Originally posted by JonGordo8:
So getting a 4inch and getting a 12inch will produce almost identical air flow IIRC.
That's not true.
Yes after a certain point it doesn't matter, but the rule of thumb is the effective surface area should be twice the size required by the engine.
Also effective surface area does not take into account the 1/4" around the edges as no flow comes from those areas.
Engine draw is determined by CFM flow at a specific rpm.
Effective surface area for a 2.5L Duratec at 7000rpm is 320cfm (so 640cfm or larger should be used)
7500rpm = 335cfm (670cfm)
Effective surface area for a 3.0L Duratec at 7000rpm is 375cfm (so 750cfm or larger should be used)
7500rpm = 395cfm (790cfm)
A 4" diameter x 4" long filter has an effective surface area of 300cfm (not even cfm the engine needs!!!)
A KKM has an effective surface area of 540cfm (yes too small again! Dyno proven at high rpms & why IMO it collapses!)
RF-1008 (4.5"x6" x 8.375" tapered conical) has an area of 833cfm (This is the filter I have BTW)
RF-1027 (4.5"x6" x 8") = 796cfm
Both fit the stock location.
The K&N drop in for the stock SVT air box has a 620-630cfm rating. Though the SVT airbox hole can only flow roughly 400cfm! (the proverbial breathing through a straw syndrome!) BTW the stock paper filter flows around 400cfm.
