You can "rule of thumb" it fairly easily. On one revolution of the engine, only half of the cylinders suck in air (the other half are on their power strokes).
So, for one revolution, a 389 that is running at 100% volumetric efficiency and also creating zero vacuum (neither of which can actually occur at the same time) will pull in at most 389/2= 194.5 cubic inches of air. Let's impose a redline of say, 6000 rpm just to be generous. At 6000 rpm (again at zero vacuum and 100% VE) it's moving 6000*194.5 = 1167000 cubic inches of air, or 1167000 / (12*12*12) = 675.35 cubic feet per minute. Again that's with zero restriction (zero vacuum) and 100% volumetric efficiency. Now we already know we need SOME vacuum for the carb to function at all, so we have to have at least a tiny bit of restriction/vacuum, and we also know that these engines only run at maximum VE at a very narrow, specific rpm (usually very close to where they make maximum torque). Let's be (very) generous and say the 389 is still at 80% VE at 6000 rpm. That drops the maximum "air capacity" down to 540 cfm and we're still not considering that we need some vacuum to make the carb work.
Things aren't that simple still, though. Carburetor manufacturers understand that they need vacuum to work, so the CFM measurements/ratings are taken while the carb is operating at a specifc standard vacuum
. (I forget what the standard is, or even if all manufacturers use the same standard.) Often, the carb will continue to function and feed fuel at a lower vacuum than what is used to obtain the rating though (up to a point).
The above is why some engines will benefit (make more power) with a carb that is rated slightly
larger than what they theoretically "need". Putting a "larger" carb on the engine will create less of a restriction (vacuum) which will allow the engine to move more air (and make more power), in other words, allow it to get "closer" to its theoretical maximum air handling capacity at that rpm - and as long as there's still enough "vacuum signal" to cause the carb to accurately
meter and feed fuel, we'll make more power. Go "too big" through and things go down the tubes (or rather, fail
to go down the tubes
) in a hurry. You'll find that the car becomes very difficult if not impossible to tune, especially at idle, because the carb isn't getting enough vacuum signal to operate consistently. It'll probably bog like a big dog whenever you hit the throttle because the off-idle transition ports won't work right, if at all. It'll be an all around miserable experience.