AIR FLOW

    In a carbureted fuel system the intake manifold must strike a happy medium between low and high speed requirements. At idle for instance, air flow is very slight and in order to keep the gasoline mixed with the air, it is necessary to have small passages to keep up the air velocity. On the other hand, when power is required, we would like to have as big a manifold passage as possible, to allow maximum breathing of the engine. Naturally, to supply both of these requirements, the manifold must be a compromise between small and large passages and results in passages of medium size which limit both the low and high speed performance, but provide enough of each to get by.

    In the case of fuel injection, the manifold does not have to carry a fuel/air mixture and, therefore, can be designed to give the best breathing possible. In fact the manifold can be made to actually supercharge the engine at certain speeds. This is done by having a rampipe for each cylinder so that the air on its way to the cylinder will be travelling in a long column, while the valve is open and air is entering the cylinder, the air flow gets quite a lot of momentum in the ram pipe. As the piston reaches bottom dead center and starts back up, air will continue to flow into the cylinder because of air velocity in the ram pipe. At the particular engine speed where the valve just closes as the air stops flowing, an extra charge of air has been trapped in the cylinder. This effect is called dynamic super charging: by design of the ram tubes a particular engine can be picked for this effect to occur and quite a boost in torque results at that particular point.

MIXTURE HEATING

    In a carburetor, mixture temperature is a problem during cold operation and it is necessary to bring exhaust heat through the heat riser in the exhaust manifold up to the base of the carburetor during warm-up. The unfortunate part is that this heat remains even when the engine is warm and often leads to vapor trouble, in addition to requiring periodic service to clean out the heat passages.

    In fuel injection, on the other hand, fuel and air are actually mixed in a very warm part of the engine and no extra heat is required during warm-up.

HORSEPOWER

    Many engineers feel that we are approaching practical limits of carburetion in size of venturi, number of cores, etc. and of course the farther we go in this direction the more difficult it becomes to maintain efficiency in the part throttle operation.

    Since a fuel injection system could supply almost unlimited quantities of fuel and air, more efficient engine performance can be realized with today's engines. This allows considerable room for further advances in engine design.