A modelling study is presented in this paper, whose objective is to obtain design criteria for optimum layouts and dimensions of exhaust manifolds in automotive engines. The first step has been the characterisation of the pulsating flow phenomena in the exhaust manifold, focusing on the pressure wave propagation process and on the interaction between cylinders across the manifold. Two relevant phenomena have been studied: the reflection of under-pressure pulses at pipe junctions and open ends, and the interference between exhaust processes of different cylinders. These phenomena have been characterised respectively by non-dimensional parameters, related to the layout and dimensions of the manifold. A parametric modelling study has been performed in order to evaluate the effects of the manifold dimensions on the engine performance. The work has been focused on a four-cylinder engine with a four-branch manifold. Significant results have been obtained from the research and some are clear design criteria, for the manifold dimensions, in terms of key values of the described non-dimensional parameters.