The amplitude of the pressure fluctuation was found to be influenced by the separation between the intake pipes in the airbox. For an n-cylinder, even-firing engine, if the intakes are coincident in the airbox, then the fundamental and all harmonics of the forcing function, apart from the (n-1)th, (2n-1)th, etc., will cancel. That is, only the multi-cylinder induction frequency and its multiples will not cancel. If the intakes are spatially separated, as in a real airbox, complete cancellation of the harmonics in the forcing function will not occur. The degree of cancellation depends on the separation distance of the intakes.
Three different intake pipe configurations were investigated numerically and experimentally. The experimental results showed good correlation with the 1D/3D predictions. At an engine speed where the single-cylinder induction frequency matched the natural frequency of the airbox, the amplitude of the low-frequency pressure fluctuation inside the airbox was found to increase as the separation of the intake pipes was increased.