 The more powerful engine was cooled by a 70% to 30% water glycol mix, rather than pure glycol used for earlier Merlin versions.This and the increased cooling requirements required a larger radiator and a redesigned, circular oil cooler housed in a deeper, slightly wider "bath".Although, by this time, production of the Spitfire had started to increase, a Merlin XX powered Hurricane Mk I was built and first flew on 11 June 1940.