Computer programs to simulate the gas dynamics of internal combustion engines are commonly used by manufacturers to aid optimization. These programs are typically one-dimensional and complex flow features are included as ‘special’ boundaries. One such boundary is the ‘pressure-loss’ junction model, which allows the inclusion of directionality effects brought about by the geometry of a manifold junction. The pressure-loss junction model requires empirical, steady-flow pressure-loss data, which is both time consuming and expensive to obtain, and also requires the junction to be manufactured before its performance can be established. This paper presents a technique for estimating the steady-flow data, thus obviating the need to perform these flow-tests.