This paper describes the use of simulation in supporting system design and in improving manufacturing performance at the Ford powertrain operations, Manufacturing Engineering division. Simulation modelling of engine assembly lines is carried out using the Ford Assembly Simulation Tool (FAST) which is an Excel based front-end user interface to the Witness simulation system. FAST is a component-based Simulation system that enables engineers and non-simulation experts to develop engine assembly models easily and quickly. The model building is also enhanced by the use of specialised sub-models (Witness modules). Assembly line models are automatically built using a Witness shell that imports data from Excel data sheets, produces Witness elements and populates the elements with relevant data. The importance of the quality of input data and their sources has been emphasised. The models are generally verified using expected cyclic throughput values, and validated with real manufacturing data from the shop floor. Validated models are used to assess and enhance performances of engine assembly lines.