Globalization has increased competition in the automotive industry and the search for new products, technologies and, in special, leaner and more economically viable manufacturing processes. In this scenario, pressures to reduce the development time of new products have made it necessary to also reduce the development time of new manufacturing processes. Manufacturing development and planning are complex processes as they engage different areas of the company, require high investment and involve different technologies. Manufacturing engineering is the area in charge of planning, project and implementation of new manufacturing for new products. The central aim of this study is the conceptualization and application of computational simulation of processes applied to the planning and project phases of manual and automated assembly lines by means of computational tools. The focus will lie on the body shop area, where the highest degree of automation and complexity are to be found. The advantages obtained in the planning and project phases are the reduction in investments, reduction in the time to introduce a new process, early identification of production bottlenecks, and definition of the required quantity of intermediary stocks, which directly influence process times and variable costs. The concepts proposed here are validated by case studies, with the presentation of actual data concerning the launch of two new models, one in an existing production line and the other in a line specifically developed for the model.