Depends what you are aiming to do, but basically your gut analysis is correct; small air filters become restrictions.
How big is needed depends on a few things:
1. Volumetric Flow That is the volume of air the engine wants to consume over time. Any screen is a restriction and the higher the velocity of the air, a higher percentage gets blocked.
2. Mass Flow If there is a pressure drop across the air cleaner, then the air will be less dense. When the mass flow of air is changed, the fuel needs will change.
3. Pressure in the Fuel Bowls Yea. Really. The air needs to be directed so it goes past the bowl vents without creating extra pressure or suction. Well, some carbs take advantage of that on purpose - usually above the venturi, but that's getting off track.
Summary. Use a filter with enough area to provide enough flow for your engine.
Use a base and lid so the airflow is not disturbing the bowl vents. If using some sort of ram-air (scoop) or the hood clearance is an issue, then make up something like shown in this
bowl vent discussion. Air flow requirement = (CID/2)x (rpm/1728) x VE
from Urich & Fisher Holley Carburetors and ManifoldsTake the air flow required in cubic feet per minute (CFM) and then calculate how big your filter area needs to be.
Figure a clean paper element will flow 4 to 5 cfm per square inch (at 1.5" of H2O, which is a very small pressure drop)
from Vizard Performance with Economy p.24Oiled cotton gauze filter may protect less from very fine stuff but is good for the last drop of efficiency in a racing motor. It's also better if the element is in situations where it will get wet, muddy or the like. Vizard's test only showed it to be
slightly more more flow per inch than the best paper element he tested when both were new.
------------------------------------------------------
Maybe some ballpark examples would help.
A 400 cid engine with 100% VE will flow around 700 cfm at 6000 rpm.
Lets take the best paper element and figure it flows 5 cfm/sq inch at 1.5 "H20.
700 cfm divided by 5 cfm/sq in = 140 square inches.
Lets assume theres room for a 3" tall air cleaner.
140 sq inches divided by 3 inches = 46.7 inches
If the engine is getting a single round air cleaner, then the diameter will equal the circumfrance divided by PI.
46.7 inches divided by 3.14 = 14.8" diameter
In reality a 14" diameter cleaner is going to work well. Even a little smaller (like 12") is not going to be noticed unless racing. Obviously a higher rpm or more efficient engine will want more at its Hp peak.
Using same example but 2 round air cleaners.
46.7 " / 2 = 23.4"
23.4 / PI = 7.5" diameter
Finally, lets use the same example but there's only room for 2.5" tall air cleaners.
140 sq in / 2.5 in = 56 inches
56 /2 air cleaners = 28"
28 in / PI = 8.9"
