The Porsche 917: The Race Car That Was Too Fast to Drive

A loophole built a legend

In 1968, motorsport's governing body raised the bar for Group 5 sports car racing: to compete, a manufacturer now had to build 25 examples of the car. It was meant to keep the field to genuine production cars, not one-off prototypes. No one expected a manufacturer to actually hit that number for a pure racing car. Porsche did anyway. Working through the winter of 1968 into 1969, the factory in Zuffenhausen assembled 25 completed examples of an entirely new car in a matter of months, then lined them up outside the factory so FIA inspectors, and the world's press, could count them by hand. The car was the 917, and Porsche had just forced its way into the top class of endurance racing on a technicality.

Beautiful, brutally fast, and nearly undriveable

The 917 was built around a lightweight aluminium spaceframe chassis and an air-cooled flat-12 engine, a configuration Porsche had never raced before. In its earliest form it displaced 4.5 litres and produced around 520 horsepower, in a car that weighed roughly 800 kg. On paper, it was the fastest thing Porsche had ever built. On the track, it was terrifying. At speeds above 300 km/h down the Mulsanne Straight at Le Mans, the tail of the car would lift and go light, and the whole rear end would start to float. Works drivers openly said they didn't trust it. One test driver reportedly refused to take it back out after a single lap.

The fix came from a roll of tape

The breakthrough didn't come from a wind tunnel. It came from John Horsman, chief engineer at John Wyer's JW Automotive Engineering, the team Porsche brought in to run the 917 in 1970. After a test session, Horsman noticed something on a 917 that had come back from a high-speed run: dead flies were spattered across the front bodywork, but the rear deck was almost completely clean. That meant the air wasn't actually flowing over the back of the car the way everyone assumed. Airflow was separating and turning to turbulence right where the tail needed to generate stability. The team reshaped the rear bodywork into a shorter, higher-kicked "Kurzheck" (short-tail) design. It looked less elegant. It also transformed the car from unmanageable to dominant.

The win, and the second act

In 1970, a 917 finally won the 24 Hours of Le Mans outright, Porsche's first overall win at the race after two decades of trying, and it repeated the win in 1971 with a version that covered more distance in 24 hours than any car had before or would again for years. The story didn't end at Le Mans. Regulation changes pushed the 917 out of European endurance racing after 1971, so Porsche sent a twin-turbocharged version, the 917/30, to compete in North America's Can-Am series instead. With boost turned up for qualifying, that car was dyno-verified at over 1,500 horsepower, in a chassis that still weighed under 900 kg. It won so decisively in 1973 that Can-Am rewrote its fuel-consumption rules specifically to stop it, and effectively ended the series a year later.

Why it still matters

The 917 is the reason Porsche is a Le Mans name at all. It's also a clean example of something every serious engineer already knows: the fastest car on paper isn't the car that wins. The car that wins is the one where every part of the system, chassis, aerodynamics, cooling, weight distribution, is actually working together, and where someone is paying close enough attention to notice what dead flies on a bumper are trying to tell you.

If engineering stories like this are the reason you build in the first place, our Cars collection is built for exactly that instinct: models that reward you for understanding why they're shaped the way they are, not just for finishing them.