Back in the day, if you wandered over to the NHRA Pro Stock Truck pits between rounds you’d find something pretty interesting. Nine times out of ten, those race trucks used some form of filter element in their air intake system. They were there to filter out dirt and dust, of course. But more critically, those filters were being used to straighten and diffuse the air before it entered the carburetors. The air cleaner elements weren’t hurting performance—they were improving it. And an air cleaner can do the same thing for your high performance street or race car.
Obviously, you need to select an air cleaner with a filter element large enough to do the job. For example, the 14-inch by three-inch element in 1960s-era Chevy muscle car air cleaners is ideally sized for many single carburetor applications. It’s so good, in fact, that aftermarket manufacturers offer clones of the Chevy filter.
Companies like K&N offer many different types of air filter elements and air cleaner assemblies to fit varied carburetor sizes and shapes. Some (especially circle track models) are even offset to clear distributors and other engine hardware. What’s more, the properly-sized air cleaner assembly with a high-flow element can actually increase the amount of airflow into the carburetor.
Shape and size dictate the ability of the air filter element to deliver maximum airflow to the engine. When it comes to the shape, K&N’s flow bench data has shown that for typical two and four barrel carburetors (and fuel injection throttle bodies), airflow is greater when the diameter of the filter element is larger in comparison to the height. For example, a 10-inch diameter by two-inch tall element will generally flow better than an element five inches in diameter and four inches tall. If space permits, K&N advises using a filter element with a height 1/5 to 1/4 of the air filter’s 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 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.
Once you have determined the proper shape, you can determine element size. Two factors dictate the filter size: the engine’s cubic inch displacement and RPM at maximum horsepower. Knowing those figures, use this formula to determine the minimum effective filtering area an engine requires, where A equals filtering area:
A = (C.I.D. x RPM) / 25,500
With the effective filtering area established, we can determine the required element height (including the rubber sealing edges), where H equals height and D the estimated maximum element diameter that will fit in the engine bay. The .75 figure is to account for the element’s rubber sealing edges:
H = A / (D x 3.14) + .75
Let’s apply these formulas to a carbureted 454 cubic inch big block V8 with a 6,700 rpm horsepower peak. Applying the formula for filter area, we came up with the following:
119.3 = (454 x 6700) / 25,500
2.71 = 119.3 / (14 x 3.14)
If you refer to the Summit Racing catalog, you’ll note that K&N part number KNN-E-3027R is 14 inches in diameter and 3 inches tall, so it would be suitable for our 454. We should point out this formula usually produces an odd element height. If that’s the case, simply select a taller element.
That wasn’t so hard, was it?