When fitting a conventional round filter on top of your engine-such as a carburetor, central fuel injector, or throttle body fuel injector-a large diameter, short filter will provide more airflow than a small diameter, tall filter. For example, a 10-inch diameter, 2-inch tall filter will provide more airflow than a 5-inch diameter, 4-inch tall filter. Where space permits, the height of the filter should be between 1/5 and 1/4 of its diameter.
The shape of the filter is less important if the application calls for a remote mounted filter, which includes many late-model fuel-injected vehicles. Typically, these vehicles will use a flat panel filter, or a conical/cylindrical shaped filter with a rubber mounting flange designed to be mounted on the end of the inlet hose.
Use the formula below to compute the minimum size filter required for your particular application. The usable portion of the filter-the effective filtering area-is calculated by multiplying the diameter of the filter times pi (3.1416), multiplying by the height of the air filter in inches, and then subtracting .75-inch. We subtract .75-inch to compensate for the rubber seals on each end of the element and the filter material near them, since very little air flows through this area.
Example: A 350 CID Chevy engine with a horsepower peak at 5,500 rpm.
If you are sizing a panel filter, multiply the width of the filter area (not the rubber seal) by its length. If you are sizing a round filter, use the following formula to determine the height of the filter.
Referencing the K&N vehicle search bar shows the proper filter for this application would be an E-1500, which is 3.5 inches tall. Keep in mind, this is the minimum size requirement. To provide an even greater volume of air to the engine, install the largest filter that will fit in the space allotted.
Off-road conditions require additional filter surface area. A filter should be sized 1-1/2 to 2 times larger than normal for any conditions that could be considered severe. In this case, the E-1500 used in our example should be replaced by an E-1120 or an E-1150. For long distance off-road events, two double-size remote mounted filters would be recommended.