The age of the Porsche 917 came to a close with a world speed record for closed circuits. In the Porsche 917/30, Mark Donohue completed a lap at Talladega with an average speed of 355.848 kilometres per hour. Porsche withdrew the Porsche 917/30 from the North American CanAm series at the end of 1973. After Mark Donohue won six of eight races on his way to a dominant victory in the drivers’ championship, subsequent changes to the rules prevented any further entries. Head of sports and PR Manfred Jantke recounts what happened next: ‘Porsche would have left it at that, but our US partner Roger Penske still had plans for the 917. He wanted to see it go out with a bang by breaking the world speed record.’ The previous mark had been set by A. J. Foyt, who reached 350.53 kilometres per hour in a Coyote-Ford
The Penske-owned 917/30 featured a 5.4-litre engine. When Mark Donohue took the car for a series of test drives on the oval at Daytona Beach in January 1975, however, it became apparent that this engine was not stable enough for flat-out driving. Following discussions with the team of Porsche technicians around engineer Helmut Flegl, the decision was taken to fall back on the more reliable 5.0-litre engine for the planned record attempt at the high-speed Talladega oval in August. Porsche performed test bench inspections with the aim of seeing how the engine performed for two minutes at full throttle and the race department in Weissach agreed with Donohue that the company would pay for any material damage that might occur during the attempt.
The body of the car also underwent an overhaul, as Donohue explained: ‘The 917/30 was designed for a huge amount of downforce. As speeds of over 320 kilometres per hour are a rare occurrence on street circuits, less emphasis had been placed on air resistance. We modified the aerodynamics in order to achieve a better top speed.’ In addition, the springs were replaced and the chassis was altered to ensure that the 917/30 pulled to the left when driving on the straights. This latter adjustment made the car more stable on the corners.
On 9 August 1975, Donohue completed a lap of the oval in 43.3 seconds. His average speed of 221.120 miles per hour, or 355.848 kilometres per hour, was a new world record. The American driver reached a top speed of 382 kilometres per hour on the straights, with the data showing that the engine delivered a total of 1,230 horsepower.
