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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Comments (1)

  • Stijn Van Damn

    There are a number of inaccuracies.

    – Increased injection pressure, compared to a curburettor
    A carburettor does not inject fuel under pressure now does it?
    The entire point is Carburettors have venturis and the engine sucks fuel in with the intake air.
    Down draft or side draft. there is no pressure involved, but vacuum. It’s the opposite of pressure.

    -creates greater atomisation of the air/fuel mixture
    No, The high pressure atomises the fuel.
    Not the mixture.
    It’s because of the atomisation that the fuel then mixes up better with the air.

    The result of this is a more a better mixture which after ignition results in a more efficient combustion.
    Practically speaking, you can mix more fuel in the same amount of air so the bang is stronger.
    A Carburettor might get the same amount of fuel in the chamber, but the mix would not be good and it would waste a lot of it in the exhaust.

    -It’s not an ever changing fuel map, in fact, it’s quit unchanging by nature.
    the 3D space cam is specific for the camshafts of the engine.
    And then the pump is calibrated to match the profile of the engine some more.

    It is not “ever changing” that’s the entire problem for MFI, everything has to be set just right for it to work.
    But when it works, it’s glorious.

    – The Solenoid valve does not enrich the pump, it just dumps raw fuel in the intake stack.
    The Mechanical thermostat, that works off the exhaust heat, that one will enrich the pump during warmup and then lean it out as the engine warms up.

    – while a solenoid valve provides automatic cold-start enrichment, doing away with the need for a manual choke.
    Except there is still an extra handle next to the handbrake.. one for the flapperbox one to enrich the engine (3 enrichments, solenoid, warmup valve, manual)


    differs on ‘E’, ‘S’ and ‘RS’ variants.

    Actually no

    It differs between the T the E and the S/RS

    the S and RS have the same camshaft on the engine, the pump has the same spacecam, only the calibration of the pump differs.

    The Space cam is the same.
    An S pump can be recalibrated to be an RS pump and vice versa
    You cannot recalibrate a T or E pump to run an S or RS engine.

    Well you can try, but you will not be able to tune your MFI to be right across the powerband.
    You’ll have to find a compromise, pick which rpm is important and accept it won’t run well on other RPM’s.