Mechanical Fuel injection Explained
Carburetors were the age-old method of combining the petrol-air mixture for combustion engines and Porsche persevered with them until the late 1960s, when US emissions regulations prompted a switch to fuel injection. The carburetor works by the suction created by the downward stroke of the pistons, with fuel delivered by a pump usually running off the crankshaft; fuel injection by contrast is a closed, pressurized delivery system. The first Boschderived injection system for the 911 was virtually a miniaturized 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 911’s six cylinders, coinciding precisely with the engine’s combustion cycle. 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 carburetor.
Injection systems were nothing new: initially developed for diesel mixture induction, they were adapted for petrol during WWII, in which Bosch made the injection systems for the Luftwaffe’s fighter planes. In the decade after the war, the injected F1 and sports racers of Mercedes were unmatchable. When Mercedes retired from the sport, Birmingham-based Lucas Industries took up the injection idea and its indirect injection would become the de facto fuel system of F1 for 20 years.
Rarely an early adopter, Porsche did not even go the injection route in motor sport until the 906 racer. However, for B-series 911s, Ferdinand Piëch had development manager Paul Hensler modify the 906’s Bosch fuel system for production. For road use, the mechanism needed several refinements. 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 overrun, 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. Instead of the slightly 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 designed to cope with a life largely at part throttle whereas the racer would only ever be driven flat out.
Barely visible beneath the air filter, the injection system required few changes to the engine compartment beyond some reshaping of the manifold. The injected engines met the 1969 US regulations so the complete 911 range could once more be sold there. While the 911T remained carb fed, the 1969 E and S with their ‘plunger pump’ injection both gained 10bhp. This increase was due to the changes Porsche was able to make thanks to the more precise fuel delivery. 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.
Mechanical fuel injection was standard production on the 911 until model year 1974. After the major revisions of the ‘B’ series, the ‘G’ series represented the next substantive change, the most obvious manifestation being the famous impact bumpers. Under the hood, the most important change was the advent of Bosch’s K-Jetronic injection. This had first been marketed on the US version of the 911T the previous year and was now extended to what had been the middle-ranking E, now called the 911S. The top-flight model was renamed the Carrera and it would keep its 2.7-litre 210bhp plunger injection engine another year, until the advent of the ‘I’ series Carrera 3-litre for MY 1976.
Emissions and the advent of the catalytic converter sounded the death knell for mechanical fuel injection. Sensitive though it was to engine demands as well as being able to strengthen the mixture at altitude, MFI lacked the finesse to achieve the delicate exhaust balance required by the noble metals of the catalyst. Even if Europe resisted the catalytic converter until 1993, Porsche opted to build largely to US specification. Fans used to the zest of the flat six complained the K-Jetronic took the edge off top-end performance and electronic metering restricted fuel flow and the way the engine revved. Progress in electronics overcame this on the 1983 L-Jetronic Carrera 3.2, top-end breathing almost free as it had been with MFI, yet the 3.2-litre car returned 24mpg against the 2.7-litre MFI’s 18.
MFI was a child of its time, but it was inevitable the precision of electronic control would succeed it and offer both better acceleration and emissions performance while using less fuel.
Injection systems were nothing new: initially developed for diesel mixture induction, they were adapted for petrol during WWII, in which Bosch made the injection systems for the Luftwaffe’s fighter planes. In the decade after the war, the injected F1 and sports racers of Mercedes were unmatchable. When Mercedes retired from the sport, Birmingham-based Lucas Industries took up the injection idea and its indirect injection would become the de facto fuel system of F1 for 20 years.
Rarely an early adopter, Porsche did not even go the injection route in motor sport until the 906 racer. However, for B-series 911s, Ferdinand Piëch had development manager Paul Hensler modify the 906’s Bosch fuel system for production. For road use, the mechanism needed several refinements. 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 overrun, 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. Instead of the slightly 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 designed to cope with a life largely at part throttle whereas the racer would only ever be driven flat out.
Barely visible beneath the air filter, the injection system required few changes to the engine compartment beyond some reshaping of the manifold. The injected engines met the 1969 US regulations so the complete 911 range could once more be sold there. While the 911T remained carb fed, the 1969 E and S with their ‘plunger pump’ injection both gained 10bhp. This increase was due to the changes Porsche was able to make thanks to the more precise fuel delivery. 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.
Mechanical fuel injection was standard production on the 911 until model year 1974. After the major revisions of the ‘B’ series, the ‘G’ series represented the next substantive change, the most obvious manifestation being the famous impact bumpers. Under the hood, the most important change was the advent of Bosch’s K-Jetronic injection. This had first been marketed on the US version of the 911T the previous year and was now extended to what had been the middle-ranking E, now called the 911S. The top-flight model was renamed the Carrera and it would keep its 2.7-litre 210bhp plunger injection engine another year, until the advent of the ‘I’ series Carrera 3-litre for MY 1976.
Emissions and the advent of the catalytic converter sounded the death knell for mechanical fuel injection. Sensitive though it was to engine demands as well as being able to strengthen the mixture at altitude, MFI lacked the finesse to achieve the delicate exhaust balance required by the noble metals of the catalyst. Even if Europe resisted the catalytic converter until 1993, Porsche opted to build largely to US specification. Fans used to the zest of the flat six complained the K-Jetronic took the edge off top-end performance and electronic metering restricted fuel flow and the way the engine revved. Progress in electronics overcame this on the 1983 L-Jetronic Carrera 3.2, top-end breathing almost free as it had been with MFI, yet the 3.2-litre car returned 24mpg against the 2.7-litre MFI’s 18.
MFI was a child of its time, but it was inevitable the precision of electronic control would succeed it and offer both better acceleration and emissions performance while using less fuel.
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