During the Battle of Britain, the RAF and the Luftwaffe realized that fighter quality was a major contributor to air superiority, and it became necessary to improve existing fighters and develop new ones at a rapid pace. Even a slight performance advantage could make a difference.

Increasing engine power was the main driver to improve WW2 day fighter performance with aerodynamic cleanness a more distant second. More power can provide more thrust and thence more speed and rate of climb. It permits increased payloads for weaponry and fuel. The price to pay is (among others) increased takeoff weight which raises stall and approach speeds because the wing area and profile of the Spitfire and Bf 109 did not change much during the war.

The Bf 109E-4 entered service in early 1940 powered by a Daimler Benz DB-601Aa engine which delivered 1050 hp at takeoff and gave the Emil a max speed of 564 km/h. By early 1941, the DB-601N equipped the 109E-7. It was the same 33.9-liter engine, but power increased to 1174 hp at TO by augmenting the compression ratio from 6.9 to 8.2 (requiring 96 in place of 87 octane fuel). The E-7 max speed was 574 km/h.

The 109F-1 was supposed to receive the DB-601E with GM-1 boost but it was not ready, and the F-1 and F-2 continued using the 601N. The aerodynamics had been improved in preparation for more engine power, so the F-2 could fly at 594 km/h. The F-2 entered service in early 1941.

When the 1350 hp (at TO) DB-601E was finally ready, it equipped the F-3 and F-4 (second semester of 1941). The latter boasted a max speed of 610 km/h.

This was the maximum power the Germans were able to extract from the 601. For the G-1 and G-2 a larger engine (35.7 liters) was introduced: the DB-605A developing 1475 hp at takeoff and giving the G-2 a max speed of 666 km/h (spring of 1942). The G-2 weighed 3107 kg at TO while the older E-4 tipped the scales at 2660 kg, almost half a ton heavier!

One year later, the Gustav still had the same 605A engine (same power), but now that the heavy American bombers appeared, the Germans were forced to increase the weight of the weaponry at the expense of speed. The G-6 could do no more than 628 km/h.

The Germans proved slow in improving the DB-605. It was not until late 1944 (more than two years after the introduction of the 605A) that the DB-605DB appeared with the 109K-4 propelling it at 676 km/h. By this time, however, it was too late. The fastest 109 in service was the G-10 with a 685 km/h speed at the end of 1944 using the 605DB.