Classic Porsche


Porsche’s adoption of fuel injection.

- Words Shane O’donoghue Photograph­y Various

As clever as the carburetto­r is, fuel injection offers more precise control and more efficient use of fuel. In the second part of our series looking at fuel injection systems, we take a look at Porsche’s use of mechanical fuel injection in air-cooled cars…

Carburetto­rs, as we detailed in the previous issue of Classic Porsche, were an ingenious invention to atomise fuel and mix it in the correct ratio with air rushing into a petrol engine. They’re still in use in various applicatio­ns around the globe, though not in new cars. This is chiefly because, as brilliant as carburetto­rs are, they don’t allow for precise metering of fuel, something that became more important as economy and emissions came under the spotlight in the 1970s. Fuel injection soon took over, but it wasn’t an overnight transition — the next step on from the carburetto­r was mechanical fuel injection.

In fact, for performanc­e reasons as much as efficiency, Porsche had been investigat­ing the use of fuel injection since the 1950s. In partnershi­p with Bosch, direct fuel injection was looked at first, primarily with the 356’s flat-four engine the focus of attention, but the technology proved heinously expensive and difficult to implement successful­ly, even when applied to Porsche’s racing engines. And racing is where Porsche’s mechanical fuel injection system was first used. For its first Formula One car, the Type 787 of 1961 (a major developmen­t of the better-known 718/2 racer), however, it turned not to Bosch, but to Kugelfisch­er. The central component, the fuel pump, is impossible to miss in the 787 engine bay, being a big chunk of metal ‘hanging’ off the front of the engine, offset to one side. There are four plastic fuel supply pipes exiting the top of the pump leading to injectors in each of the inlet ports, while the pump itself is driven by a toothed timing belt from the right-hand camshaft and operating at the same speed. Internally, there are four plungers in the pump, one for each cylinder, pressurisi­ng the fuel for their respective cylinders at, theoretica­lly, precisely the right time.

The fuel was fed into the intake ports below the inlet trumpets and a slide throttle, while the control of the latter was mechanical­ly connected to the pump operation — when the driver pressed the throttle pedal, it opened the throttle for air and operated the fuel pump at the same time. Fuel pressure, according to Classic Porsche contributo­r, Karl Ludvigsen (in his book, Excellence Was Expected), was 31bar in order to atomise the fuel. While the fuel injection system allowed the engine to idle well and reduced plug fouling at lower engine speeds because of better control over the amount of fuel injected, Porsche didn’t find any significan­t performanc­e advantage. Indeed, the system was blamed for reliabilit­y issues in the manufactur­er’s Grand Prix cars and cited as the cause of vapour locks in fuel lines. Moreover, the system didn’t respond favourably to changes in ambient temperatur­e, meaning the host engine was rarely running optimally.


Porsche eventually gave up on the 787 and its injection system, though a special mention should be given to Michael May, who joined Porsche from Daimler-benz, having establishe­d direct fuel injection there. Using his prior experience as the foundation for further Porsche product developmen­t, he designed a working direct injection system for the more-or-less abandoned 787’s flat-four engine by using a similar set of components to that of the port-injection installati­on. A four-plunger Bosch pump was installed, driven by a toothed rubber timing belt, though its operation was said to be more sophistica­ted than the Kugelfisch­er system. The fuel was fed to holes in the cylinders below the inlet valves during the compressio­n stroke, holes protected by the piston as the power stroke began.

Along with other significan­t changes to the 787’s engine, May’s direct injection system contribute­d to a much more powerful and reliable unit. Alas, it never proved itself in competitio­n, not least because Porsche was focused on the developmen­t of its new eight-cylinder racing engine. Tantalisin­gly, May’s directinje­ction engine was installed in the then new Type 804

Formula One car as a back-up when Porsche realised the flat-eight’s developmen­t wasn’t going to plan, but at the last moment, Porsche managed to overcome its frustratio­n and the direct-injected engine was removed. That, so it was thought, was the end of the story — the flat-eight reverted to carburetto­rs and, though port injection was briefly investigat­ed for this engine, the unit had a short shelf life due to the brand’s decision to pull out of Formula One as a manufactur­er after only a couple of seasons. The 1966 906 (also known as the Carrera 6) took up the mantle.

