Classic Inlines CFM Calcs and Volumetric Efficiency

The larger the engine, the more air it's capable of gulping.
In math terms it looks like this:

When calculating the CFM requirements, the volumetric efficiency must also be taken ito consideration. Volumetric efficiency in internal combustion engine design refers to the efficiency the engine moves the air/fuel charge into and out of the cylinders. More correctly, volumetric efficiency is a ratio (or percentage) of what volume of fuel and air actually enters the cylinder during induction, to the actual capacity of the cylinder under static conditions. Therefore engines that create higher induction manifold pressures, above ambient, will have efficiencies greater than 100%. Engines with higher volumetric efficiency will generally be able to run at higher speeds (commonly measured in RPM) and produce more overall power due to less parasitic power loss moving air in and out of the engine.

There are several ways to improve volumetric efficiency of an engine. A common approach is to use a camshaft profile that has more duration and/or lift to increase air flow. Another method is to use larger valves, however larger valves increase the weight of the valvetrain which in turn increases the engine parasitic losses. Streamlining and/or polishing the ports increases flow capability. This is referred to as Porting & Polishing, and is usually done with the aid of a flow bench. Improving the air flow or induction, by swapping to a high performance intake manifold can also help. And finally, increasing the exhaust flow with headers or improving the exhaust system. While naturally aspirated motors generally reach 80% VE, they can hit 100% if properly built and tuned. However using forced induction, such as supercharging or turbocharging, can easily push the volumetric efficiencies up to 120% or more.

In our example a 250ci @ 6000 RPMs can use 434 CFM

Calculations using volumetric efficiency look like this:

434 CFM x 80% (.80) volumetric efficiency = 347 CFM
434 CFM x 90% (.90) volumetric efficiency = 391 CFM
434 CFM x 100% (1.00) volumetric efficiency = 434 CFM
434 CFM x 120% (1.20) volumetric efficiency = 521 CFM

Click here for an online calculator that will do the math for you.

Remember calculated results are just that, calculated.
Consider the following "Rules of Thumb"
before you make the final decision.