indicative of the speed at which the engine 11 is operated this engine speed signal is transferred to a computer circuit shown schematically at 43 that is programmed with the control for the main exhaust control valve 32 and the auxiliary exhaust control valves 35 these control signals include a timing varying signal e that is transmitted to the servo motor 34 for the main control valve 32 and either an opening control signal f 1 or a closing control signal f 2 to the servo motor 37 for the auxiliary control valves 35 although a wide variety of sequensive operations may be employed in conjunction with the invention a particular sequence is shown in fig6 which is fairly typical of the way the invention may be practiced as will be noted from the curve a the auxiliary control valves 35 are normally maintained in a closed position until the engine reaches a predetermined speed 8 000 rpm in the illustrated embodiment at which time the valves 35 are fully opened on the other hand the main control valve 32 is normally held in its lower most position h 1 which provides the maximum retard of the opening of the main exhaust port 26 at low and mid range speeds at some speed 5 000 rpm in the illustrated embodiment the main control valve 32 is opened with a ramp like function until the valve is fully opened at some pre determined higher range speed however in accordance with the invention and to avoid the.