By proper choice of the length of the intake and exhaust piping, the performance of internal combustion engines can be increased. Inlet and exhaust valve opening and closing creates a compressible flow process in which pressure waves flow back and forth through the inlet and exhaust system. The appropriate pipe length can be estimated through solution of the compressible flow equations. Alternatively, a number of heuristic pipe length equations have been developed.
A pressure wave is created when an intake or exhaust valve is opened. The wave propagates through the the fluid in the pipe at the speed of sound c. When this wave encounters a change in cross sectional area, such as the end of the pipe, a wave of opposite sign will be reflected from the end of the pipe. Based on the time it takes for this wave to return back to the valve, the length of the pipe can be determined.
For example,when the inlet valve opens, a rarefaction wave is sent upstream from the valve. When this wave encounters a change in area such as the intake manifold, a compression wave is generated and sent downstream back to the inlet valve. This compression wave increases the local density of the inlet flow, a process called the "ram effect".
Experimentally, it has been found that a significant gain in volumetric efficiency is attained when the reflected compression
wave returns when the piston is at a crank angle of 90o. At this
point the piston velocity is maximum. Matching the time it takes for the
wave to return with the characteristic piston time, the required length of the pipe can be
The velocity of the wave is given by: