The Birth of the 917

The Porsche 917 was born from regulations aimed to make sure such a car was never built. In 1967, alarmed by the expense of prototypes like Ferrari's P4, the CSI decreed that from 1968 all prototypes would be limited to 3 litres. It was a rule you could only duck if you produced 50 examples, something no-one building three or four highly specialised racers could countenance.

Fortunately for Porsche and marques such as Lola who stood no chance of manufacturing 50 of T70s, the CSI cut the requirement to 25 examples.This would allow cars like the Lola to race while restricting the big boys to a 3-litre limit.

At Porsche, Ferdinand Piech thought differently and presented a 917 plus 24 sets of parts for inspection.When this was rejected, he turned the parts into cars, ready for inspection in April 1969.

It was Piech's masterpiece, a car created from an almost uniquely harmonious marriage of technical innovation and pragmatism.The result was a super-light racing car, with huge power and, ultimately, almost impregnable reliability.

The lightness came not simply through the use of trick materials such as titanium and magnesium (both feature extensively) but by pushing more mundane materials to unprecedented levels.The outer skin of the glass-fibre bodywork, for instance, is just 1.2mm thick and complete with the windscreen, perspex, seats, wings and even the wiring loom, weighs just 95kgs.

The flat-12 engine, far from being a leap into the unknown, was based around technology Porsche had known for years. Each cylinder was fed by just two valves and the reason Porsche did not bother at first to use the full 5-litre allowance was simply that the engine was, at its heart, that of a 3-litre 908 with another four cylinders grafted on. A wide array of components were common to both, the engines differing most in the method of power take up which was taken from the end of the 908 and the middle of the 917 unit.This meant the 12-cylinder car could run with, in essence, two short and stiff crankshafts thereby allowing the engine to rev higher and produce more power.

According to Porsche figures, a 1969,4.5-litre 917 engine produced over 580bhp, rising to more than 620bhp for the 1971 5-litre motor for 1971. Kerbweights rarely rose much above 800kgs.

Take an intermediate 4.9-litre 917 from 1970 possessing a power-to-weight ratio of around 750bhp per tonne and compare it to the only car to offer opposition, the Ferrari 512S. That weighed 880kgs and produced just 550bhp to give a power-to-weight ratio of just 625bhp per tonne. Looked at in such terms, the dominance of the 917 was not so much surprising as inevitable.