The E-1 and E-4 saw the most heavy action during the Battle of Britain and most of the E-3s were already converted to E-4 standard. The fuel-injected DB601 engine of the Bf 109 proved most useful against the British Supermarine Spitfire and Hawker Hurricane fighters, as the British fighters used gravity carburetor engines, which would cut out under negative g forces whereas the DB601 did not. The Bf 109s thus had the initial advantage in dives, either during attack or to escape it was able to get out of gun range. The Spitfire proved a formidable opponent, being approximately as fast and is claimed somewhat more maneuverable in turns at medium to high speeds than the Bf 109 (the latter due to the Bf 109's high wing loading). On the question of comparative turning circles in combat, Spitfires and Hurricanes benefited from their lower wing loading compared with the Bf109; 22 to 24 pounds per square foot on the RAF machines against 32 pounds per square foot for the Bf 109. Royal Aircraft Establishment tests with a captured Bf 109 showed the Spitfire's turning circle without height loss was 696 feet (212 m) in radius (the Hurricane's would be slightly tighter) while the 109's was 885 feet (270 m) radius according to British calculations using assumed values as basis. According to the German manuals however, the smallest turning circle was 170 m, and fighter pilots on both sides claim they would out-turn their opponents in combat. In roll rates the Bf 109 enjoyed an advantage at dogfight speeds, though at high speeds the maneuverability of all three fighters, especially the Spitfire was severely limited in this regard. The Bf 109 enjoyed good handling near stalling speeds as it was particularly forgiving then. Firepower between the antagonists was comparable, with the Spitfire and Hurricane having eight .303 inch machine guns versus the Bf 109's two 7.92 mm MG17 machine guns and two 20 mm MG FF cannon. However, the MG FF occasionally jammed and had a small (60-round) ammunition capacity. To be fair, when the Spitfires were later upgraded to two 20 mm Hispano-Suiza cannon, the British initially had serious jamming problems of their own with the new weapon. RAF pilots who tested captured Bf 109s liked the engine and throttle response but criticised the high speed handling characteristics, poorer turning circle, greater force required on the control column at speed and the thick framing of the cockpit glazing which they felt created blindsots in the pilot's field of vision.