For vibration and acoustics vehicle development, one of the main challenges is the identification and the analysis of the noise sources, which is required in order to increase the driving comfort and to meet the stringent legislative requirements for the vehicle noise emission. Transfer Path Analysis (TPA) is a fairly well established technique for estimating and ranking individual low-frequency noise or vibration contributions via the different transmission paths. This technique is commonly applied on test measurements, based on prototypes, at the end of the design process. In order to apply such methodology already within the design process, a contribution analysis method based on dynamic substructuring of a multibody system is proposed with the aim of improving the quality of the design process for vehicle NVH assessment and to shorten development time and cost.The methodology here proposed is applied to assess vibrational contributions of an internal combustion engine without considering the corresponding Frequency Response Functions (FRF). Hence, the different excitations of the system (e.g. combustion, piston-liner interaction, bearing contact forces, etc.) are directly applied on the investigated geometry and their contributions are computed through numerical simulation. A comparison of the influence on the overall vibrations of the different excitations acting on an I3 engine will be done for structural vibration paths. The applicability and the accuracy of the methodology is finally discussed with reference to experimental measurements of a V6 engine. Two variants of the engine are investigated: the main differences between the two engine variants, mostly associated with piston-liner interaction forces, are investigated at mid-low frequency main engine orders for the entire speed range at full load condition. The proposed methodology is performed to assess the influence of these differences in terms of acceleration level on the external surface of the numerical model of the engine.