Date: Sat, 18 Oct 1997 11:52:52 -0700
From: Robert Harris 
Subject: Dumb resonance idea - maybe.
To: 'Dave Williams' 

Has anyone ever tuned an exhaust system by using resonant stubs - closed end
pipes added to the system??  Correct me if I am wrong (probably) but don't the
equal length headers try to work like an organ with tuned pipes to get the
pressure wave reflections to arrive at the valve at the proper time?   Adding a
resonant stub could change the resonant frequency without changing the headers.
Add a butterfly valve at the H pipe, and a tuned resonant stub pipe.  When the
engine is in the resonant range - open the butterfly and now the exhaust
resonates at the current rpm - not the header tuned RPM.  Say maybe tune the
headers for mid - upper and the stub for converter stall.  Nice boost coming
off line and better mid upper to be lethal.  Would have to use higher rpm
tuning for headers as stub can lower resonant frequency but not shure about
raising resonant frequency.  Wish I had the math background and some acoustic
engineering data to back me up.

Got the idea from a mid eighties Honda accord EFI.  Uses a curved intake
manifold with two separate runners per cyl. One a short radius inner runner and
two, a long radius outer runner.  The join at the port and the inner has a
butterfly that the EFI computer opens into the plenum at the right rpm.   Honda
claimed it greatly broadened the usable power region, which I think might be
true.  Reading Vizard and his discussions of resonant wave effects gave me the
mind fart to vary the resonance while running and thus broaden the range.
Probably would be easy to tune into a given range once approximated also.

Bad ASCII Drawing

      E E E   E      E E E   E     exhaust ports
|   |  |    |        |   |  |    |       headers
\   |  |   /         \   |  |   /
  \ |  | /             \ |  | /
     \/                   V
       | -------+------- |            Cross over
      |         X         |             Outer - exhaust pipes
      |         |          |              Inner - tuned stub with butterfly
      |         :          |
      |         :          |
       o        X        o