04
Apr
10

Sunday night sports-cars: Porsche 917

(Rather appropriate that my 911th post should discuss a Porsche.)

Images grabbed from RSC to avoid stealing their bandwidth; all image links go to source.  This is Porsche 917K #053, the car that won Le Mans in 1971

In the late 1960s, Porsche was a specialized manufacturer of small, light, and agile sports cars — much like Lotus is now.  Ferdinand Piech — Ferry Porsche’s ambitious nephew — had pushed the company to develop, build, and sell the now-iconic 911 at the start of the decade, and now pushed it to bolster its image with racing successes.  Cars like the 904, 907, and 910 were slippery and agile featherweights, capable of winning outright at tight, winding venues like the Targa Florio and the (“old”) Nürburgring, but lacking the power to win outright at venues like (“old”) Spa-Francorchamps and Le Mans against the larger, more powerful Ferrari 330s and Ford GT40s (though class wins were rarely a problem).

In 1968, however, the FIA’s competition board reduced the minimal production number for cars in the “sports” category from 50 to 25.  Piech saw an opportunity, and by April 21st 1969 the Zuffenhausen factory had built 25 Porsche 917 “sports cars”, each with vestigial passenger seating, spare tires, and cargo compartments, which were dutifully homologated by the FIA.

The 917 was designed to be fast above all else.  Its original 4.5l flat-twelve engine was partially derived from the earlier 908’s 3.0l flat-eight, at least in terms of cylinder head and camshaft arrangement, but the flat-eight could not easily be stretched to a flat-twelve — the 908’s engine already suffered from vibration problems, and extending it would only make things worse.  Porsche’s engineers solved the problem simply by placing the engine’s power take-off in the middle, where the second harmonic mode was zero; a pinion gear in the centre of the crankshaft drove the output shaft (below) and accessory shaft (above).  The engine was later bored out to 4.9l (1970) and 5.0l (1971), and would eventually expand to the 5.4l twin-turbo 1500hp monster at the core of Mark Donohue’s 1973 Can-Am “Turbo Panzer”.  All this without a water pump: the 917’s engine was air/oil-cooled.

Porsche 917 langheck #045 at Le Mans in 1971

Piech’s insistence on top speed über alles also drove the 917’s aerodynamics, with long-tailed (langheck) bodywork being constructed and standard on the cars to reduce flow separation (and thus drag) at high speeds.  Unfortunately, this led to cars that were notoriously twitchy, particularly in 1969 before rear tires were widened from 12in to 15in, and drivers complained that their 917s were impossible to control at high speed.  Fortunately, Porsche had contracted Britain’s John Wyer to run the “works” team in 1970 and ’71, and at a three-day test in Austria engineers from Wyer’s team and the Porsche factory cobbled together a shorter “kurz” tail that produced meaningful amounts of downforce and essentially fixed the car.  Porsche would continue to develop the langheck bodywork, and Wyer’s team would continue to run them in competition, but most of the 917’s successes would come from the kurz car.  (Aside from aerodynamics, the shorter car had the advantage of open rear bodywork to lower the car’s chronically-high transmission temperatures.)

In this way the Porsche 917 represents a turning point for motorsports in general: it marks the transition from a focus on high speed to a focus on low lap time (through increased downforce and thus corner speed).  The factory’s insistence on developing the langheck bodywork in the face of its demonstrable inferiority to the kurz tail is the most obvious example of this disconnect, but there were others.  For example, most 917s were set up with positive camber at the rear wheels; this would tend to keep the rear tires vertical (for higher grip) under squat in straight-line acceleration, but must have drastically reduced rear mid-corner grip, where the outside (loaded) tire would lean way out*.  It’s easy to look back on these things and wonder how the Porsche race engineers could possibly have missed them, but at the time these things can’t have been obvious — or someone would have done them earlier.

Nevertheless, Piech didn’t leave behind Porsche’s tradition of “adding lightness” (or wait; that’s Colin Chapman’s tradition…) and the 917 shows it.  The frame was made from aluminum tube sections — these being very difficult to weld in 1968 — and was pressurized to detect leaks.  (One wonders if the engineers ever considered filling it with helium.)  Nothing was off-limits in the quest to reduce weight to the class-minimum 800kg — even the gearshift knob was made of balsa wood.  Mass distribution was also carefully considered: the early car’s fuel tanks were located in the door sills (much to the driver’s dismay, one imagines), so that the centre of mass barely changed as fuel burnt off and the car grew lighter.  In later models, the twin aluminum tanks were replaced by a single aviation-grade rubber tank on the right-hand side, which must have better balanced the driver’s mass when full (not to mention eased his mind!).

Mark Donohue’s Turbo Panzer winning at Riverside in 1973

Porsche 917 Ks won Le Mans outright in 1970 and 1971, as well as endurance races at Spa-Francorchamps, the Nürburgring, and the 24 Hours of Daytona. The car, in its World Championship of Makes trim, won a total of 30 races in two and a half years.  Ultimately the Porsche 917 received the highest honour in motorsports: Its whole class was banned from FIA competition in 1972, and when John Penske and Mark Donohue brought the 917/10 to the Can-Am series that year, they so dominated the field that by 1974 the series had essentially disintegrated.  Having Steve McQueen turn it into a movie star doesn’t hurt the car’s reputation, either.

In the late 1990s, the Porsche 911 GT1 would follow a similar path: ACO and FIA loopholes and a decaying true-prototype class would permit barely street-legal “grand touring” cars to be homologated with a 25-car production run.  Unfortunately, the 911 GT1 would not be competitive against the Mercedes CLK-GTR (which killed its class in much the same way as the 917), and while it would take an overall victory in the 1998 24 Heures du Mans it never achieved the spectacular success of the 917.

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References:

* This may also have been due to the 917’s cam and pawl-type ZF differential, which locked up fully under power and would have made getting on the throttle before the corner’s apex somewhat problematic.  JWA’s drivers and race engineers may have simply decided not to bother trying to fix mid-corner behaviour and optimize for exit speed instead.


9 Responses to “Sunday night sports-cars: Porsche 917”


  1. 1 aczarnowski
    April 5, 2010 at 06:13

    A 1500 hp flat twelve without water all over the place? Do want.

    • April 5, 2010 at 13:47

      Gotta love an engine that makes 900hp Chevy big-blocks look limp and insipid, eh? You have to wonder what kind of fuel they were running to get that level of output — the boost pressure figures I’ve seen for the turbo are insane, along the lines of 39psi in qualifying trim. (The turbo panzer ran races in a much more sedate configuration — “only” 1100bhp.)

  2. April 25, 2010 at 18:10

    sports information needs have I got from you … thanks

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