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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. Origins of the 917 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. Well aware that few manufacturers were ready to take up the challenge immediately, the CSI also allowed the participation of five-litre Group 4 sports cars, of which a minimum of 50 units had to be manufactured. This targeted existing cars like the aging Ford GT40 Mk.I and the newer Lola T70 coupé. In April 1968, facing few entrants in races, the CSI announced that the minimum production figure to compete in the sport category of the International Championship of Makes (later the World Sportscar Championship) was reduced from 50 to 25, starting in 1969 through the planned end of the rules in 1971. With Ferrari absent in 1968, mainly Porsche 908s and Ford P68s were entered there, with the Ford being a total failure. As a result, old 2.2-litre Porsche 907s often won that category, with John Wyer's 4.7-litre Ford GT40 Mk.I taking wins at faster tracks. Starting in July 1968, Porsche made a surprising and expensive effort to take advantage of this rule. As they were rebuilding race cars with new chassis every race or two anyway, selling the used cars to customers, they decided to conceive, design and build 25 versions of a whole new car with 4.5-litre for the sport category with one underlying goal: to win its first overall victory in the 24 Hours of Le Mans on June 14, 1970. In only ten months the Porsche 917 was developed, based on the Porsche 908. 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. On March 12, 1969, a 917 was displayed at the Geneva Motor Show, painted white with a green nose and a black No. 917. Brief literature on the car detailed a cash price of DM 140,000, approximately £16,000 at period exchange rates, or the price of about ten Porsche 911s. This price did not cover the costs of development. On April 20 Porsche's head of motorsports Ferdinand Piëch displayed 25 917s parked in front of the Porsche factory to the CSI inspectors. Piëch even offered the opportunity to drive any of the cars, which was declined. Construction 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. Power came from a new 4.5-litre air-cooled engine designed by Mezger. A completely new flat-12 block was designed without the cylinder offset needed by a boxer design to keep the engine compact. The 'Type 912' engine featured a 180° flat-12 cylinder layout with six crank throws (each conrod journal shared by opposing piston pair) as opposed to twelve on a boxer, twin overhead camshafts gear-driven from centrally mounted gears and twin spark plugs fed from two distributors. The large horizontally mounted cooling fan was also driven from centrally mounted gears. The longitudinally mounted type 920 gearbox was designed to take a set of four or five gears. To keep the car compact despite the large engine, the driving position was so far forward that the feet of the driver were beyond the front wheel axle. The car had remarkable technology. It used many components made of titanium, magnesium and exotic alloys that had been developed for lightweight "Bergspyder" hill climb racers. Other methods of weight reduction were rather simple, such as making the gear shift knob out of birch wood, some methods were not simple, such as using the tubular frame itself as oil piping to the front oil cooler (a design used successfully by Lotus beginning with Lotus 22 of 1962). 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". 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. 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 ca.... Discover the Roger Wyer popular books. Find the top 100 most popular Roger Wyer books.