Under the skin - Porsche 917

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During the 917’s competitive life Porsche continued to develop and experiment. Our technical editor looks at some of those iterations, focusing on the 917K. Jake Boxall-Legge.


Under the skin - PORSCHE 917 AT 50 1969-2019 TECH FOCUS


Fast fact

Pedro Rodriguez and Jackie Oliver won the 1971 Spa 1000Km at an average speed of 154.8mph – even now faster than the top world championship F1 winning speed – despite driver changes and fuel stops


When the Porsche 917 hit the track for the first time 50 years ago, there was something gravely amiss. The handling (or lack thereof) was a colossal problem for the first generation of the 917, and Porsche took time to get to the bottom of the issue. Arguably, the development paths Porsche took throughout 1969 were a case of barking up the wrong tree; initially, the engineers surmised there was too much chassis flex, and the car couldn’t cope with the weighty 4.5-litre flat-12 engine mounted in the back. Although the frame was beefed up, the car’s wayward and unpredictable behaviour continued to make it a handful. It wasn’t until Porsche elected to tie up with sportscar guru John Wyer to run its works outfit for 1970 that the instability issues were solved.


Under the skin - Porsche 917 / cutway
Under the skin - Porsche 917

Aerodynamics

John Wyer’s JW Automotive team, which had been one of Porsche’s chief rivals, was present at an Osterreichring test in the late stages of 1969 when it became clear that the 917’s Achilles’ heel was not the torsional rigidity of the chassis but the aerodynamics. With the low tail, the airflow tended to separate as it moved towards the rear of the car, resulting in an unhealthy amount of rear-end lift . Accounts on exactly who spotted what differ, but legend has it that dirt and detritus had gathered everywhere on the car except its rear deck during the course of testing. Although race-car aerodynamics were still in their nascent stages, the team realised this was strange and that it indicated sub-optimal airflow. A new ‘aero package’ was fabricated on the spot out of sheet aluminium, essentially aimed at raising the tail. It worked, addressing the stability issues perfectly. Porsche was tasked with building the new bodywork for 1970, and the car became known as the 917K, the ‘K’ standing for ‘Kurzheck’, or ‘short-tail’. JWA later made its own developments too, such as introducing a small aerofoil (below) within the valley of the tail, during its two-season campaign. The shape of the cars gradually changed (partly to accommodate bigger wheel rims and tyres) and vertical tail fins in 1971 allowed a lower rear deck. Porsche also worked hard on developing the long-tail, lower-drag concept, designed for the demands of Le Mans and its long straights. The JWA squad and the 1970 Le Mans-winning Porsche Salzburg pairing stuck with the 917K thanks to its greater advantage in the corners. Meanwhile the Martini-backed works effort took the long-bodied variant to second place, albeit five laps down, though the polesitting Vic Elford/Kurt Ahrens example had been in the fight at the front while it lasted.


Engine

The initial 4.5-litre flat-12 engine produced a little over 540bhp, before a 4.9-litre edition became available in time for the Monza 1000Km round of the world sportscar championship in April 1970. It was more powerful, with an output of over 575bhp, and was used in the front-running 917s at Le Mans. But it was the 4.5-litre unit that gave Porsche its 1-2 in the 24 Hours. Over-revving was a danger with the 917 and attempts to rev-limit the engine resulted in random misfires as the potentiometers used would quickly lose calibration. Porsche did find reliability, experimenting with different materials used within the engine components to develop titanium valves. For most of 1970, Porsche also used only four speeds in the transmission to avoid missed shift s, particularly grabbing third instead of fifth. A new five-speed gearbox was developed for 1971, though four speeds were still oft en used, and even the 1973 917/30 was a four-speeder. A full five-litre engine had been developed by the end of 1970 to combat the Ferrari threat. Even more exotic materials helped push the power output to around 630bhp, but later turbocharging (and expanding to 5.4 litres) for Can-Am would double that.


Chassis and brakes

One of the last successful tubular-spaceframe chassis designs before the mass gravitation towards aluminium monocoques, the 917’s skeleton would not be considered stiff by modern standards. Porsche had ensured that every tube could be pressurised without leaking air (so a loss of pressure would indicate cracks). That meant the chassis could be used as the pipework for the oil cooler, though JWA wasn’t keen on this and used flexible pipes instead. Teves/ATE brake discs were used initially, but JWA (and some others) switched to Girling early in 1970 to improve spongy brake feel and uneven-wear issues. Drilled brake discs with chamfers were also introduced during the world sportscar championship programme as Porsche continued its pursuit of light weight. Carbon discs were considered too, though this didn’t come to fruition.


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