Miata Mailing List: May 1994, Message #61
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   From: jwa@yog-sothoth.dcrt.nih.gov (James W. Adams)
Subject: Re: Weber DCOE update
   Date: Tue, 3 May 1994 16:17:44 -0400

A couple people's comments/questions about my mention of velocity stacks in
my earlier post suggest some explanation:

There are basically three reasons for the fitting of stacks (trumpet-like
air horns) to carburetors:

1) Air "straighteners": To smooth the flow of air into the carburetor and
reduce turbulence.  This is the reason for using the very stubby ones inside
the filter that I mentioned.

2) Induction ramming: The intake runner to each cylinder acts as an
organ pipe of sorts.  At the right engine speed, the intake pulses will
be reflected at the open end (or a plenum, as in the stock Miata FI
intake manifold) such that a negative pressure pulse will arrive at the
intake valve seat just before valve closure.  This permits a slight
additional charging effect, giving as much as a few percent boost in
output near the speed for which the length is tuned.  In racing,
different length air horns were sometimes fitted to adjust the effect
for short tracks or longer race circuits.

3) Fire safety: In typical Weber DCOE applications, the intake air
pulses may result in a column of fuel mist which extends a short
distance out past the front of the carb.  This creates a severe fire
hazard.  In racing applications where no air filters are fitted, stacks
are fitted to contain this mist, reducing the risk of fire.