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The Early Days of Turbo - Part Three

A tribute to the Porsche 917 - a classic racecar that morphed into a V12 twin-turbo developing over 1100hp.

By Michael Knowling

Click on pics to view larger images

This article was first published in 2003.

Le Mans is one of the most prestigious motoring events in the world. It was Porsche, however, that first got ultra serious with their 917 - a car that became so fast and dangerous it was eventually banned. Following this, Porsche's 917 effort was focused on the Canadian American (Can Am) series where - for a while - they were free to develop perhaps the most spectacular turbocar of all time...

The First Porsche 917s

The first incarnatio of the Porsche 917 - to replace to 908 racecar - was sprung in 1969. It shocked critics being the largest air-cooled race engine in history and with a remarkably low aerodynamic co-efficient; to the best of our knowledge no other racecar to date has matched the early 917's low drag figure.

Part of the reason the 917 stayed faithful to the air-cooled configuration came through political interests - Volkswagen were investing significantly in the 917 and were keen to publicly explore the limits of an air-cooled engine. This was important to keep the aging Beetle competitive in a rapidly changing market.

While the air-cooled engine had its advantages - largely reduced weight, the lack of a water pump and reduced complexity - Porsche's engine designers had to contend with the massive amounts of engine heat. For this reason, the team ran with essentially the same DOHC, 2-valve design heads as seen on the 908; 4-valve heads, it was said, would have run too hot.

Porsche conceded its archrival Ferrari would be making greater specific power with their water-cooled 4-valve engine, so they had to level the playing field in other areas. A key design philosophy was therefore adopted - maximum possible aerodynamic efficiency and minimal weight (the minimum allowable under regulations). A bonus of these two attributes was reduced fuel consumption.

As mentioned, the heads on the new 917 engine - dubbed Project 912 - were based on the design of those on the older 908; in fact, the entire engine was based on much the same architecture except with one major difference. The 917 engine was a 180-degree flat (boxer) 12 displacing 4.5-litres - 50 percent larger than the 908's 3.0-litre eight-cylinder engine.

At 4.5-litres, the 917 was by far the largest air-cooled race engine ever built and, as such, it relied heavily on a large fan mounted atop the cylinders. At an engine speed of 8400 rpm, this gear-driven fan sucked a massive 2400-litres of cooling air with 65 percent devoted to the finned cylinders. It is also said that the oil lubrication system was more heavily relied upon to remove heat; a massive oil cooler (with the chassis tubes used to circulate the oil) was essential. In certain models, oil squirters also cooled the pistons.

The crankcase - cast in magnesium to reduce weight - contained titanium conrods and forged alloy Mahle pistons for a compression ratio of 10.5:1. The crankshaft was forged from high-grade steel and a dry sump system was incorporated.

The cylinder heads were made from aluminium and contained 47.4mm titanium inlet valves plus 40.5mm sodium filled exhaust valves. The air intake arrangement comprised fibre composite trumpets with complex slide throttles. Magnificent extractors were used to scavenge exhaust gasses.

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Bosch developed a new fuel injection pump to suit the engine and the injectors were mounted high in the fibre composite trumpets. Bosch also devised a special capacitor ignition system working with twin distributors and twin spark plugs per cylinder.

Just five moths after engine designing began, Porsche's flagship race engine was fired into life in a dyno cell and generated 542hp at 8400 rpm. At the time, Porsche had no gearbox to cope with the 4.5-litre's torque so it in-house developed a 4-speed (with an optional 5th speed) synchro gearbox. Drive was then put through a ZF 75 percent self-locking diff.

Like the out-going 908, the 917 swallowed its engine in a longitudinal mid-mount configuration - drive was to the rear wheels, of course. The relatively short wheelbase (drawn from the 908) was said to help keep weight to a minimum and aid rigidity. The chassis was constructed from ARC welded aluminium frame, draped with lightweight fibre composite body panels.

The body form is the product of extensive wind tunnel testing of both 1/5 scale and full-size models. Two versions were built - the conventional 917 and the 917 Le Mans. Without any add-on spoilers and the optional Le Mans 'long tail' attached, the 917 achieved a remarkably low (for a racecar) 0.33Cd. Note that part of this low Cd can be attributed to the relatively narrow wheels fitted to the 917 - 15 x 9 at the front and 15 x 12 at the rear (much narrower than F1 cars of the time).

The 917 came suspended on fairly conventional outboard wishbone arrangement using inverted Bilstein dampers (to reduce unsprung mass) lightweight titanium springs, roll bars and hubs. Fifty percent anti-dive geometry was dialled in and the rack-and-pinion steering arranging was just 1 ¾ turns lock to lock.

