The original contribution of this paper in this field of research is the successful thermodynamical modelling of a complex geometry of a Diesel Engine utilizing the Method of Characteristics. For this purpose, a 16 cylinder, ‘V’ type, turbocharged, 4 ported (2-intake, 2-exhaust) High Speed Marine Diesel Engine was tested at 1900 rpm. For the analytical investigation, one dimensional, time dependant gas flow throughout the inlet and exhaust manifolds of a Marine Diesel Engine is shown to be successfully modelled by the non homentropic gas flow theory. In order to model the four ports on a cylinder, the narrow passages towards the ports inside the cylinder head are considered as a couple of short ducts connected to each other through a junction. Analytical and experimental results are compared with the gas pressure variations at the middle of the inlet pipes before cylinders and in the corresponding cylinders. It is concluded that the theory and assumptions mentioned in the paper for CAD research works are more economical and effective compared with the multi dimensional, modelling because of the storage capacity and calculation speed of the computer.