performance eg fuel flow rate in cylinder pressure torque or it might be derived indirectly via a measure of one or more engine parameters eg speed throttle position in conjunction with a model of engine performance optionally a sensor or sensors 8 that can be used to infer the pressure in the exhaust manifold may form part of the first sensor means and provide inputs to the controller means in some cases the performance measure of the engine may be derivable from this type of sensor alone in which case it may be the only type of sensor in the first sensor means an additional sensor 6 that can be used to define the position of the engine in its cycle may also form an input to the controller this signal can be used to synchronise the output of the controller to the engine 39 s cycle when there is not provided second sensor means it is possible to control only the performance of the engine however the radiated sound could be reduced by a standard passive attenuator placed downstream of the actuator thereby passively improving the noise performance the passive attenuator may be a conventional silencer adapted to reduce high frequency noise the control system of the invention being adapted to deal with low frequency noise this embodiment of the invention is shown in fig2 this embodiment is different from that of fig1 wherein the actuator means is extended to include actuators 4a and a second sensor means 9 that measures the pressure close to the exit of the exhaust is provided normally only a single sensor would be.