A method of operating an IC engine in an agricultural harvester includes the steps of: operating the IC engine in a normal mode with a base torque curve as a function of engine operating speed and engine power output, the base torque curve being generally isochronous at a rated operating speed over a power output range terminating at a rated power output; and operating the IC engine in a boost mode with a boost torque curve when a power boost is required above the rated power output, the boost torque curve having a power output which is above the base torque curve over a predefined range of the operating speed.