[NStimac]

I believe this formula should work for both types of combustion. All the equation basically says is Displacement (volume) x RPM (how many times the volume is filled) = Cubic feet per min.(total air flowed through the said volume). Of course as VTcruiser states we must divide by 2 because it’s a 4-stroke engine. The volumetric efficiency (VE) takes into account energy lost. It’s kind of a fudge factor. I think it’s for loss due to stuff like the timing of valves opening and closing, restrictions to airflow (intake and exhaust), and heat generated. From what I have read diesel engines have a higher VE than gassers but I am not sure how much.
I think problems with multiple variables (RPM, Displacement, and VE) are best understood by looking at them graphically. So I Exceled it. I just made some VE numbers up for the sake of argument. I think 75% is reasonable for a gasser and I gave the diesel only a 5% advantage.
If you compare the CFM’s between a theoretical 3B (3.4L at 4000 RPM with 100% VE) and a theoretical gasser (2.5L at 5500 RPM (redline?) with 100% VE) the numbers are very close (about 240CFM upper yellow line). That’s why the 2.0-2.5L rule-of-thumb Brownbear stated works.
To say a gasser out flows a diesel can be slightly misleading. A gas motor will be able to rev higher than a diesel. So assuming the same displacement, then yes the gas motor is able to flow more air. But diesels have a higher VE. Because of this if you compare a gasser and a diesel with the same displacement at the same RPM then the diesel is flowing more air. Also look at the more “realistic” 3B with 80% VE at 4000RPM vs. the gasser with 75% VE at 5500 RPM, now the 3B is actually out flowing the gasser by about 10CFM (lower yellow line).

To summarize:
Graphs allow us to understand complex relationships between variables.
A gas engine can rev higher therefore flowing more air than a diesel with the same displacement.
But a diesel flows more air than a gas engine with the same displacement at the same RPM due to their higher VE.
Diesels Rule.
I am a physics geek.

-Noah