This paper presents a methodology for tool-assisted development of software for electronic control units in cars. Because of increasing complexity the developer requires a set of tools that cover the whole development process.In this paper the process is separated into several phases. Ideally after analysis there is a specification phase, where executable but hardware independent models are created. After that in the Design phase the hardware architecture is designed and partitioning of the models onto the network of ECUs is performed. Here the appropriate architecture is chosen.This paper shows the seamless methodology from the Specification phase tool ‘ASCET-SD’ to the Design phase tool ‘Virtual Component Co-Design’ (VCC) with respect to the overall design flow. Several ways of doing the transition considering intellectual property protection, target dependency accordingly independency, and model semantics are considered.It is interesting what information can be translated from ASCET-SD to VCC. VCC provides three ways of importing models: A model imported as Clearbox can be simulated, automatically estimated and synthesized to hardware. Whitebox models can be simulated and analysed but not synthesized, while Blackbox models can only be simulated. But Blackbox import mechanism provides intellectual property protection, which is one of the most important aspects in automotive industry, because of the enormous number of suppliers. Further in this paper it will be discussed on which level of granularity the model transition should be carried out to achieve preservation of model semantics.