Filter Selection

If maximum horsepower is the objective, the size and shape of the air filter element is paramount.

cartoon figure Let's first consider shape. When a fitting a conventional round filter on top of the engine, such as a carburetor, central fuel injection or throttle body fuel injection, we have found a large diameter, short filter will flow more air than a small diameter, tall filter. For example, a 10-inch diameter filter 2-inches tall will flow more air than a 5-inch diameter filter that is 4-inches tall. 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 models. Typically these vehicles will use a flat panel filter or a conical or cylindrical shaped filter with a rubber mounting flange designed to be mounted on the end of the inlet hose.

That brings us to size.

Pleated K&N filter material will flow 6.03 cfm of air per square inch. By comparison, a single square inch of the highest flowing paper will allow 4.95 cfm of air to pass and the freest flowing foam will flow 4.38 cfm. Use the formula below to compute the minimum size filter required for your particular application. The usable portion of the filter is called the EFFECTIVE FILTERING AREA which is determined by multiplying the diameter of the filter times Pi (3.1416) times the height of the air filter in inches, 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.

A=(CID X  RPM) / 20,839
A = effective filtering area
CID = cubic inch displacement
RPM = revolutions per minute at maximum power
Example: A 350 CID Chevy engine with a horsepower peak at 5,500 rpm.
A=(350 X  5500) / 20,839 = 92.4 square inches

If you are sizing a panel filter, multiply the width of the filter area (not the rubber seal) times its length. If you are sizing a round filter, use the following formula to determine the height of the filter.

A = effective filtering area
H = height
D = outside diameter of the filter
3.14 = pi
.75 = the rubber end caps
H=(92.4 / 12 * 3.14)+0.75 = 3.20 inches

Referencing the K&N Flow Comparison Chart 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 extend the service interval and to provide an even greater volume of air to the engine, install the largest filter that will fit in the space allotted. If the space above the engine is restrictive, perhaps a remote filter arrangement could be used to gain space.

Foam Amsoil LT-31;
376 CFM
K&N (used) (42,000 miles)
K&N E-1500
Paper AC A348C; Fram CA326;
Hastings AC-145; K-Mart KA-12;
Motorcraft FA-71R
508 CFM
K&N E-1500 887 CFM
TEST NOTE #1: All air filter elements were flow tested at .3" H20 (water) restriction and then converted to 11/2" H20 restriction by using the square law formula.
TEST NOTE #2: All elements tested were of equal size (12" od x 31/2" H) and were specified by their respective manufacturer for the same application.

Off-road conditions require added filter 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 best.

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