You can get a rough idea by measuring the venturi area of the single bbl and comparing it to a the verturi area of a known two bbl carb. There is some effect based on the design of the boosters etc in the venturi but for an approximation calculate the ratio of the areas. Remember that a two bbl carb is rated at 3"hg, I would guess that the single throat YH would be rated at 3" also.A 4 bbl is rated at 1.5"hg. This means that a 2 bbl would flow about 71% of a 4 bbl with the same nominal CFM rating. For example a 3 deuce setup with carbs rated at 150 CFM would not be the same as a 4 bbl with a 450 CFM rating. The 3/2 setup will flow 318 CFM if rated at the same differential pressure as the 4bbl.