three-litre Group 6 prototypes for four years from 1968 through 1971. This capacity reduction would also serve to entice manufacturers who were already building three-litre Formula One engines to adapt them for endurance racing.
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When Porsche was first visited by the CSI inspectors only three cars were completed, while 18 were being assembled and seven additional sets of parts were present. Porsche argued that if they assembled the cars they would then have to take them apart again to prepare the cars for racing. The inspectors refused the homologation and asked to see 25 assembled and working cars.
The car's chassis was designed by Helmuth Bott and the engine was designed by Hans Mezger, both under the leadership of Ferdinand Piëch. The car was built around a very light spaceframe chassis (42 kg (93 lb)) which was permanently pressurised with gas to detect cracks in the welded structure.[4] Power came from a new 4.5-litre air-cooled engine designed by Mezger, which was a combination of two of Porsche's 2.25L flat-6 engines used in previous racing cars. The 'Type 912' engine featured a 180 flat-12 cylinder layout, twin overhead camshafts driven from centrally mounted gears and twin spark plugs fed from two distributors.[5] The large horizontally mounted cooling fan was also driven from centrally mounted gears. The longitudinally mounted gearbox was designed to take a set of four or five gears.
There are at least eleven variants of the 917. The original version had a removable long tail/medium tail with active rear wing flaps, but had considerable handling problems at high speed because of significant rear lift. The handling problems were investigated at a joint test at the Österreichring by the factory engineers and their new race team partners JW Automotive. After exhaustive experimentation by both groups, a shorter, more upswept tail was found to give the car more aerodynamic stability at speed. The changes were quickly adopted into the 917K for Kurzheck, or "short-tail".[6]
In 1971, a variant of the 917K appeared with a less upswept tail and vertical fins, and featured the concave rear deck that had proved so effective on the 1970 version of the 917L. The fins kept the clean downforce-inducing air on the top of the tail and allowed the angle of the deck to be reduced, reducing the drag in direct proportion. The result was a more attractive looking car that maintained down force for less drag and higher top speed.[7]
By this time the original 4.5-litre engine, which had produced around 520 bhp in 1969, had been enlarged through 4.9-litres (600 bhp) to 5-litres and produced a maximum of 630 bhp. The 917K models were generally used for the shorter road courses such as Sebring, Brands Hatch, Monza and Spa-Francorchamps. The big prize for Porsche however, was Le Mans. For the French circuit's long, high speed straights, the factory developed special long tail bodywork that was designed for minimum drag and thus highest maximum speed. On the car's debut in 1969, the 917L proved to be nearly uncontrollable as there was so little down force. In fact, they generated aerodynamic lift at the highest speeds. For 1970, an improved version was raced by the factory and for 1971, after very significant development in the wind tunnel, the definitive 917L was raced by both factory and JW.
The original Porsche 917 was first run at the Le Mans Test in March 1969 and right from the start showed considerable handling problems due to aerodynamic lift. The original specification of the car included a detachable long-tail (Langheck), that was designed using experience from the previous 907 long-tail coupes for minimum aerodynamic drag, with suspension controlled moving flaps on the tail. At Le Mans the CSI baulked at this, moving aerodynamic aids having been banned in motorsport. It was only when Rolf Stommelen demonstrated how undriveable the car was without the moving flaps that they relented and allowed them for Le Mans only. Throughout 1969 the car's speed was countered by the handling problems and it won only one race, the Zeltweg 1000km. Following that event, JW Automotive, who would be acting as a semi-works team in 1970, requested a test session with Porsche to try and sort out the car's problems.
The 917PA was an open-topped and short-tailed version of the original 917 and was built to compete in Can-Am racing. Only two cars were built by Porsche. The first never raced, and later became a test mule for an experimental flat-16 engine. The second car was entered in the 1969 Can-Am season by Porsche Audi, the North American distributors for Porsche (hence the PA designation), and driven by Jo Siffert. Compared to the dominant McLarens, the car was underpowered and overweight. Siffert's best result was 3rd at Bridgehampton, and he finished 4th in the championship. For 1971 the car was obtained by Vasek Polak who ran it for the next three years with the car gaining ever more aerodynamic aids, until by 1973 it resembled the 917/10 variant.
The 917K was an evolution of the original 1969 car. After the first 917s were run in 1969, it was clear the car's aerodynamics made it nearly undriveable at higher speeds. After the 1969 championship season had finished, John Wyer requested a 3-day test session at the Austrian Österreichring course. The Porsche technical team turned out ready to do some serious panel work on the coupe and in order to make a comparison, brought along the Can-Am 917PA Spyder. The drivers present instantly preferred the PA and together, the JW Automotive and Porsche engineers came up with the idea of a more upswept tail (as on the 917PA). The JW team had had similar high speed handling problems with the early Ford GT40 models. With gaffer tape and aluminium sheet, a completely new short tail was evolved at the racetrack. This was quickly converted into a 'production' design back at Porsche and the 917K (Kurzheck) made its public debut at the season opening 1970 24 Hours of Daytona. Such was the improvement in the stability of the car at high speed, the 917K became the standard configuration for all races except Le Mans, the Nürburgring 1000km and the Targa Florio. This car was raced at every event by JW Automotive and Porsche Salzburg in the 1970 season except the Targa Florio and the Nürburgring 1000 km. The smaller, more nimble and generally better suited 908/03s were used for those races, but privateers used the 917K at the Nürburgring 1000 km, and Vic Elford drove a lap of the 44-mile Targa Florio course in the 917K at Ferdinand Piëch's request. The 917K won 7 out of 10 races; all the races it competed in. Later on in the 1970 season, the 4.5 litre flat-12 was bored out to 4.9 litres, then 5 litres.
1970 917L: This long tail, low drag version of the 1969 917L was purpose-built for the 1970 24 Hours of Le Mans. Le Mans in 1970 was almost entirely made up of long straights and this version was designed to maximise the speed capability resulting from the increased power developed by the flat-12 engine over the previous Porsche types. The 1970 917L was largely based on the initial 1969 car. Nevertheless, factory driver Vic Elford had found the car's ultimate speed an advantage enough over its still questionable handling in the braking and cornering sections of Le Mans. It was 25 mph faster down the straights than the 917K and the Ferrari 512Ss.[10][better source needed] Two were entered in the 1970 Le Mans race, one by Porsche Salzburg and the other by Martini Racing. The Porsche Salzburg 917L was qualified in pole position by Elford, but retired with engine failure after 18 hours and the Martini 917L finished 2nd, five laps behind the winning Salzburg 917K of Hans Herrmann and Richard Attwood.
The 917/10 was built for Can-Am racing, with a 5 litre engine, new bodywork and weight pared to a minimum. It was run in the latter part of the 1971 Can-Am season by Jo Siffert, with moderately successful results.
In testing, it soon appeared that the Porsche 917 did not work well on the race track. Porsche factory driver Brian Redman recalled that "it was incredibly unstable, using all the road at speed." Many thought that the 4.5-litre engine was too much for the frame. The suspension and the stability of the frame were suspected, but modifications did not improve the problem. It was finally determined that the "long tail" body was generating significant lift on the straights, as the 917 was 30 km/h (19 mph) faster than anything previously built for Le Mans[citation needed]. As with former under-powered Porsches, the 917 aerodynamics had been optimized for low drag in order to do well on the fast straights of Le Mans, Spa, Monza and elsewhere. The significance of downforce for racing was not yet fully realised although Can-Am and F1 cars were using wings by that time. 2ff7e9595c
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