The engines were unusually quiet because the exhausts were muffled by the General Electric turbo-superchargers on the twin Allison V12s. There were early problems with cockpit temperature regulation; pilots were often too hot in the tropical sun as the canopy could not be fully opened without severe buffeting and were often too cold in northern Europe and at high altitude, as the distance of the engines from the cockpit prevented easy heat transfer. Later variants received modifications (such as electrically heated flight suits) to solve these problems.