Theory as to why you cant run a K&N filter on a spree

[noiseguy]

OK, I researched this to refresh my memory:

Here's a simple formula: 42000/RPM = length in inches. Length from reeds to intake mouth. For 4000 RPM, that's 10.5" Guys at mopedarmy were discussing this; this equation in similar form is in at least one internal combustion engineering books I've read so I think it's right:

http://www.mopedarmy.com/forums/discuss ... 57/105532/

For 4-stroke, it's double the above (2 rotations per intake breath.)

Then you can argue about whether the carb works better in the center of the tract or off-center, and how much off center. More centered = more tuned to one RPM, as I recall, but don't quote me on that.

It's the length mainly; cross-section is more of a sizing thing, and then you can get into the dynamics of the airbox, etc. If you really want to get into it, look up "Helmholtz resonator" online. Here's a taste. Empirical work is really better for this kind of stuff.

http://www.phys.unsw.edu.au/~jw/Helmholtz.html

When I worked in automotive, there were a group of guys that spent a lot of time tuning the boxes to minimize noise; external noise is one of the main reason these boxes are used.