[BN]

For a naturally aspirated 4 cycle engine, the cfm consumption may be calculated as follows:
Since 2 revs are required for 1 intake stroke, divide the RPM by 2.

Since we are dealing with cubic feet, divide the cubic inch displacement of the engine by 1728 cubic inches in 1 cubic foot.

The volumetric efficiency can vary widely. 75% - 85% for more modern engines. Model A engine is probably less than 75%, especially with an air filter.

Example:
Use 205 cubic inch displacement, since most Model A engines have been bored.
Use 2400 RPM.
Use 73% vol eff

205/1728 = .1215 ; 2400/2 = 1200

Required CFM = .1215 x 1200 x .73 = 106 cubic feet/min

Compare this with Vince Faller's chart.