To approach the problem of the design of more correct geometries for exhaust pipes, two consecutive analysis are necessary. In the first part of this report it is intended to establish the relationship among the pressure-alpha plots of a point of the exhaust duct near the exhaust port and the fundamental parameters which characterize the scavenging process. In that way, the most suitable pressure-alpha plot for each specific use can be determined via experimental work.In the second part of the report, a calculation model is presented. The model based on the method of characteristics, allows to compute analytically the plots of pressure at the exhaust port versus crankangle for a given exhaust duct configuration. With this calculation model different outlines can be performed analytically and choose the one that complies with the required coefficients of the scavenging process for several engine rpm.