Top of the range, with lightweight construction and 12-cylinder engine

Porsche 917 – development, technology and racing in 1969

In 1968 Porsche develops the new 917 model Group 4 sports car for the 5-litre class in just a few months and under enormous time constraints

For the 1968 race season, the new CSI (Commission Sportive International) regulations stipulate a maximum engine displacement of three litres for sports car prototypes such as the newly constructed Porsche 908, and at the same time, a maximum displacement of five litres is specified for the Group 4 sports car category. This new 5-litre limit initially comes with a minimum manufacturing requirement of 50 vehicles – though this is later reduced to 25.

By now the Porsche motorsport development department under Ferdinand Piëch has full confidence that the 908 is the right race car to win the World Sports Car Championship and the much-coveted overall win at 24 hours of Le Mans. However, the new rules are expected to result in many more Group 4 sports cars appearing on the grid in the near future, compromising the strategy favoured by Porsche up to this point.

Hans Mezger constructs the 912 model 12-cylinder race engine

Specifying a 12-cylinder engine as a requirement, Ferdinand Piëch entrusts Chief Engineer Hans Mezger with the project of constructing a new 5-litre sports car, and by spring 1968 the car starts to take shape. With a V configuration and a 180-degree bank angle, the new 12-cylinder engine breaks the mould of the Porsche boxer, as the V-12 design features a shorter overall length and reduced friction from fewer main crankshaft bearings. Using components from the 3-litre 908 engine (pistons, cylinder, valves), Hans Mezger develops the 12-cylinder engine with 4,494 cc displacement.

Special design features of the new dry-sump-lubricated race engine include the lightweight magnesium housing and central power take-off that drives the overhead camshafts over each cylinder bank using gears. Like all Porsche engines up to that point, it is air-cooled and features a horizontally mounted cooling fan. Fuel is regulated by mechanical induction tube injection using a Bosch injector pump for a 12-cylinder V engine. The 912 engine designation comes from more recent Porsche race car nomenclature, whereby the number of cylinders in that particular car’s engine is also included, i.e. 904, 906 and 908; however, the vehicle as a whole with 5-speed transmission, is given the 917 designation.

Learning from the 908 to construct the 917

The 908 sports car prototype is given a powerful lightweight construction to keep to the 800 kg minimum weight requirement, and it becomes the blueprint for the new Porsche 917. As such, the 917 features a lightweight aluminium tube frame chassis that weighs only 45 kilogrammes. Shifting the cockpit and driver’s seat further forward means that the 908 wheelbase is kept at 2,300 mm, despite the considerably longer 12-cylinder engine. As with the 908, the outer skin of the 917 is also constructed from fibreglass laminate and firmly bonded to the aluminium frame in the front and roof sections, and along the door and window frames.

Since its inception, Hans Mezger’s layout for the 917 has been intended as a basic version with an optional long-tail section to be mounted separately; the rear extension (long-tail) – which is included in the homologation specifications and used for the high-speed stretches at Le Mans – can easily be detached for use on other tracks, transforming the 917 into a short-tail version.

Aerodynamic development of the 917 in wind tunnels

With technical specifications as the starting point, the body is developed in the Porsche design department. A plasticine model of the car is first created at 1:5 scale, shortly followed by a 1:1 model. This full-scale model is used for testing in wind tunnels at the Research Institute of Automotive Engineering and Vehicle Engines (Forschungsinstitutes für Kraftfahrwesen und Fahrzeugmotoren an der Universität Stuttgart – FKFS) at the University of Stuttgart. Additional support for testing the aerodynamic development of the 917 comes from Charles Deutsch and his Paris-based research institute SERA (Société d’études et de réalisations automobiles).

Finally the body design is drawn up with two adjustable side-mounted air flaps included in the rear and in the front respectively – these feature on both the short-tail and long-tail versions. The flaps operate using kinematics based on the left and right wheel suspension: if the vehicle experiences downward force, the flaps are level; if downward force is reduced and potentially creates lift, the flaps tilt upwards to generate downward force.

