The original 1969 Porsche 917 suffered from significant aerodynamic lift at high speeds, generated by its long-tail bodywork. The handling problems were so severe that drivers described the car as nearly undriveable. After the 1969 championship season, John Wyer of JW Automotive requested a test session at the Osterreichring in Austria to address these issues.
During testing, Porsche chief engineer John Horsman observed that the lack of insects on the tail surface revealed air was not flowing cleanly over the rear bodywork. Engineers improvised a new short-tail shape on the spot using aluminium sheet and gaffer tape, inspired partly by the gently upswept tail of the 917PA Can-Am spyder. The modification dramatically improved aerodynamic stability. This improvised solution was quickly converted into production-specification bodywork at the Porsche factory, and the result was designated the 917K. The car made its public debut at the season-opening 1970 24 Hours of Daytona.
The 917K became the standard configuration for all races except Le Mans, the Nurburgring 1000 km, and the Targa Florio, where the nimbler Porsche 908/03 was preferred. JW Automotive and Porsche Salzburg campaigned 917Ks throughout the 1970 season.
At the 1970 24 Hours of Le Mans, a special low-drag long-tail 917LH was developed for the straight-heavy circuit, while the 917K acted as the backup configuration. The winning car was the red and white number 23 917K of Porsche Salzburg, driven by Hans Herrmann and Richard Attwood, which gave Porsche its first overall victory at Le Mans. The 4.5-litre flat-12 engine was progressively enlarged during the season to 4.9 and then 5 litres. In total, the 917K won seven of the ten championship races it contested that year.
For 1971, the 917K was further developed with vertical fins on the tail section and dual air intakes for improved aerodynamics and engine cooling. The fins maintained clean airflow over the rear bodywork, allowing the deck height to be reduced while retaining equivalent downforce levels, resulting in lower drag and higher top speeds than the 1970 model. A version of the finned 917K fitted with a lightweight magnesium frame chassis won the 1971 24 Hours of Le Mans for the Martini Racing team, driven by Helmut Marko and Gijs van Lennep. This car set an overall distance record at Le Mans that stood until 2010. The 1971 917Ks won seven of the eleven World Sportscar Championship races.
The 917K was built around an extremely light spaceframe chassis weighing just 42 kilograms, permanently pressurised with gas to detect structural cracks. Power came from the Mezger-designed Type 912 flat-12 engine, a 180-degree layout with six shared crankshaft throws and twin overhead camshafts driven by central gears. The engine was progressively enlarged from 4.5 litres producing around 520 bhp in 1969 to 5 litres producing a maximum of approximately 630 bhp by 1971. The car made extensive use of titanium, magnesium, and exotic alloys for weight reduction.
The 917K's transformation from an unmanageable prototype to an outright Le Mans winner across just one development winter stands as one of the most successful rapid engineering corrections in motorsport history. Its domination helped Porsche secure the International Championship for Makes in both 1970 and 1971. The Gulf Oil-liveried 917K was prominently featured in Steve McQueen's 1971 film Le Mans, cementing its place in popular motorsport culture. The car's influence extended beyond racing, as many 917 suspension, chassis, and brake components were later used in constructing the Porsche 936 in 1976.