An Automotive Exhaust System Structural Key Life Test has been successfully developed, in part, due to investigations into the boundary conditions of powertrain input. The powertrain (engine and trans-mission) degree-of-freedom study (here after referred to as engine) was investigated in order to determine the sensitivity of the exhaust system to engine motion. Understanding engine motion was necessary in order to establish proper control strategy in the laboratory simulation process. Accurate reproduction of exhaust system response to input road load events was crucial to reproducing known exhaust system fracture modes in early life wear-out conditions.A method multiple coherence analysis has been used to analytically measure the degree of severity between engine input motion and exhaust system output response by analyzing dynamic strain and acceleration. Removing one engine control input at a time, a multiple coherence function was calculated and the exhaust response computed. In all, eighteen case studies were investigated for one vehicle line. Results showed that engine longitudinal translation, and roll were not as critical as engine vertical, lateral, pitch, and yaw motions. This information was then used to drive the real time simulation control strategies for engine motion.