I wish K&N had a CFM rating on their filters.
CFM is in the secret [Vizard] recipe at filter selection: http://www.knfilters.com/filter_facts.htm#MYTHS
I was just taking a wild-as s guess as to the needed filter size based off CFM requirement. I was just using "round" numbers:
180" x 9,000 rpm/3,456 = 469 cfm
at 20 lbs boost the engine "thinks" it's 2.4 times bigger than it is: 2.4 x 470 cfm = 1,128 cfm.
looking at it another way: if we are making 900hp ( rounded up numbers) with a BSFC of .55 we are burning 495 lbs fuel an hour. At an AF ratio of 11:1 we would be burning 5,445 lbs air an hour or 91 lbs air per minute. 91lbs/.0807( weight of a cubic foot of air) = 1,124 cf.
So my wild a ss guess was kind of close. We need at least 1,125 cfm to feed 900hp.
A 9" diameter round filter has a circumference of approx. 28" 28" x 5''(H) = 140 x 2 (filters) = 280 sq. in. of filter area to feed 900 hp.
If I use the K&N formula where A= filter area and CID x rpm/20,839 =A I'd get 187" filter area.
180" x 2.4 ( boosted engine) 432" x 9,000 / 20,839 = 187" filter area. So my filter area of 280" would have a minimum of a 50% over-size factor in it.
I think that all works out .....
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