Lightweight chassis and high-performance brakes

The 917 chassis features double wishbone suspension, bringing the advantages of reduced weight and broad scope to configure the wheel kinematics, as freely rotatable uniball joints offer a multitude of options for adjustment in this area. In order to maintain minimal sideways tilt and maximum wheel surface contact with the track, anti-roll bars are built into the front and rear axles; these are connected to the wheel knuckles with joint rods, and enable greater alignment of the chassis through adjustment of the lever arms’ length. The 15-inch wheels are also made from the same lightweight cast magnesium alloy as the knuckles. The titanium wheel hubs fit each wheel using a central lock with large lightweight metal alloy wheel nuts, and have arisen from the ongoing pursuit of lightweight construction and reduced unsprung weight;  the tapered coil springs for the four gas-filled shock absorbers are also made from titanium.

The braking system on the 917, which is designed to be a right-hand-drive model, has also been completely overhauled, with three-part, four-piston brake calipers at its heart. The mid-section, which contains the brake discs and is bolted to the knuckles, is also made from titanium, while both cylinder bridges on the outside house two brake pistons each, and are made from a cast aluminium alloy. The 917 features two separate brake circuits, each with a master cylinder; a rocker switch between the two enables the brake balance between the front and rear axle to be finely adjusted. Internally vented cast iron brake discs are designed for top braking performance and durability. The aluminium brake disc chamber allows for lightweight construction and lowest possible rotating weight, with the brake discs attached to it using machined grooves, a mounting ring and 12 screws; this design has the benefit of drastically reducing the risk of cracks forming, through the use of one-piece cast iron brake discs.

Race against time in Stuttgart-Zuffenhausen

From the outset, Ferdinand Piëch is set on exhibiting the Porsche 917 at the Geneva Motor Show in March 1969, leaving precious little time for preparation as work to construct the 25 units required by homologation rules begins in December 1968. To meet the target, 13 working groups are formed, with a total of 45 race mechanics assembling the vehicle in the testing and racing department at plant 1 in Stuttgart-Zuffenhausen, as 10 other mechanics take care of component pre-assembly. While the engine, transmission and chassis are constructed in Stuttgart-Zuffenhausen, the aluminium frame goes into production at Baur assembly works in Stuttgart, and the plastic bodywork is manufactured at Waggonfabrik Rastatt.

By now, the first of Hans Mezger’s 12-cylinder engines constructed from prototype components has completed testing and delivered 542 PS on its first run – a great initial result for the 4.5-litre engine, which goes on to achieve approximately 580 PS in April 1969.

With just over two months to go before the car is unveiled in Geneva, all the mechanics are frantically working, virtually around the clock, to meet the deadline – stopping only briefly for Christmas. Baur delivers the first aluminium frame to Waggonfabrik Rastatt at the end of 1969; then finally, on 1 March, the frame body for the first Porsche 917 with chassis number 001 arrives at Stuttgart-Zuffenhausen for final assembly. The 917-001 is completed on 10 March 1969, the night before it is transported to Geneva.

The new 917 is exhibited at the Geneva Motor Show

The clock strikes 3 on the afternoon of 12 March 1969, and the whole world’s media are gathered at Porsche KG’s trade fair stand, drawn by the buzz that followed the sports car manufacturer’s announcement on 3 March: “A new sports car with designation 917 is currently in production at the Porsche works in Zuffenhausen. This spectacular model will be unveiled to the public at the Geneva Motor Show. The Geneva Salon de l’Auto takes place from 13 to 23 March, and the Porsche 917 will be exhibited on the official press day on 12 March.” The announcement wins worldwide coverage for Porsche, and generates high expectations among industry figures and racing fans alike.

The key information for the new Group 4 sports car makes for impressive reading: 520 PS at 8,000 rpm with a maximum torque of 46 mkp (451 Nm) – all at a selling price of DM 140,000 ex-works. In fact, these performance specifications do not reflect the actual performance of the 4.5-litre engine at this time; the figures are calculated on the basis of the 916 model six-cylinder test engine, which has delivered 260 PS with a 2.25-litre displacement.