ATE came to the party with 12 x 1.1-inch cast iron ventilated discs with 4-pot calipers. Adjustable front-to-rear brake bias system was also fitted.

Stability at high speed was given particular design attention - especially given the projected 236 mph top speed. Interestingly, a pair of large flaps (similar to aircraft trim tabs) were attached to the rear deck. A bell crank system lifted the flaps as the rear uprights were extended (such as during heavy braking or at very high speed).

Overall mass for the 4.5-litre Porsche 917 was spot-on the legal minimum - 800 kilograms. Note that the front-to-rear distribution was 40/60.

The car had not yet been properly run when it was first unveiled at a very public '69 Le Mans test weekend - up until then it had only been driven around the factory's forecourt a week earlier... Not surprisingly there were a few lessons learnt across that weekend; there was a throttle linkage failure, starting problems, an oil leak, brake problems, excessive cabin temperature, the window wiper lifted at high speed and - amazingly - a rear window broke and was sucked through the engine's massive cooling fan! The "evil" handling was also a challenge for drivers - despite fitment of the long tail, the instability at high speed was such that nobody was brave enough to push the car past 200 mph...

Revisions were made and, in order to qualify for prototype status in the Group 5 Le Mans series, a minimum of twenty-five examples had to be manufactured. These vehicles were hastily thrown together for inspection to qualify for homologation and - once achieved - Porsche had to rebuild the last few cars that were thrown together in haste...

At last, though, the 917 was finally ready for competition.

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Up against earlier Porsche 908 models, the Ferrari 312P spyder, Matra, Chev/Lola and the aging GT40 the 917's first race was at the high-speed Spa circuit. Its debut was not very successful, with engineers unable to tune the chassis due to inclement weather; one thing remained clear, though, the 917 had to be driven cautiously.

Over the rest of the 1969 Sportscar season the 917 won only one race. During the season, however, a number of engineering changes were made - this included additional air intakes, improved brakes, much reduced anti-dive geometry and numerous aerodynamic tweaks (such as front side tabs). Still, the early 917s were very easily unsettled and difficult to drive - drivers required a water-cooled suit to stay cool...

Spreading Into Can Am...

Porsche's 917 open-top spyder chassis (dubbed the 917-Porsche/Audi) was a rework to suit the '69 Can Am series. America was a major export market for Porsche and there were also ties with Volkswagen and Audi that would benefit from US competition. Up against the unlimited capacity Can Am vehicles (dominated by 7.0-litre V8s) it was accepted that the 917-PA would, at best, be reasonably competitive and prominent.

Most of the 917-PA's mechanicals were carried over from the coupe, but it did receive lighter titanium/iron discs, wider wheels and, as a result, a slightly pumped body. Aerodynamically, the PA was very different to the hardtop coupe - it had a long, flat rear deck, a wedge shape nose and, of course, it had no cabin. Furthermore, under Can Am rules, headlights and a full windscreen were not required. Power output was reputedly tweaked to 580hp at 8400 rpm - still around 100hp less than the opposition...

Driven in anger, the 917-PA was said to handle better than the coupe and put in some very good showings. On-going alterations included front spoilers and tabs, rear spoiler extensions, fitment of several air vents and, later, a completely reworked nose. The 917-PA managed a third place finish and, overall, performed reasonably well against the brutal opposition.

Porsche Handballs the 917!

For a variety of reasons, Porsche decided it wouldn't run a works team in the 1970 European Sportscar series; instead, it supplied their cars and parts to the small but highly successful JW Automotive team. Porsche would also continue to provide engineering upgrades essential to keep 'their' vehicle competitive.

At the end of the '69 season, the JWA team were involved in an extensive test session with Porsche engineers. The team realised that 917's biggest shortcoming was aerodynamic stability, so decided to feel the effect of attaching sections of sheet metal to the hindquarters of the 917. It was discovered that a flatter rear deck (similar to the 917-PA) reduced rear-end lift, improving driver confidence and stability. These aerodynamic changes were then adopted for the 1970 update, which became known as the 917 Kutz (aka 917K).

During the 1970 season, the inherent twitchiness and instability of the 917 continued being remedied with various stages of aerodynamic guises, suspension revisions and chassis strengthening. Wheels size, too, was increased for the 1970 season. The bigger rims and altered aerodynamics - intended for more downforce - saw the 917's Cd increase massively to 0.464. The original 917 objective of aerodynamic efficiency, it seems, was forgotten.

Engine wise, changes through the '69 season saw power increase to 565hp at 8500 rpm.

