Main aim of this paper was to exploit the well-known Willans line method in a twofold manner: indeed, beyond the usual identification of Willans line parameters to enable internal combustion engine scaling, it is also proposed to infer further information from identified parameters and correlations, particularly aiming at characterizing mechanical and frictional losses of different engine technologies. The above objectives were pursued relying on extended experimental performance data, which were gathered on different engine families, including turbo-charged Diesel and naturally aspirated gasoline engines. The matching between Willans line scaled performance and experimental ones was extensively tested, thus allowing to reliably proceed to the subsequent objective of characterizing mechanical losses on the basis of identified Willans parameters. This latter task was accomplished by comparing Willans derived losses to those estimated via the Bishop model, extensively adopted in the related literature to predict such a key performance variable. The comparisons highlight the usefulness of Willans line-based estimation of mechanical losses as a function of engine speed, thus also contributing to adding new exploitation means to a key methodology for optimal design of advanced automotive propulsion systems, in view of prospective diffusion of optimally sized and cost-effective solutions for the reduction of CO2 emissions, including hybridization devices