When it comes to sizing filter elements, bigger is always better. If you can squeeze a 14-inch diameter air cleaner base on your carbureted or throttle body EFI engine combination, go for it. If you can fit a 4-inch tall (or taller) element under the hood of your car run with it. Shape and size dictate the ability of the air filter element to deliver maximum airflow.

K&N Engineering’s flow bench data has shown that for typical carburetors and EFI throttle bodies, airflow is greater when the diameter of the filter element is larger in comparison to the height. Following that idea, a 10- x 2-inch element will generally flow better than a 5- x 4-inch element. If space permits, K&N advises using a filter element with a height 1/5 to 1/4 of its diameter. Element diameter in relation to carburetor or throttle body throat is also important. The element should be at least three times larger than the throat.

The one exception to K&N’s rule is when a filter element is clamped to then end of a long runner, like with cold air intakes for EFI engines. The straighter the airflow shot into the intake tube, the less restriction there will be. In this case, an element that has a height two to four times greater than the diameter will work best.

The engine’s cubic inch displacement and RPM at peak horsepower are required to determine filter size. Knowing those figures, use this formula to determine the minimum effective filtering area an engine requires. A represents the effective filtering area, the usable portion of the filter:

A = (CID x RPM) / 20,839

After determining the effective filtering area, we can determine the required element height. H equals height and D is the estimated maximum element diameter. The .75 figure accounts for the element’s rubber sealing edges:

H = A / (D x 3.14) + .75

Let’s apply these formulas to a carbureted 350 cubic inch V8 with a 5,500 RPM horsepower peak. Applying the formula for filter area, we came up with the following:

A = (350 x 5500) / 20,839

A= 92.4 inches

To determine element height, let’s assume the element will be 12 inches in diameter. The correct height is as follows:

H = 92.4 / (12 x 3.14) + .75

H = 3.20 inches

If you are using a panel filter, simply multiply the length of the filter by the width, not including the rubber seals, to determine the correct filter size.

With the minimum height figured out, you can go filter element shopping on the Summit Racing website. It shows elements that are 3.43-, 3.44-, 3.47, and 5.50 inches in diameter. Since the 3.20-inch figure is the minimum filter height our theoretical 350 requires, the right filter is the one that fits under your hood (if you run one).

Let’s take a look at some filters I had for my big block-powered 1969 Nova and see which one I ended up using.

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