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Despite the introduction of the new 48-valve Ferrari 512S, the Porsche 917 completely dominated the 1970 Sportscar season winning all-important Le Mans 24 hour race in a spectacular battle. Note that for Le Mans, Porsche designed the so-called 917 Langheck (German for Longtail). The Langheck is often referred to as the 917LH. The Langheck used an extended nose, revised engine ventilation and a longer tail with stabilising fins. With its remarkably low 0.30Cd (better than the original 917), the LH could run to a terrific top speed but was beaten at Le Mans by the less streamlined 917 Kurz model. Mechanical difficulties were the cause of the LH's lack of success.

Although exact details are sketchy, it appears that a stroked 4.9-litre version of the 917 was released some time during the 1970 Sportscar season. This engine cranked out 580 horsepower at a lower 8400 rpm.

Moving onto 1971, the Porsche 917K received yet another increase in engine size. Certain vehicles were equipped with a 5.0-litre version of the 180-degree flat 12, capable of an increasingly serious 630 horsepower. Running changes to the car during '71 include beryllium brake components, a magnesium frame and, again, revised aerodynamics.

The 1971 season saw the 917 destroy the opposition once again and achieving a back-to-back victory at Le Mans. This season saw Porsche again employ a reworked version of the 917LH at Le Mans but history repeated with mechanical failures handling the lead back to the 917K. Despite breaking all records with 240 mph speeds attained, the LH suffered massive brake pad wear.

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Meanwhile a new 917/10 was released for the Can Am series. It embodied all of the upgrades made to the coupe plus its own unique aerodynamics - almost a copy of the old Porsche 908/3, but with a more chisel-like nose. With its 5.0-litre 630hp engine and - later - a 5.4-litre 660hp engine combined with a low 750kg kerb mass, the 917/10 was considerably quicker than the original 917-PA. It put in some good races - again, though, it never won.


For 1972 the FIA had essentially banned 240 mph supercars, so Porsche decided to focus more attention on the popular Can Am series; great for the export market from Porsche's point of view.

In the early stages - in order to compete with the big block American V8s - Porsche was to employ a 7.0-litre 16-cylinder engine. It had already got to the stage of design when plans were shelved in order to pursue a smaller capacity turbo engine; there were many advantages in turbocharging the existing PA10 engine.

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The carried over 4.5-litre 180-degree flat 12 received new pistons to reduce the compression ratio from 10.5:1 to just 6.5:1, cam specifications were altered, new exhaust manifolds were created to locate twin Eberspacher turbochargers and the plumbing to the engine incorporated a pressure balancing plenum chamber. The turbochargers were based on a diesel design and featured a roller bearing centre, while a shared external wastegate regulated turbine speed.

The turbo engine suffered appalling drivability, however. Torque was very much either 'on' or off' and the car was a huge effort to drive quickly in tight conditions. As a result, in its earliest non Can Am appearances, the turbocar was not particularly successful. Subsequent changes to the injection system and fitment of what we now call a blow-off valve improved the situation markedly.

With drivability now largely acceptable, the well-respected Penske team received a 5.0-litre version of the turbo engine, making around 900hp depending on boost pressure. The driveline was accordingly beefed up and the chassis was strengthened in order to improve handling. Still, the turbo 917 was a twitchy vehicle to drive.

With massive torque and power though, the turbo 917 shocked its opposition - nobody had expected anything to beat the established big-cube front-runners!

Banned Again!

Nineteen seventy-three was the last year of the evolution of the 917.

Just when audiences thought they'd seen what must be the ultimate 917, along came the 917/30. With - once again - revised aerodynamics and engine capacity increased to 5.4-litres, it is said that this monster made 1100hp at 7800 rpm on 1.3 Bar boost. With the in-cabin boost adjustment wound out, though, up to 32 psi could be used to force out 1500hp. This makes the 917/30 the most powerful racecar to tear up a road circuit. Top speed was 240 mph, and it could accelerate from standstill to 60 and 100 mph in 2.1 and 3.9-seconds respectively. Quarter mile? About 7.3-seconds!

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Interestingly, the wheelbase of the 917/30 was longer than its predecessor. An adjustable wheelbase 9017 had been built for testing, where it was found the optimal wheelbase was some 7.5-inches longer than previously... This alone got rid of the 917's nervousness, allowing drivers to relatively safely push the vehicle to its fullest potential. Needless to say, the 917/30 completely owned the 1973 Can Am season with what many call an unfair advantage.

With heavy fuel rations placed on turbocars for the 1974 Can Am season, the 917 was essentially killed off; Can Am was to return to its routes that saw thundering V8s dominating. Nobody, it seems, wanted to be responsible for hosting these awesome 240 mph Porsche 917 racecars...

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