 Better ventilation, insulation and heating were also added.The powerplants were replaced by R-3350-745C18BA-1 engines (the civilian equivalent to the wartime R-3350-35).Design tests did not need to be conducted, as Lockheed had already tasked them to the C-69 aircraft during the war (one of the C-69s also completed the trials for the civilian airworthiness certificate on December 11, 1945).