Novel functionality, configurability and higher efficiency in automotive systems require sophisticated embedded software as well as distributed software development between manufacturers and control unit suppliers. However, at least for engine control units (ECU), there exists today no well-defined software integration process that satisfies all key requirements of automotive manufacturers. We propose a methodology for safe integration of automotive software functions where required performance information is exchanged while each partner's IP is protected. We claim that, in principle, performance requirements and constraints (timing, memory consumption) for each software component and for the complete ECU can be formally validated, and believe that ultimately such formal analysis will be required for legal certification of an ECU.