Just how important the overall intake system is for the mixture formation and the engine's performance delivery, was only recognised relatively late. In racing vehicles it was different. The shape that the choke-funnel should have can be seen in the picture right at the bottom, it is shaped like this so that the air flow does not lift off it's walls, thus avoiding the development of a vortex.
The manufacturers had already started testing with the exhaust system much earlier. Also the truck Diesel direct-injector was equipped with swirl-ports almost from the beginning. In most petrol engines, the inlet-manifold only differs then, when instead of a single- a register carburetor has been installed. In four-cylinder engines with two-, or six-cylinder engines with three twin side-draught carburetors, there is hardly any inlet-manifold left at all.
The only rule that was valid was: - a long inlet manifold provides fuller torque (lower RPM-range), - a shorter inlet manifold provides higher performance (upper RPM-range).
Nowadays, one wants both, and this is even achieved through an inlet-system which is, either in stages or continuously adjustable. The (multi-point-) injection system also offers far more design possibilities. Quite often there are two successively opening throttle valves, to provide the continuous performance thrust. In the meantime, both the throttle valve(s) (E-Gas) and also the intake length-varying are controlled by the engine management.
A very imortant phenomenon in the creation of inlet manifolds, is the resonance effect, which is also used in the exhaust system. Particular attention must be paid to this when deciding on the length of the piping. Just how important it is, is shown by the fact that the BMW company, even years after the changeover to air-mass-meters, still remained with air-volume-meters for their four-cylinder models. The reason why, was that at certain operational ranges, the oscillations of the fresh-gas column were so strong, that they were registered several times by the far more sensitive air-mass-meters, thus they were incorrectly measured. It is probably still the same today, however, the mismeasurement is taken into consideration electronically.
Additional amounts of air, for an improved filling, can be helpful in combustion engines. In the Helmholtz-Resonator, through the elasticity of a larger volume of air, and together with the inertia of the air-column, one can achieve autonomic oscillations which supply more air to the individual cylinders. This type of system is found, e.g., in the Porsche Carrera GT.
Not only the oscillations in the intake system (dynamic charging) are important. In the meantime, even the outside air-stream is used, more or less, for charging. While two-track vehicles mostly suck in the air from the side or from the wheel housings, motorcycles gather the enormous wind pressure occurring at higher speeds in Ram-Air boxes which then supply the engine. To achieve this, a special design of front fairing is necessary. The air box was probably spearheaded by motor-racing (Formula 1). 09/11