The influences of VVT on engine power performance were well summarized in previous studies from various aspects. However, most of the previous investigations were qualitative analysis and focused on the total effects of VVT without decoupling the influence factors. The primary cause is due to lack of an effective mathematic model to predict the IMEP of VVT engine. In this study, an approach to detect the VVT signals of engine was proposed, and the influence factors of power performance of VVT engines were quantitatively studied. The fundamental relationships among IMEP and its influencing parameters, i.e., intake density, intake VVT, RGF and thermal efficiency, were decoupled and the governing equation of IMEP was derived with the influences of VVT considered, the calculated results through which are in good agreement with tested data. Results indicate that, IMEP is directly proportional to effective cylinder volume at IVC timing and inversely proportional to RGF. The change rate of RGF influence coefficient is almost three times as that of RGF, and the combined effects of these parameters on IMEP equals the product of the coefficients of four influence factors. All these extended engine thermodynamics and provided theory basis for improving VVT engine's power performance. (C) 2018 Elsevier Ltd. All rights reserved.