RAC1812 - I would GUESS the volumetric efficiency of a stock 261 at 70 - 75 percent.
Unless the engine is GROSSLY undercarbureted (or the original carburetor is not functioning properly); simply bolting on 2 or 3 one-barrels or a small 4-barrel is going to result in modest gains at best.
More substantial gains would be realized by also adding higher compression (more mechanical advantage), a longer duration or higher lift camshaft (more inlet air flow) in conjunction with better exhaust (if you have better inlet flow, you need better exhaust flow). Strengthening the lower end of the engine with the above for greater RPM will also raise the airflow.
Also, when applying the equation, remember that it is JUST an equation. If you get in the 95 percent or better volumetric efficiency (racing) the equation is not necessarily correct. Example - I own a "production" car that came from the manufacturer with (2) 500 CFM 2-barrel carbs. The engine is a 121 CID 4-banger. Car weighs 1900 pounds, has a 5.13 rear gear and will not pull itself in low gear below 2500 RPM. Always understand what the equation means, and what you are trying to do with it.
Jon.
