K&N Universal Air Filters

K&N manufactures their Universal Air Filters (UAF) for a wide variety of applications. If there is no replacement air filter or FIPK for any specific model & type, then there will still be a Universal Air Filter model that will fit the engine. The UAF are available in various angle, offset and diameter and are constructed with ultra-strong molded pliable rubber flanges. These rubber flanges absorbs vibration thus allowing for secure attachment and can additionally be stretched up to 1/16" (or 1.5mm) to fit in-between sizes. K&N UAF are available in round, tapered and oval designs.

The K&N catalog lists about 10 pages of various different UAF design and sizes which should fit just about any Honda engines that does not have specific direct replacement or FIPK available. There are four basic designs for the UAF; Round Straight, Round Tapered, Oval, and Dual Flange Oval. These 4 designs comes in many variations on length, height and with various flange sizes. The 4 designs are shown below with some variations of each design being shown immediately below the base design.

As an illustration, to fit a K&N UAF to the JDM EK3 Civic Ferio Mi which uses a 1.5l SOHC PGM-Fi with the air-filter box directly on top of the engine, we can use either Round-Straight or Oval UAF. We will first need to measure the diameter of the flange and then calculate the size of the filter element we will need based on the engine displacement. To do this calculation, K&N gives the some guidelines

K&N have found that a large diameter short filter will flow more air than a small diameter tall filter. Thus we should aim for the widest and shortest UAF we can find. However, the relation of height to diameter of the filter should be also between 1/5 to 1/4 and this relationship should be maintained as much as possible, space permitting. K&N gives the air-flow capacity of their pleated filter element as 6.03 cfm of air per square inch.

First the usable portion of the filter element needs to be calculated (what K&N calls the Effective Filtering Area). This is done by calculating the circumference of the filter times the height and then subtracting 0.75 inch to compensate for the rubber seals on each end of the filter element.

Displacement conversion from litres (L) to cubic inch (CID)	Next we need to calculate the air requirement of the engine in terms of this effective filtering area. The formula K&N gave in the catalog is reproduced as equation 1 here. CID is the engine displacement in cubic inch because the U.S. still measures engine displacement in cubic inches. 1 litre is about 61.02 cubic inches and K&N provides a conversion table in their catalog which is reproduced on the left. K&N recommends using the max power rpm for this calculation though the redline rpm can theoretically also be used. Using the calculated area requirement, we can then calculate the height of the UAF given the diameter wish to use. The formula K&N gave in their catalog for calculating the height is reproduced as equation 2 in here
	Equation 1	Equation 2

Using the calculated height and diameter required, the required UAF can then be determined from the UAF catalog. We do not re-create the UAF catalog listing here because it will probably be changing constantly as K&N adds new models. Thus it would be advisable to go to the K&N dealer with the calculated height and diameter requirements to determine the proper UAF from the most current catalog. The calculated dimensions represents the smallest that will satisfy the air-flow requirement of the engine and K&N recommends installing the largest UAF that will fit into the engine bay to extend service interval and for higher air-flow capacity.

Conclusion

We have tried to cover comprehensively the many types of K&N air-filters that can be used on our Hondas in this article. Of course the information in this article being derived from the official K&N catalog will be dependent on our interpretation. If any reader finds errors in this article, please feel free to correct us by contacting us.

The information in this article is provided on an as-is basis only. TOVA's opinion is that you the reader is the best person to determine the validity and applicability of this information for your particular application. There may also be unintended errors while compiling this article from the catalog. TOVA apologizes for any inconveniences that such errors may create but will not accept any responsibilities arising from their use.

Copyright Temple of VTEC
May 1999.