Automotive systems often employ a variety of intercoupled embedded controllers. To ensure that the controller actions are coordinated and that unwanted interaction does not occur, system testing is necessary over the entire operating range of the automotive systems. A cost effective, safe, and convenient means to perform much of the testing of embedded controller code is to use real-time simulation for all or major parts of the automotive system. However, the integration of various automotive system models and controllers into a single simulation model for hard real-time is a difficult task which is complicated by the usual approach of synchronizing the various computational tasks and I/O activites. This paper describes a new methodology involving asynchronous operation of processors and processes. The methodology produces substantial simplification and gains in efficiency for both real-time simulations interfaced to actual hardware and simulations involving multiple subsystems which must be interfaced. The automotive application used to demonstrate the asynchronous simulation technique is composed of an event-based engine model interfaced to a nonlinear 17-degree-of-freedom vehicle dynamics model running in real-time on two processors.