Wednesday, 29 March 2017

The SU float-chamber.

The following is a short blog relating to that most humble of instruments, the SU float chamber.

Without wishing to state the obvious, a float chamber’s primary purpose is to provide a constant head of fuel to the carburettor under all circumstances. Clearly there will be a difference in the head of a gravity fed fuel system, between the tank full and the tank empty. Less clear are the effects due to an ever changing demand for fuel under engine running conditions and the gravitational forces generated under heavy acceleration, braking and cornering. Quite simply, it is impossible to calibrate a carburettor unless you can ensure a constant fuel supply.

The SU did this very well.
The design was simple, it was easy to manufacture and was robust in use. All the above guaranteed it had a long life, both in terms of production and under normal working use. It was available in many different sizes and forms, but the design was always basically the same. As such it was fitted to a large proportion of UK car production, from when it was initially conceived in 1910, up until its demise due to the coming of fuel injection in the 1980s.

The SU float chamber was designed to be pressure fed, either from a mechanical fuel pump driven off an eccentric lobe on the camshaft, or from an electrical pump wired to the vehicle power supply. For this purpose SU manufactured a range of constant pressure electric pumps with differing flow rates to suit all applications. The fuel is ‘top-fed’ via a gauze filter into the lid of the float chamber. Here a pivoted fork controls the opening and closing of a needle valve, which allows the fuel to enter and fill the float chamber. This causes the hollow brass float to rise, closing the valve and halting the fuel supply when the correct level is attained. The fuel is ‘bottom-fed’ to the carburettor and as the supply within the chamber is used, the level drops opening the needle valve once again. The design of the later series of SU float chambers was amended to incorporate a pivoted a plastic float within the chamber lid, but the operating principle remained the same.

Due to simple functionality, the SU float chamber was also used for a great number of applications for which it was never intended. It was a very popular choice in motorcycle racing, both solos and sidecars. On a solo it would normally be gravity fed from the fuel tank above, but on a sidecar two float chambers would be employed, one either side of the carburettor. This would ensure the carburettor could never be deprived of fuel due to the corning forces generated on a three-wheeled machine. The fuel was normally supplied to these chambers via a fuel pump of the type described previously.