You can only get out what you can suck in. The engine is just a big air pump. At some point, the larger exhaust starts hurting HP #'s. If you are running a 2bbl, I would put on 1 5/8 primaries with long tubes and collecotrs. If you are running a 4bbl, you can run 1 3/4 primaries with whatever length tubes and collectors will fit. This is a gross generalization, and this assumes you have the internals to take advantage os said carburation. All that being said, the previous post is probably in the ballpark.