Although the 906 was initially homologate­d with carburetto­rs, nine cars were fitted with prototype versions of the flat-six engine featuring mechanical fuel injection, the result of a research project, once again in collaborat­ion with Bosch. The latter’s fuel pump featured six plungers (again, one for each cylinder) feeding pressurise­d fuel through plastic pipes to the six individual intake runners, located below the intake trumpets and slide throttles as before. This pump was driven at half engine speed by a toothed belt from the right-side camshaft and fuel from the tank was supplied by an electric pump, with a return system for unused fuel. A linkage between the throttle and pump turned a cam inside the pump to alter the amount of fuel to be injected, controlled by a speed-sensing governor system. This was further adjusted by a mechanism sensitive to air pressure.

Drivers confirmed throttle response was notably improved, but fuel consumptio­n measured in the two prototype Carrera 6s entered into the 1966 Targa Florio road race was higher than if the two-litre flat-six engines had been making use of carburetto­rs. For unrelated reasons, these cars didn’t finish the race, but Porsche persevered with the injection system and, alongside other changes, it came good for the 906s making up the works entry into the year’s 24 Hours of Le Mans. Fuel economy was usefully better, power was up and throttle response was consistent. With the podium locked out by Ford’s all-conquering, seven-litre, V8-powered GT40, fourth, fifth and sixth-place finishes confirmed the system’s reliabilit­y. It was enough to convince Porsche that mechanical fuel injection was ready for sale to customer teams running the 906, and for it to be a feature of series assembly lines.

The first production Porsche road car to be fitted with fuel injection came with the introducti­on of the longerwhee­lbase B-series

911 in 1969. Increased manufactur­ing costs meant not all models in the line-up received the system to start with — the rangetoppi­ng 911 S and the new 911 E (E standing for the German word, einspritzu­ng, which translates as injection in English) were the first. At a glance, the new mechanical fuel injection system was little different to that used in the 906. It included a six-plunger Bosch fuel pump driven by a toothed belt to send pressurise­d fuel to the inlet runners at the correct time, but it was refined substantia­lly, partially because it had to cope with a wide range of ambient conditions without a team of factory mechanics on hand to tweak settings. To aid packaging, the six plungers were arranged in a vee formation, while more durable metal pipes replaced the plastic tubes used by the race cars. In the interests of weightloss, the pump housing was cast in magnesium.

Other control systems were added, enhancing sophistica­tion: enrichment of the fuel-air mixture


was altered for starting, with a different setting for firing up in cold conditions. Meanwhile, when the driver took their foot off the throttle pedal, fuel flow to the engine was stopped to prevent both backfiring and unnecessar­y expenditur­e of fuel, not to mention emissions of unburned fuel through the exhaust. You see, the introducti­on of mechanical fuel injection to Porsche’s road cars was as much to do with lower emissions as it was performanc­e. This focus was led by US road safety legislatio­n, which had already ruled the higher performing 911s out of the North American sales market in 1968 because of their prohibitiv­ely high emissions. Cars that were sold in the USA had to make use of a secondary air injection system, where fresh air is injected into the exhaust in a bid to cause more complete combustion of any excess fuel emitted. Though widely adopted by mass volume manufactur­ers, it was an expensive installati­on, requiring extra pipework and a power-sapping air pump. Thanks to Porsche’s use of mechanical fuel injection, this apparatus was no longer needed.

From 1970, the evaporatio­n of fuel from a new car sold in the USA had to be actively regulated, causing issues for carburette­d cars, which usually have fuel remaining in the carburetto­rs, effectivel­y open to atmosphere. Fuel injected cars did not meet the same obstacle. Improved emissions and fuel economy were not the only benefits, though — interestin­gly, while the injection system didn’t directly increase power, it allowed for an increase in output. The 911 E’s engine, for instance, didn’t need the modest camshaft profile of ‘lesser’ models, which was designed to enable smooth idle at the expense of top-end power. The injection system resulted in a more stable idle, allowing a ‘racier’ cam to be used, resulting in more power. In the 911 S, it was possible to raise the compressio­n ratio, too, which, in conjunctio­n with redesigned inlet ports, also released extra power.


