My referances are for a typical downleg booster Holley carb in a single 4 barrel tune.Of course we would like no vacum in the manifold,this is the biggest bonus of fuel injection,you dont need vacum since you dont need boosters restricting the airflow in order to move fuel.If you selected a throttle bore size that offered no restriction to airflow at wide open throttle and therfore had no vacum at peak load,and then installed a set of downleg boosters inside the venturies you would have at least an inch of vacum at the same load.So we select larger throttle bores to reduce this loss in flow,but in reality,as long as those boosters are inside those bores,you have some sort of restiction.The engine is moving air and that air is being used to create a pressure change that will draw fuel out of the boosters.Whenever airflow is used to create a pressure differential across a surface,some of that flow is lost.
Since we arent talking about changine the boosters or air bleeds here,we have to stay focused on carbs that will deliever thier rated flow at 1.5" of pressure drop,being used on an engine that will tolerate a bit less of a drop.Stock engines are teamed up with torque convertors that have too low of a stall speed for even a stock engine,so soo much throttle area can easily leave you with an engine that will never move enough air to create a good signal at low engine speeds under a high load,so regardless of how much power the engine makes,if the carb is within my guideline,it will cause the car to be soft at low speed/high load.This isnt the carbs fault,the combination is mismatched.Even a bone stock 350 smog motor would perform better with a 2500 stall convertor,so it isnt the carbs fault if the engine performs poorly.In a street car,convertor selection is still a bit conservitive.In reality an engine that really needs 3500 stall will be teamed with at least 1000 rpm less then that,and you need to keep this in mind when selecting a carb,but even a mild race motor could stand less then that if it doenst have to operated outside of it's operating range.
The long and the short of it is that no matter how you slice it,most people dont know of cant figure their volumetric effiency when building a street car,so simply using the rpm and engine speed formulas leads then to believe they need a smaller carb they they can really use,and most would never have a clue about how cfm is rated and where and engine wants to be rated at.Look at most good running engines that have the optimum size carb on them,and see where they fall in the hp per cfm scale.Look at most common bracket race and street cars that run well for their respective combination,and see how their hp to cfm ratio scales up.No rocket science just a quick way to put the right size off the shelf carb on a well matched street car.