The Mirage IV is powered by two Snecma Atar turbojets, fed by two air intakes on either side of the fuselage that had intake half-cone shock diffusers, known as souris ("mice"), which were moved forward as speed increased to trim the inlet for the shock wave angle. It can reach high supersonic speeds: the aircraft is redlined at Mach 2.2 at altitude because of airframe temperature restrictions, although it is capable of higher speeds. While broadly similar to the model used on the Mirage III, the Atar engine had a greater airflow and an elevated overspeed limit from 8,400 rpm to 8,700 rpm for greater thrust during high altitude supersonic flight. While the first Mirage IV prototype was fitted with double-eyelid engine nozzles, production aircraft featured a complicated variable geometry nozzle that automatically varied in response to the descent rate and airspeed.