I had decided to add "ram air" cool air induction to my single cylinder BSA powered partial streamliner and had already started on an airbox when I remembered reading about a Helmholtz resonator. After searching the web, the best article I found was by Kevin Cameron who stated as follows:
"a forward-facing intake, operating at an unlikely 100% pressure recovery, is only worth about a 3% pressure gain at 150 miles per hour. Yet these “ram airboxes” were regularly showing much larger gains of 10 or even 15% more torque. Where was the extra power coming from?"(
http://www.thunderproducts.com/airboxesdynotech.htm)
He went on to state that some of the improvement undoubtedly came from the cool air, but that more of the gains came from the Helmholtz resonator effect. The formula for determining the frequency expected was: R(cps) = 5280(a number that even I can remember) x square root of A/(VL), where A is the area of the inlet(s) in square cm, V is the volume of the airbox in cubic cm,and L is the length of the inlet pipes in cm (only the length of one pipe, if more than one is used).
In my case, "V" will be 15.24x15.24x15.24cm, "A" will be two 2" diameter tubes, or about 40.5 square cms, and L will be the distance from the carburetor to the front of the box, which is about 76cm. Doing the math results in a frequency of 64.77 cps, or 3886 cpm. Because I have a 4 stroke motor, I'm assuming that this will be equivalent to 7772 rpm (an intake pulse every other revolution). Not too far off, but I would like to move this up to 8400 rpm. I can do this by reducing the area of my inlets, but I won't until I can put it on a dyno.
So far, so good, but the air filter is a problem. I built my box around the filter, which will probably kill the resonance. I've read that the filter should be upstream from the resonator box, which is a little difficult unless I can find some that will fit in a 2" diameter tube. I'm also wondering about my inlets, which are actually only 1-1/2" at present.
I'm looking for confirmation of my idea, and suggestions for air filters. Also, since my airbox is .030" aluminum, I'm wondering if I'll get any resonance at all. I could wrap it in fiberglass and resin.
Tom