properly sized air filter for FI
 

i have seen this subject mentioned only rarely. It is an important subject after you FI this car.
The general rule is to put as big an air filter as you can get, and that's not a bad rule. But, folks may want to know the facts behind this?
Air flow is king. So the flow rate of the filter is actually more important than the size.
There are 4 types of material used for air filters.
1- cotton ( K&N type)
2- Paper (oem, aem etc)
3- steel mesh( Blitz etc)
4- Foam
Flow rates are different for them all.
Maxium flow rate is influenced by the size of the filter--right? Correct.

Now what size of filter is needs for 330-350 flywheel HP. You want to use flywheel horsepower for this not rwhp for the obvious reason.
here is a formula to help decide the size of air filter that you need per K&N:

Square inches of filter required =

(lbs of boost / 14.7) + 1 x CID x Max RPM / 20,839.

On my car with 9 lbs boost and a 8K redline--it comes out to approx 112 sq inches .
Now, how to determine the filter diameter (remember that it is pleated, this is why you need to use the equations provided). I am using a cone shaped filter. To estimate the average diameter, add the large diameter and the small diameter and divide by 2.

My K+N

6” OD at large end, 4.625 at small end / 2 = 5.31” average diameter

For a round filter, no calculations are required, simply use the O.D.

Now to determine the filter length (or height) required, based on our average diameter, to fulfill the square inch requirement

Formula #2:

Filter length (or height) =

(square inches of filter / (filter diameter x 3.14)) + .75 (compensate for rubber endcaps)

In MY specific case this is:

(112 square inches / (5.31 inch average O.D. x 3.14)) + .75 = filter length required

(112 / 16.68) + .75 = 7.46 Inches length (or height) required



In terms of CFM, the pleated K+N filters flow 6.03 or air per square inch. We previously calculated that the correctly sized air filter for my car needs to have at least 112 square inches of material to flow the required air.

112 square inches x 6.03 cfm per square inches = cfm flow requirement 675 in my case

In using this formula and assessment I found that my 6 inch long K&N was to short by approx 1 inch. I will be calling summit racing--sigh.

I do not know the flow rates for the other filter mediums.
Now --one point--K&N filters do not filter the best. So if you use one--keep it INSIDE the engine bay. Eric Meyer had some really bad outcomes by placing it outside the bay.

Now intake air flow testing can be taken further with differential pressure flow testing etc--but I dont want to go into that in this thread.
Hope this little thread can help someone

What happenned to the " pleated filter" in your calculations? You have calculated for the surface area of a smooth cylinder not accounting for the pleats ;)

couldn't you simply mock up a filter on a bench and actually test how much it's flowing? Going by manufacture flow rates for what their filter is "suppose" to be flowing seems dubious.

Seems to me that if you're getting to the point you're actually testing how much CFM you're able to pull through your filter then mocking up your intake system would be worth the trouble.

you can call the manufacturer too. They will give you the flow rate.

Pleated surface area is another head scratcher. Triangle shape is by far the best--but no true triangle pleat is out there. Close but no potato.

the reason i didnt use the pleated area in my sizing ( and that is an excellant point) is because you actually want an air filter that is 1.5 to 2 times bigger than normal. Air filters will get dirty and as that happens the max flow rate goes down. The bigger the filter the longer the service life also. So by not including the pleated area helps to issue I have an oversized filter.
Air filters can cause a restriction. It is actually "Negitive Boost". I have seen turbo cars ( not rx8's) with a little foam filter that was taking 1-2 lbs of boost away from their setup.