An agricultural combine (102) includes a rotor (114), a concave (116), a force sensor (134) to indicate the force applied to a crop mat located between the rotor and the concave, a position sensor (132) to indicate the size of a gap (130) between the rotor (114) and the concave (116), an actuator (122, 124) to position the concave (116) with respect to the rotor (114), and an ECU (144) coupled to the sensors (132, 134) and the actuator (122, 124). The ECU (144) contains an economic operating curve (154, 156, 158) that relates the force to the size of the gap. The ECU (144) is configured to receive a signal from the force sensor (134) and a signal from the position sensor (132), and based upon those signals, to control the actuator (122, 124) to change the force and the size of the gap (130) to a point on the economic operating curve (154, 156, 158).
