Large diesel engines are a challenging product for engineer: they are large, heavy, dangerous. Modelling is lengthy and cumbersome, testing is expensive and specific. In this paper, the authors will highlight some of these problems and their influences in the correlation between test and numerical data set, and at the same time show that a good match between numerical simulations and test can be achieved with the appropriate procedure. The paper reports about the analysis and updating of a "genset", an aggregate formed by tree main parts: the diesel engine, the electric generator and the common base frame. These three elements combined weight roughly 200 tons, occupying a space of about 260 cubic meters.