The cylinder heads had an indentation like an inverted top hat, which was finned, but it was difficult to get air down into this hollow to adequately cool the head. During development, the engineers contacted ICI Metals Division, Birmingham, to enquire whether a copper-chromium alloy with higher thermal conductivity would have sufficient high temperature strength to be used for this purpose. Tests were successful and with the same volume of cylinder, these modified heads enabled the horsepower per cylinder to be raised from 110 hp to 220 hp.