I did a lot of carb and intake testing on a 261 Chevy engine a few years ago,wrote an article in Inliner's on the test.
The CFM charts are pretty accurate for street engines.My engine was moderately tuned,3/4 cam,Fenton's,more compression,always used heated intakes,a Offy 2 carb and then a Clifford single 4 bbl intake.The carbs were 2 Rochesters,2 Holley-Webers and a variety of single four and two barrels on the Clifford,Holley 390 4 bbl,Holley 500 cfm 2 bbl,a rare Holley 500 cfm staged 2 bbl,Carter 400 cfm 4 bbl,Edelbrock 500 cfm 4bbl,Carter WCFB and a Rochester 4GC.
All the carbs were tuned for use on my engine,some requiring internal changes to run their best.
I measured acceleration in second gear of a 3 spd from 2000 to 4500 rpm,measured the time it took and measured time between two road markers.And also took in how the carb "felt" ,like part throttle response and fuel mileage.
After all this testing,the 261 didn't need more than 350-400 cfm to run it's best.Both single and multible carbs performed the same one sorted out.The best response and power in my testing came from a single Holley 2305 500 cfm staged two barrel mounted on a Clifford intake.The Holley Webers worked just fine,but gave no more power than two stock jetted matching Rochester B's,the larger size from mid 50's Chevy cars.
The Holley Webers are good for relatively cheap money they cost,but they are ugly as hell.
I didn't get into Carter YF's or Strombergs cause in my opinion they are far too expensive.Same for Quadra-jets,good one cost 400 bucks.But thery both will work well id money is no object ot you get the carbs on the cheap.
For a simple test to judge your carb needs,hook up a vacuum gauge.Then using a lower gear,like second,apply full throttle and watch the guage,if your engine can tolerate more carb,the guage will go to nearly zero when ya first floor the throttle,then rise up to above 4 inches or so of vacuum as the rpm's near redline.
