ok here goes, a carburetor needs to match it's air flow with what the engine is capable of using. this is measured in cubic feet per minute(CFM). what ever size engine the CFM of the carb must match the engines max CFM. the larger the CFM of a carb the more gas it uses. so if you only have a 2bbl carb you must have 2 very huge throats in the carb to match the engines max CFM. on a 2bbl this is your only option if you try to use a smaller CFM carb you will starve the engine at full throttle. so now on to the 4bbl carb, a 4bbl carb can get the large CFM with 4 smaller throats in fact 1/2 the size as a 2bbl. what this allows you to do is to drive around basically on a much smaller carb because most of the time you are only using the 2 front throats. it's like you to installed a 2bbl carb that was 1/2 the CFM of the 2bbl. that came with the 390. you will have much better throttle response at low end and much better fuel economy. then when you need the full CFM flow of the carb under full throttle the secondary's kick in. to give you full CFM. I have owned a few of the 390 2bbl set ups in a truck I was lucky to get 10mpg. once a 4bbl was installed with the correct manifold this would go up to maybe 12 mpg. but have much better throttle response. btw a spread bore 4bbl makes this even more pronounce because the primaries are even smaller CFM flow, the total CFM is made up by even larger secondary's. hope that clears it up. highdesertranger