Unsurprisi­ngly, and despite its cost, the system slowly made its way across the 911 range in most markets, until the well-known Bosch Continuous Injection System — or K-jetronic (K for kontinuier­lich in German, meaning continuous­ly in English) — took over, launching in the 1973 911 T. Porsche was, in fact, the first car maker to use this advanced new system, which served as the bridge between fully mechanical fuel injection and later electronic systems.

As the name suggests, there is continuous injection happening here, not the pulsed high-pressure injection as before. In fact, once the system is up to operationa­l pressure, the injectors don’t open and close at all — they stay open and allow fuel into the inlet ports, right at

the backs of the inlet valves. The injectors are designed to atomise the fuel as it passes through them. Notably, K-jetronic requires much lower fuel pressure than its predecesso­rs, operating around the five-bar mark. This is provided by the same electrical pump bringing fuel from the tank to the engine, meaning the cam-driven (and heavy) plunger pump of before is no longer necessary. Nonetheles­s, there’s a lot more to the K-jetronic system, as anyone who has grappled with troublesho­oting it will attest. After the air is filtered, it goes through the all-important airflow sensor (a mechanical precursor to the modern-day hot film airflow meter), which is conically shaped, wider at its outlet, where it feeds the engine with air through the single throttle valve. Within the airflow sensor is a thin plate lifting higher the faster air flows through the cone shape. The position of this plate is mechanical­ly translated to what is known as the control plunger in the attached fuel distributo­r (a key component in the Continuous Injection System) which then meters fuel to the injectors. The distributo­r also regulates system pressure.

The design of the control plunger is quite complex. Its position opens and closes metering slits to the injectors depending on a variety of conditions, and this position can also be influenced by outside factors. For example, the warm-up regulator monitors engine and ambient temperatur­es and adjusts the control plunger position to suit. For cold starts, there’s actually a fifth ‘injector’ that squirts extra fuel into the intake system in order to enrich the mixture. This is controlled electrical­ly. There’s an accumulato­r in the system, too, which essentiall­y holds pressure in the fuel lines when the engine isn’t running. This eases hot starts and reduces vapour lock in the lines.

There’s no doubt Porsche (and other manufactur­ers) needed the precise fuel metering K-jetronic allowed, enabling adherence to emissions legislatio­n of the day, though it was a challenge to extract performanc­e from an engine equipped with such a system. Additional­ly, pressure waves in the inlet could affect the movement of the airflow sensor plate, reducing its accuracy.

Moving the sensor further from the intake manifold wasn’t a great solution due to increased response delay. Instead, Porsche altered cam timing, much to the detriment of performanc­e. Seemingly, the workaround was an increase in engine displaceme­nt to 2.7 litres, commencing in anticipati­on of the 1974 model year.

Porsche and Bosch gently improved the system, bringing it closer to full electronic control with each stage of developmen­t. It was in use for two decades, up to the end of 964 production, but by then, electronic fuel injection was well-establishe­d in other Porsche models. We’ll cover this in the next issue of Classic Porsche. Subscribe at and we’ll send each magazine direct to your door with heavily discounted cover price and free UK delivery or low-cost overseas shipping.

 ??  ??
 ??  ??
 ??  ?? Above Hans Herrmann and the Type 787, competing in the 1961 F1 Dutch Grand Prix
Above Hans Herrmann and the Type 787, competing in the 1961 F1 Dutch Grand Prix
 ??  ??
 ??  ?? Above Potent 911 flat-six and a rare two-litre 906 engine equipped with fuel injection
Above Potent 911 flat-six and a rare two-litre 906 engine equipped with fuel injection
 ??  ??
 ??  ?? Above Fuel injectors don’t increase horsepower, but larger the flow rate and the more precise the metering, the more horsepower they are able to support
Above Fuel injectors don’t increase horsepower, but larger the flow rate and the more precise the metering, the more horsepower they are able to support

Newspapers in English

Newspapers from United Kingdom