Computer software, which simulates the thermodynamic and gas dynamic of internal combustion engines, are used extensively during design and development process. This paper analyzes the 1D boundary multi-pipe junctions calculations using the Method of Characteristics (MOC). Sonic flows can be encountered in the exhaust manifolds of internal combustion engines (especially racing engines) and in the model a check if the flow is sonic or not have been made. Flows with more than one manifold have flow toward the junction, need an equivalent "Datum" manifold, with an airflow as the sum of all flows, an averaged area and stagnation enthalpy has been defined in order to calculate the pressure loss when crossing the junction. The pressure loss terms have been calculated as function of the flow-ratio of the gas flowing to the manifold to the total incoming flow and the pipe angle. Such terms take into account of the flow ratio referred to the "Datum" flow and the pipe angle term is the average of all the pressure losses of every duct with incoming flow. The main model used to calculate the wave actions in the manifolds is the Two Step Lax-Wenfroff scheme, second order in space and time with the TVD flux limiter, needed to smooth the instabilities typical of second order hyperbolic schemes. Two set of tests have been designed in order to show the advantages of the present formulation. The first is "Y" junction with an inlet duct. Increasing the inlet pressure, the flow increase up to reach the sonic flow. The second test is a Y junction with 2 inlet ducts with the third duct that goes to sonic flow. A racing engine has also been simulated comparing the results with those from a dynamometer, showing good accordance between model and measured data. Copyright © 2013 SAE International.