This paper investigates the effect of the powertrain mounting system on the linear and nonlinear torsional dynamical behaviour of a transmission system. To this aim, two dynamic models, namely rigid mounts model and flexible mounts model, are presented and compared, the first considering only the torsional dynamics of the transmission and driveline, the second considering also a 3 degrees-of-freedom powertrain block dynamics. The mechanical coupling and interaction between the powertrain block and transmission system is discussed and formulated. These models are then analysed in terms of vibrational mode shapes, natural frequencies and Frequency Response Functions (FRFs), considering also the sensitivity to the main transmission parameters on the system vibrations. From time and frequency domain comparison of the two simulated configurations (with and without powertrain mounts) a significant interaction between the two subsystem has been noticed which is particularly visible in the vehicle acceleration signal. Neglecting the powertrain mount may lead to underestimating the real vibration level. A nonlinear model of a dual clutch transmission, which takes into account the main backlashes present along the transmission path (clutch spline, gear meshes, synchronizers), is coupled to the powertrain mounting system model. The effect of mounts damping on transmission NVH during critical manoeuvres is finally shown.