The number 3456 represents 1728 inches per cubic foot(volume) X 2(four stroke). If this were a 2 stroke, then the number 1728 would stand alone. Note in an attempt to simplify, we didn't take volumetric efficienty into consideration. since an ordinary engine runs at about 75% VE, the formula extends to this CFM X .75 =adjusted CFM. It goes on even farther when you begin to factor in altitude, temp, ect. Be advised that the lower in the RPM band you go, the smaller the carb you want to use.This is due to the effect of the cylinder pulses on the carb bores. It is importiant that you try to keep the primary bores as small as possible. Holly,Carter, and Rochester did this by desiging the "spread bore carb". This carb uses small primaries to keep the pulses high at low RPM's, and larg secondaries for better breathing at higher RPM's. A good example is the Carter Thermo-quad.As to wether you should change carbs or not, that is a decision you will have to make your self.