Technology explained: Mechanical Fuel Injection

While early Porsche 911s used carburettors to regulate the air/fuel mixture, changing emissions regulations at the end of the 1960s and the quest for greater power soon saw Zuffenhausen turn to mechanical fuel injection on its iconic rear-engined sports car.

Originally introduced on the 906 sports racer, the MFI fuel pump (developed by Kugelfischer and later built by Bosch) was later fitted to the 911R before making its road-going debut on the 2.0-litre Porsche 911S of 1969.

Built in an age before electronic sensors and regulators, the MFI pump uses a series of mechanical ‘sensors’ to create an ever-changing fuel map based on throttle position, engine speed and barometric pressure.

Orange Porsche 911 2.7 Carrera

The bottom half of the Kugelfischer pump features a camshaft (phased to match the engine’s firing order) while in the top half, six plungers – one for each cylinder – sit in their own individual barrels.

As the pump’s camshaft (belt-driven off of the crankshaft) spins, it acts on tappets that in turn operate the plungers via pushrods. As each plunger drops, it exposes a suction valve, where fuel is drawn through towards the injectors. A return spring keeps the plunger in position when closed.

Injected at a pressure of between 225-250psi, the increased injection pressure of the MFI unit (compared to a carburettor) creates greater atomisation of the fuel. The result of this is a more even flame front during ignition, producing more efficient combustion.

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To regulate the amount of fuel depending on the throttle position, a pull rod on the throttle linkage adjusts a 3D ‘space cam’ housed in the bottom of the pump. The irregular profile of this cam was shaped to match Porsche’s desired fuel map for each 911 and differs on ‘E’, ‘S’ and ‘RS’ variants.

Via a rack-and-pinion gear, the space cam rotates the plungers on a corkscrew-like trajectory, providing more or less fuel during their movement within the barrels.

On top of this a centrifugal governor, connected to the camshaft, helped to regulate the overall fuel flow through the pump depending on the engine speed, while a solenoid valve provides automatic cold-start enrichment, doing away with the need for a manual choke.

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