At the start of the war the Luftwaffe standardized on 87 octane aviation gasoline, called "B4", made from leaded hydro-petrol extracted from brown coal. In 1940 an improved fuel, designated "C2" was introduced having a higher aromatic content of 35–38% and giving performance equivalent to Allied 100 octane grade of that time. C2 was used in small quantities by aircraft such as the Messerschmitt Bf 109E-4/N and E-7/N and the Messerschmitt Bf 110C when equipped with the DB 601N engine, that entered series production in October 1939. The power was increased by 20% over that of the DB 601A, to 1,260 hp at 6,900 feet (2,100 m) at 1.35 atm boost pressure and 2,400 rpm. By July, nine Bf 110 and three Bf 109 fighter "Staffeln" (squadrons) were equipped with the new engines, By the end of October around 1,200 DB 601N engines had been delivered. and the number of aircraft equipped with the improved engine gradually increased through the second half of the year. However, due to leaking valves there was relatively high wear on the 601N-engines, which had a life of about 40 hours.