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 ageing 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