So thinking about this a little more:
Using a common rule of thumb formula for air consumption rate, then Engine CFM=(engine cubic inch displacement x max rpm x volumetric efficiency)/3456. So a 121 cubic inch engine at 5500 rpm and 85% VE (admittedly a SWAG) will flow about 165 cfm. I chose 85% VE as that is a reasonable number for a normally aspirated modest performance two valve engine.
So any air filter assembly that can flow 165 cfm or greater (as installed) should be more than adequate for our M3W for street driven use. And unless the air filter assembly is somehow deficient in design (see note on stock unit, below) using only the total flow capacity of the filter element itself gets us close enough for a street engine.
K&N in a published whitepaper indicate their testing shows oiled gauze type filter medium such as used in their products flow 6 cfm per square inch on average. So to evaluate an air filter for an intended application just calculate the square inches of the filter and apply the cfm per square inch rating for the particular medium.
In the case of the Stealth unit, the air filter has a mean diameter of about 5.25" and an effective filter height of 1.5" (measure only the fully exposed filter medium) so I get 24.7" square inches and about 148 cfm, which is just adequate for 5000 rpm use, so just a bit undersized if you are running to max rpm all the time. Going to a 1" taller filter (about 2.5" height of effective filter area) gives us about 247 cfm which, while overkill, is useful for higher rpm work (good to 8000 rpm!), but no real practical benefit to those running 5000 rpm or less.
As a side note, while the stock air filter has a larger element than the Stealth unit, it is also closely shrouded around most of it's circumference which can have a noticeable impact on 'as installed' cfm. The stock air filter can be significantly improved by reducing the shrouding effect. One way to do this is increasing the bottom 'cutout' area further up the sides of the cover and reducing the aluminum backplate radius on the upper half to match the filter element radius.
In any case, at idle, where the engine is only pumping about 33 cfm, just about any air cleaner would do.
Last edited by Bitsobrits; 11/04/21 03:35 PM.
