Steve Foley wrote:
My O-320 cruises at around 2500RPM. That's 340 cubic inches * 2500 RPM * 60
Minutes per hour to arrive at about 400,000 gallons of air per hour.

I used 128 cubic inches per gallon. I think that may be right.

Not to be picky, but there are 231 cubic inches in a gallon. Your
engine is a four-stroke, so it will only displace it's volume in two
revolutions, so you need to divide by two. The volumetric efficiency
will depend on throttle setting, making the calculation more difficult.
It is easier to go by way of fuel burn, since the proper ratio of fuel
to air doesn't change much in a gas engine. See above for an example.

tom