Air intake filter sizing

[pb3]

I found on the K&N website a calculation for determining the minimum filter size requirement. I was curious what I actually needed, partly because I have been looking into ducting outside air / possible use of the ram effect.

The required filter area (A) is CID * RPM / 20839, where CID is the displacement in cubic inches and RPM is the maximum revs. In my case 383 * 7000 / 20839 = 129sq. inches.

Now the effective filter area of a round K&N filter (E) is Dia. * Pi * height – 75, where Dia is the filter diameter in inches, height in inches and the 75 is to allow for the filter top and bottom edges that do not flow any air. In my case 14” * 3.14 * 5.5” – 75 = 167sq. inches.

So to confirm that you have the correct filter A must be less than E, for example in my case 129 is less than 167 so I am happy.

I guess that for us GTR owners this will not be an issue as we have lots of headroom, but how are you Canam people fairing. I’m thinking of one in particular that has a rev limit of 8000 and bored to 6.3L, that is A = 147, so min filter height would be around the 5.5” mark!

[Graham-P]

I'm going for a K&N air filter that has the top filtering as well as the sides. Can't remember how high it is, about two or three inches, I think.
Not sure how you would calculate that one.


Graham

[Stig]

I used a flow top on the GTR as I had to fit such a short filter (about 25mm IIRC). I did a calculation at the time and found that the flowtop actually gave me a larger filter area than the 5" job.

Didn't stop it getting blimmin hot though

[jacko460c]

The flow tops have to have quite a large space above them to work. Can't remember the exact spec but if it is too close to the hood/scoop the actual flow through the entire filter is LESS than if a solid top is fitted.