A rotordynamic study for an engine with an ultra-high bypass ratio (BPR=17) was performed using a multibody (Simpack) and a finite element (Ansys) description. The previous validation based on an analytical system of a coaxial counter rotating twin rotor model was performed to legitimize the use of Simpack and the methodological approach, which included the modal reduction of finite element bodies and their integration in the multibody description. Therefore the eigenfrequencies as a function of the rotational speed and the unbalance respond due to eccentricity were compared. The same methodology was then applied to the UHBR engine. Performing an evaluation of the critical speed map, Campbell diagram and an unbalance respond analysis to identify the system behavior. By examining the system respond due to an unbalance force the need of a full modal description of the rotating parts with respect to a suspension point of the complete engine was shown.