The use of the throttle valve in the intake of the spark ignition engine is one reason for the lower part load efficiency of this engine when compared with the compression ignition engine. The need for the throttle valve can be avoided by the disablement of individual cylinders.A microprocessor is being used to operate on the fuel injection to the inlet ports as a means to disable the cylinders. It is being programmed such that the cylinders disabled, cycle by cycle, can be varied in order to keep all the cylinders hot and to minimise the amplititude of the vibrations of the power unit on its mountings. Further, the number of cylinders disabled, cycle by cycle, can be varied in order to provide a fine control of engine power output.The control system is described and the results of the experimental test work are presented. The paper also contains the theoretical modelling of the dynamics of the power unit on its mountings excited by the range of forcing torques expected from an engine with disabled cylinders. A six degree of freedom model is being used.