Cid x rpm / 3456 = cfm x VE% = actual cfm (displacement over time)
Eg.
318 @ 6113 rpm 90% ve = 506 cfm
360 @ 5400 rpm 90% ve = 506 cfm
440 @ 4418 rpm 90% ve = 506 cfm
Ideal for a highly competitive racing series or Joe Blows average street strip car/weekend burnout machine? I think to many worry about the 1st but are building the 2nd.
The cfm formula above calculates the total cfm demand for the entire engine, the flow that the carburetor would see.
In the second video you posted there is another cfm demand calculation. Cfm=(cid x rpm x 0.0009875)/(# of cylinders). That’s the formula that equates to the demand of 1 cylinder on a flow bench at 28”.
For instance, a 360 engine at 6000 rpm would require a port capable of 266 cfm at 28” on the flow bench. Then, if you know the target velocity, the average CSA can be calculated from the cfm.