In its original form, the B-339 had a theoretical maximum speed of 323 mph (520 km/h) at a rather unrealistic 21,000 ft (6,400 m), but fuel starvation problems and poor supercharger performance at higher altitudes meant that this figure was never achieved in combat; the B-339E was no different in this regard. Its maneuverability was severely impaired (the aircraft was unable to perform loops), and initial rate of climb was reduced to 2,300 ft/min. The Wright Cyclone 1890-G-105 engine designated for use in the Brewster Mk I was in short supply; many aircraft were fitted with secondhand Wright engines sourced from Douglas DC-3 airliners and rebuilt to G105 or G102A specifications by Wright. In service, some effort was made by at least one Brewster squadron to improve the type's sluggish performance; a few aircraft were lightened by some 1,000 lb (450 kg) by removing armor plate, armored windshields, radios, gun camera, and all other unnecessary equipment, and by replacing the .50 in (12.7 mm) machine guns with .303 in (7.7 mm) machine guns. The fuselage tanks were filled with a minimum of fuel, and run on high-octane aviation petrol where available. At Alor Star airfield in Malaya, the Japanese captured over of high-octane aviation petrol from British forces, which they promptly used in their own fighter aircraft.