1/4 Wave Tuning

[Donald Whisnant]

> From: Chuck Tomlinson <tomlinsc at ix.netcom.com>
> Date: Tue, 18 Jun 1996 14:05:41 -0400
> Subject: Re: 1/4 Wave Tuning
> 

> The module is called a Helmholtz resonator, and its primary (I think only) purpose
> is noise reduction near a particular frequency.
> 
> According to the Bosch Automotive Handbook,
> 
> Volume = (Sm/l)*(c/omega)^2, where
> 
> Sm = avg. cross-section area of the runner
> l = runner length
> c = speed of sound in air
> omega = resonant frequency = 2.pi.f, for f in Hz.
> 
> AFAIK, it adds no power (maybe reduces power slightly), and the shape does _not_
> matter.  I don't know why the GM manual calls them "1/4 wave" modules, because
> the volume (not the length) is the important parameter for a Helmholtz resonator.
> Chuck Tomlinson
> tomlinsc at ix.netcom.com
> 

Thanks Chuck...
I thaught that maybe they called it a "1/4 Wave Module/Box" because it had
something to do with the 1/4 wave of the frequency of concern ... I was also
trying to figure out if it had some function as a "reservoir" for the air that
could maybe provide for a smoother air induction from the filter box into the
intake...  Since I retrofitted this engine into an older car, I of course had
no such "1/4 wave box".  Instead I did some fiberglassing to modify an air
cleaner box, and used an industrial/aircraft grade 3" duct (as the intake
opening measured 2-7/8") to run between the aircleaner box and the intake.  The
result looks rather "different" -- as I had to route this large Carolina Blue
duct in a long circular fashion (it is about 4-5 feet long) to clear the cooling
fan (which I had to switch the an electric unit) and to clear the belt and many
pulleys...  I would like to find a "1/4 wave box" (or something similar) used for
that engine off of a truck or something just to clean things up and make it look a
little neater ....  Though right now it is most definitely a true cold air
induction system -- plus as smooth as the inside of the duct is, there should be
no turbulance at all from "ripples" in the sidewalls (since this duct uses a hard
plastic core wrapped external to the inner wall) --- I'm almost afraid that
switching to such a box may even hurt the effecientcy/performance...

Thanks again...
Donald Whisnant
dewhisna at ix.netcom.com