Technology explained: Carburettors
From the Solex of pre-1967 cars, to the Webers of early 911Ss and Zeniths of later 911Ts, the air/fuel mixture in Porsche’s early flat six engines was fed into each cylinder by a pair of three-choke carburettors.
A carburettor’s job is to mix together air and fuel before it is pulled into the cylinder during the intake phase of the internal combustion cycle. Making use of the Bernoulli principle (the same scientific tenet that explains how an aerofoil creates lift/downforce), the inside of a carburettor is shaped like an hourglass, with a narrow section at its centre creating a venturi.
At this narrowing, the air travelling through the carburettor is forced to speed up. When this happens, thanks to Bernoulli’s Principle, the higher velocity air creates a low-pressure zone, forcing a vacuum effect that draws petrol out of the float chamber, through a jet and into the intake tract of the carburettor.
Airflow through a traditional carburettor is controlled by two butterfly valves. On a downdraft unit, where air flows in from the top-mounted velocity stacks as per a Porsche 911, the uppermost butterfly valve, mounted about the venturi, is the choke.
This is used to prevent airflow during cold starts when engines need a rich mixture (more fuel per unit of air) in order to start. However, in all carburetted 911s, the choke is removed, simply leaving the throttle butterfly valve mounted below the venturi and main intake jet.
Whenever the intake valves in the cylinder head opens, the suction created by the downward movement of the piston, sucks air into the top of the carburettor.
As the throttle pedal is pressed, the valve inside the carburettor rotates, increasing allowing more air to flow through the venturi, sucking in more fuel from the float chamber.
Mounted to the side of the carburettor’s body, the float chamber is a reservoir filled with fuel, from which the petrol is sucked into the venturi. The chamber’s name comes from the float that is used to control the level of fuel stored inside the reservoir. As the float drops, it pulls down on the float arm, opening up a float valve. Fuel can then be fed into the float chamber via the pressurised fuel line.
Carburettors were a simple mechanical system for mixing fuel and air however, Porsche quickly began phasing them out in favour of mechanical fuel injection in a move designed to improve throttle response.
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