This paper demonstrates the use of a system level model that includes torsional models of a Cummins diesel engine and an Allison transmission to study and improve system NVH behavior. The study is a case where the two suppliers of key powertrain components, Cummins Inc. and Allison Transmission Inc., have collaborated to solve an observed NVH problem for a vehicle customer. A common commercial tool, Siemens' AMESim, was used to develop the drivetrain torsional system model. This paper describes a method of modelling and calibration of baseline engine and transmission models to identify the source of vibration. Natural frequencies, modal shapes, and forced response were calculated for each vehicle drive gear ratio to study the torsional vibration. Several parametric studies such as damping, inertia, and stiffness were carried out to understand their impact on torsional vibration of the system. The best solution was then selected based on the system study and implemented in the vehicle for validation.Finally, the aim of this paper is to demonstrate the successful use of an analysis led design approach by collaborating suppliers (Cummins and Allison) to identify drivetrain NVH problems and options for solutions. Such an approach can be used in early phases of vehicle integration to achieve optimized drivetrain system design, to the benefit of the end customer and the component suppliers.