Subject: Re: air horns


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 On Sun, 13 Nov 1994 dcoleman@osiris.ac.hmc.edu wrote:
>   Mark,
>   Yes, you did miss when this was brought up before.  What I mentioned
>   last time that I didn't mention this time was that these air hors
>   are part of a project for a class I am taking.  I have to make 10 of
>   them for this class and will probably stop at that simply because I
>   don't have time to do very many more, and, like you said, there
>   really isn't very good money in the air horn business.
> 
>   So what were these interesting results you found?  I'm curious.
>   Also, how does your modular assembly work?  Are they tapered or do
>   they have a straight bore like mine?  I pretty much bagged the
>   tapered bore when I decided to make them modular, though I suspect a
>   tapered bore would flow a little bit better.

Well I did some testing with an assortment of horns and filters trying
to determine how long a horn you could use for a given filter and
whether a horn alone made more or less power that a horn and filter. I
also looked at which had a greater effect; tract length or air filter
type, tract length or horn to filter clearance and finally tract
length or air filter use.

The first and most obvious observation was that without an air filter
power droped over time due to accelerated engine wear. This power loss
was greater than any improvement we could achieve with horns, filters,
etc.  Conclusion, whatever is done, a filter must be incorporated into
the design. (this was not really a surprise).

Next observation, the damping action of an air filter smoothed out
power pulses, and stabilized power output due to its interfering with
pressure pulses and buffeting associated with turbulent, unstable
airflow accross the unfiltered carb top or air horn. Conclusion, if
properly designed a filter will actually increase power output over an
open horn.

The next observation was that when fitting horns into a filter with an
impermeable top (such as the K&N's) the theoretical minimum distance
from the topmost portion of the horn to the underside of the top cover
of the air filter was equal to 1/2 the diameter of the carb throat. In
some instances even this distance had a negative effect on air flow,
and so a distance of 3/4 the diameter of the carb throat was set as a
minimum and a full diameter was considered ideal. It was found that in
some instances even when the air horn to fliter top distance was great
enough that no drop in air flow was measured, turbulence was created
which caused an eratic vacum signal at the jet. Many air horn/air
filter combinations were found to engender a net power loss due to
these two phenomenon. Often a greater power loss was realized by using
horns that were too close to the top of the air filter than the gains
of extra tract length could justify.

Although not germane to your situation, since based on you description
your horns have a nearly optimum shape, we did find that horn shape
was in fact more significant to air flow potential, and therefore to
power production, than tract length tuning, in most instances. We
found that optimum tract length was often impossible to fit within the
physical limits of the car body, and that compromise lengths seldom
equaled horn shape for power production.

The last thing I think you will find interesting becuase it relates to
your question of straight vs. tapered configuration. I also opted for
a straight section design, but seldom sell the horns with the
extension.  We found that when the total horn length begins to come
near to that of the carb throat diameter, a straight cross section
extension presented a restriction to air flow at higher RPM's. A
tapered or flaring cross section of aproximately 7 degrees (if I
remember right, I am pulling this stuff out of my memory here so
forgive me if my figures aren't quite right) was needed to ensure no
restriction. The problem with this was that it quickly became
impossible to fit the air horn into the filter with properly shaped
horns, of a length sufficient to have a significant positive effect on
tract length tuning.

If a customer must have a long horn to contain a fuel stand off (due
to a radical cam etc.), I will make him one but I try to get customers
to do their tract length tuning with manifolds/spacers.  Spacers still
have a restrictive effect if made too long and with too small a cross
section but at least the restriction is before the fuel jet and so
will not effect mixtures.

>   Anyway, since I am only making four sets for sale (one is for me), I
>   don't think you have to worry too much about me cutting into your
>   (non)profits.
>   -Dave
> 
> PS.  Now wait a minute...  Did you always sell your air horns for $1 less
> than mine?  :-)

As to my pricing see my other post on this question. I must admit to a
certain relief when I saw your price though. :-) And I am glad you
won't be competing with me. If you'd be willing to make 11 horns I
would like one just to see and display, but I don't want to keep
someone who wants a set to actually use, from being able to get
them. If this is possible let me know.

Hey here's a question for you. Did you check to see if your horn
design affects the air pressure/flow at the dashpot air intake? I ask
because I once saw a horn/air filter from England that created a
slight pressure drop at the dashpot intake at high speeds. This lead
to some rather erratic operation. Only saw it that one time but its
such and off the wall thing I though you'd enjoy.

Marc Sayer