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Air Filter Size Calculation

The air filter you have on your engine is most likely a compromise between what fits under the hood and what looks cool…but is it the right size for your engine?

The air filter element can do more than just filter out dirt and dust, it can help straighten and diffuse the air before it enters the carburetor and actually enhance performance.

Usually one of the first performance upgrades, that cool looking small-diameter, low-profile, open-element air filter could be hurting performance. Surprisingly, the boring 14 x 3 element seen on most 1960s era muscle cars is ideally sized for many single carburetor applications.

Shape and size dictate the ability of the air filter element to deliver maximum airflow to the engine. K&N’s flow bench data has shown that for typical two and four barrel carburetors, airflow is greater when the diameter of the filter element is larger in comparison to the height. For example, a 10 inch diameter, 2 inch tall element will generally flow better than a 5 inch diameter, 2 inch tall element.

If space permits, K&N advises using a filter element with a height 1/5 to 1/4 of the filter diameter. Element diameter in relation to carburetor or throttle body throat is also important. The element should be at about three times larger than the carburetor throat.

Two factors dictate the filter size: the engine’s cubic inch displacement and RPM at maximum horsepower.

With these figures, the minimum effective filtering area and the required element height (including the rubber sealing edges) can be calculated.

H equals height and D equals the estimated element diameter. Adding the additional .75 is to account for the element’s top and bottom sealing edges.

H = ( (CID x RPM) / 25,500 ) / ( D x Pi ) + .75

In this example, a 327 cubic-inch engine revving to 6800 RPM would require a minimum filter element size of 14 x 3, but since hot-rodding is all about excess… an additional factor of 1.7 brings the filter size up to 14 X 5.