Well the guy that has the OH6A for sale was saying that the C18 works just as well as the C20 in the 4 blade model and that the C20 was used when they went to the 5 blade D,E models for the extra power and lift. He said if your not trying to use the OH6 or C model for line work there is no real benefit to having a C20. In fact you use less gallons per hr with the C18. This particular turbine is the military version a 250-C10D which he says has a continuous 317 hp.