At a bench depression of no less than 28 inches of water and using a velocity probe (straight section of tubing of roughly 0.020-inch i.d. and connected to manometer, "map" the velocity profile around the circumference of the carburetor inlet. In particular, note the amount of flow "separation" (possible turbulence or unstable flow) at the junction when virtually horizontal air turns downward into the carburetor throat. Keep in mind that this same "pulsing" flow can be read by the carburetor's bowl vents and/or main metering system, thereby not always providing controlled and proper fuel calibration (flow). Now, by removing the lid portion of the cleaner and remapping the entry pressure profile, you'll discover it is often the case that air cleaner lids have marginal influence on net inlet flow. It's the lower portion's shape and its relationship to the carburetor throat that has the greatest effect.

So, armed with this information, you can experimentally reshape the cleaner base to both optimize and stabilize flow into the carburetor. Even if you are not able to create measureable increases in total flow, reconfiguring the base in a way that minimizes "unsteady" flow will help vent tube and main fuel metering functions. Once on the dyno, you'll note that this translates to a more precise and repeatable fuel calibration as typically evidenced by a more controllable BSFC curve and improved power. For purposes of further clarification, we've included a little sketch intended to show areas of sensitivity in a typical air cleaner bottom.

Finally, I thought you might like a comment on this subject from a noted circle track engine builder who is no stranger to the CT readership, Dennis Wells of Wells Racing Engines. I brought Dennis into a little air cleaner design exercise I was conducting a few years ago and he got some first-hand experience on how air cleaner shape can impact engine performance. In his words, "To me, it's all about the design (shape) of the air cleaner's base. I still have the one you and I worked out several years ago and it's the one I use to evaluate all the 'new' stuff that comes along. At the time, it was worth 4-5 horsepower at the tire patch over any of the best stuff I'd been using, and the gain is clearly a function of the base shape.

"In fact, where I do see the power improvement in an air cleaner is in the radius in the bottom of the cleaner. When the airflow is relatively stable, you can see a reduction in the brake specific numbers and, of course, the slight boost in power.

"Proper turning of the air into the carburetor is the critical element. Overall, I've not seen any particular gains when switching to one of the 'top loading' air cleaners that flow through the top. The short path for the air is along the base of the air cleaner and how it's able to turn air into the carburetor. I can't emphasize the importance of this."

Engine builders of Dennis' experience simply don't use (or recommend) parts that are unproven on the track. In this particular case, you stand to gain from his experience. As a well-known camshaft manufacturer friend of mine of many years ago drilled into me, "It's not what you do, but the way how you do it that counts." Racer Brown was seldom incorrect. The term "air cleaner" is almost a misnomer, relative to the other benefits it can provide.