An engine testing system has a speed control system which controls at least one of an engine speed and a dynamo speed. The control system comprises a mechanical transfer function which receives an engine torque and a dynamo torque and outputs an engine speed, the axial torque and a dynamo speed, an electric transfer function which receives a command dynamo torque and outputs the dynamo torque, and a speed controller which receives one of a command engine speed and a command dynamo speed and at least one of the engine speed and the dynamo speed, and outputs the command dynamo torque. A transfer function representative of the speed controller is designed using the structured singular value synthesis method so as to be adapted to the mechanical transfer function and the electric transfer function.