NACA cowling
The Mark IV introduced the much more powerful 825 hp (615 kW) Bristol Perseus sleeve valve radial engine enclosed in a NACA cowling which significantly improved performance, increasing maximum speed to 156 mph (251 km/h) and rate of climb to 840 ft/min (4.3 m/s). In this version, the Perseus had overheating problems and was deemed unsuitable for tropical service with production limited to 18 aircraft, all of which served with the home based squadrons.