Final Zuffenhausen sprint to homologation

Following the successful unveiling of the Porsche 917, work in Stuttgart-Zuffenhausen continues, as 24 more vehicles must be completed to meet homologation requirements. The 917 is accepted for technical inspection on 20 March 1969 and the day finally arrives on 21 April 1969: the English FIA delegate, Dean Delamont, and the German ONS representative, Herbert Schmitz, arrive at plant 1 in Stuttgart-Zuffenhausen to examine the 25 regulation-compliant roadworthy units – replicas of the “White Giant” (the name given to the 917 at Porsche). The next day, the Porsche press department declares: “The Porsche 917 is homologated as a sports car from 1 May 1969 and is expected to make its début appearance at the 1,000 Kilometres of Spa-Francorchamps on 11 May.”

The first race in Spa and at the Nürburgring

Though the car completes preliminary test runs for the 24 hours of Le Mans on 29 and 30 March, with a few issues being ironed out, Gerhard Mitter is forced to abandon the car’s racing début in Spa after the first lap, because of a broken valve spring. In the end, the Nürburgring 1,000 km race on 1 June is the final chance to rehearse for Le Mans. British drivers David Piper and Frank Gardner drive the 917 over the finish line for the first time, taking eighth place to round off a day of excellent results, including a terrific win for the Porsche 908 and eight Porsche vehicles placing in the top ten.

Dominance and drama at Le Mans

With the implementation of several fine tuning adjustments to the chassis, Porsche competes in the Le Mans race on 14 and 15 June with the usual adjustable rear air flaps, but also with fixed spoiler flaps on the front. Porsche brings a total of four 917 cars to Le Mans: two long-tail versions to be used as works cars, another long-tail version for training, and one more long-tail 917 to be provided to the John Woolfe racing team as a customer.

Four Porsche works cars are out in front when the race starts at 2 p.m. on the Saturday afternoon; leading the pack is the 917 driven by Rolf Stommelen and Kurt Ahrens, closely followed by the second 917 with Vic Elford and Richard Attwood, then a 908 long-tail Spyder with Joseph Siffert and Brian Redman, and a 908 long-tail Coupé driven by Rudi Lins and Willi Kauhsen. The third Porsche 917, driven by John Woolfe and Herbert Linge, starts the race from ninth on the grid,

but before the first lap is over it suffers an accident that is fatal for John Woolfe. More than 14 hours in, the 917 with Stommelen and Ahrens retires from the race with a slipping clutch; the 917 driven by Elford and Attwood is still on the track and prospects are looking good – after 21 hours, the car is leading by six laps. “The engine was running like clockwork until that point. No problems whatsoever!” Hans Mezger notes, recalling taking his lunch break around this moment. “When I returned to the pit, I could tell straight away by the sound that something wasn’t right,” he says, remembering the moment that the glory-bound 917 is forced to retire from the race with a cracked gearbox. The Porsche team’s second place in the overall standings, with the 908 long-tail Coupé driven by Hans Herrmann and Gerard Larrousse, offers cold comfort. After driving 372 laps, the Porsche works drivers miss out on the eagerly awaited win just 100 metres or so from the finish line.

Building on experiences from Le Mans

Le Mans certainly throws into stark relief those areas of the 917 that still require improvement. This leads to more extensive testing and test drives being carried out before the last race of the year: the Austrian Grand Prix in Zeltweg (Styria) on 10 August. A scheme to improve driving stability, instigated by race engineer Peter Falk, is one of the first steps to be tested in the car on the South Loop of the Nürburgring. Further tests are later conducted on the skid pad in Weissach and in Hockenheim. Adjustments to the aluminium frame and modifications to the body are intended to enhance the driving characteristics of the 917.

The first 917 win at the last race in Zeltweg

Porsche sends five cars out to race on the fully rebuilt track in Zeltweg: two 908 long-tail Spyders; one 908 short-tail Spyder (customer commission); and two 917 short-tails driven by Joseph Siffert with Kurt Ahrens, and Brian Redman with Richard Attwood. The official opening of the Österreichring eventually turns out to be a big day for Porsche and the indefatigable 917 team, as Siffert and Ahrens successfully secure the coveted first place slot after 170 laps, followed by the other 917 which takes second place on the podium. The first win for the 917, at the last race of the year, is a glorious conclusion to a season marked by extensive testing and development work. Zeltweg is seen as precursor to further intensive development work and an unprecedented series of successes – the pinnacle, of course, being the first overall win for Porsche at the 1970 24 hours of Le Mans.

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