On the 1957 Chevrolet's , the power pac option included ; 4 barrel power pac heads, with bigger valves. 4- barrel intake, and carburetor. 4 barrel oil bath air cleaner,and dual exhaust. Bama can you tell me if the 4 barrel power pac version also used a different cam? How about the carburetor linkage? Is their something else different, besides the gas lines running on the outside of the frame on the 4 barrel car's ? I believe the 2 barrel versions put out 185 HP and the 4 barrel car's 220HP. I have heard that the 2 barrel car's will leave faster, because the 2 barrel carb's have bigger primary's , but when your rolling along at 40 or 50 and you get into it and the secondary's kick in, you know you have a 4 barrel . Also the Dual exhaust , must really make a difference.