911 tech: MFI v CIS
Porsche turned to fellow Stuttgart company, Bosch, for both mechanical and electronic fuel injection. Here’s how both systems work
Total 911 takes a look at Porsche’s early injection systems: how they work, how they differ, and what their advantages are
Like all car manufacturers, Porsche used carburettors. The early 911 had bespoke Solex units designed for it, but these proved unsatisfactory and were soon replaced by Weber items. Carburettors are the age-old method of combining the petrol-air mixture for combustion engines and work by the suction created by the downward stroke of the pistons, with fuel delivered by a pump usually running off the crankshaft. Cheap and reliable, the drawbacks of the carburettor, essentially a non-pressurised system, are a tendency for fuel starvation under high cornering forces and their inability to provide optimum fuelling at very high RPM. In motorsport, Porsche had been fielding class winners for a decade, but as its competition ambitions increased under Ferdinand Piëch, extracting as much power as possible from its relatively small-capacity engines became a priority. The 906 was the first sports racer to experiment with a Bosch-derived injection system, and competition advances soon found their way into production engines.
Differing from the classic carburettor, fuel injection is pressurised: it replaces entirely the rabbit warren-like series of vessels, jets and floats of the carburettor. The first 911 injection system was virtually a miniaturised motor with twin rows of tiny pistons actuated by a roller cam. This turned on a shaft belt driven from the engine at half crank speed and the effect was an exact dosage of fuel at constant pressure delivered to each of the six cylinders, coinciding precisely with the engine’s combustion cycle. Because the system was closed, not only was there more efficient use of fuel – no possibility of fumes evaporating as they could from a carburettor’s float chamber – but combustion was more complete. Essentially injection offered the precision a carb could not, providing the fuel the engine demanded rather than what it could manage to ingest through a carburettor. Moreover, pressure injection atomised fuel spray far more comprehensively than a carb could, creating more efficient combustion.
Mechanical fuel injection was hardly new – diesel engines (which are of course compression-ignition) had long used it, but its application on production cars was much slower. For one thing, low volume meant it was very expensive to manufacture – Mercedes-benz used it for its stunning straight-eight 300 SLR racer in the 1950s, but not on the majority of the street version. The 300 SL and mechanical injection itself was bulky and heavy as it was derived, like the early turbochargers, from diesel engines. Injection was of only incidental interest to a Zuffenhausen where the comparatively low revving flat four with torque essentially in the midrange rather than at high rpm was perfectly suited to traditional carburation.
A decade later, if the production 911S was producing peak power of 160bhp at 6,600rpm, and if this was more than enough to see off competition, concerns about tail pipe emissions were becoming a serious consideration for Porsche: with half its sales in the US and half that in California where the clean air movement began, reducing hydrocarbon output was becoming a priority. Mechanical injection replaced the triple Webers on the 911S and 911E.
MFI was not a simple changing of a component: if requirements for the competition 906 were one-dimensional, for road use the mechanism needed considerable refinement. A solenoid valve ensured mixture enrichment for starting and another dealt with completely cold starts, while a third solenoid intervened to cut off fuel supply on over run, restoring it only when engine speed had reached 1,300rpm. As with the 906’s injection, the main method of fuel metering was by a circular cam rotated by throttle movement, but instead of the crude slide throttles of the race engine, the production car had six individual butterflies for better control of the metering cam: the 911’s injection was obviously designed to cope with a life largely at part throttle whereas the racer would only ever be driven flat out.
The injection system required few changes to the engine itself beyond some reshaping of the manifold. Both power and torque went up due to fine-tuning that Porsche was able to effect thanks to more precise fuel delivery: the steadier idle of the E allowed re-timing of the ignition through the use of a higher lift cam; on the S, minor redesign of inlet ports raised compression ratio fractionally and peak power from 6,600rpm to 6,800rpm. 170bhp from 2 litres was unprecedented at the time and Porsche took the precaution of adding a second, thermostatically controlled oil cooler to the S.
The injected engines met the 1969 US regulations for a couple of seasons, but only a couple: if pressurised fuel delivery enhanced both combustion efficiency and performance, it could also have an even greater appetite for fuel. With further and stricter tailpipe regulation on the near horizon, as well as the infamous ‘CAFÉ’ corporate average fuel efficiency standards which threatened non-compliant manufacturers with massive fines (as with CO2 today), mechanical fuel injection would disappear from Us-bound Porsches in January 1973 and from European deliveries with the demise of the 2.7S from the 1977 model year.