The complexity of automobile powertrains grows continuously. At the same time, development time and budget are limited. Shifting development tasks to earlier phases (frontloading) increases the efficiency by utilizing test benches instead of prototype vehicles (road-to-rig approach). Early system verification of powertrain components requires a closed-loop coupling to real-time simulation models, comparable to hardware-in-the-loop testing (HiL). The international research project Advanced Co-Simulation Open System Architecture (ACOSAR) has the goal to develop a non-proprietary communication architecture between real-time and non-real-time systems in order to speed up the commissioning process and to decrease the monetary effort for testing and validation. One major outcome will be a generic interface for coupling different simulation tools and real-time systems (e.g. HiL simulators or test benches). In this paper, we show the seamless transition from a purely simulated vehicle in a MiL (model in the loop) co-simulation to a heterogeneous testing scenario with an engine test bench linked to real-time models. First, the offline co-simulation consisting of a GT-POWER engine model, a SimulationX transmission model and a dSPACE ASM vehicle dynamics model is set up. All models are parametrized based on extensive vehicle measurements. Second, the engine model is substituted by an engine test bench which is coupled with the same real-time simulation of the transmission and the vehicle. A suitable closed-loop interface between the simulation, the dSPACE HiL simulator SCALEXIO® and a test bench automation system is created. Finally, the simulation results and test bench measurements are compared with a real test vehicle. The results show a good congruence and a high potential for the use of test benches and MiL co-simulation in early project phases. Standardization requirements for a real-time capable simulation interface are derived from this use case.