02GF74
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 posted on 16/1/06 at 12:33 PM |
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poor man's carb trumpets?
now that I am a fibre glassing god  can I not make some fibre glass trumpets for dellorto carbs?
There is some theroy about the length and shape but a tapering out cone, smooth inside and curved outer lip should be reasonably ok?
Possible problem with petrol getting on to the fibre glass perhaps but a lick of paint should sort that out?
You can get carbon fibre ones so why not GRP?
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smart51
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| posted on 16/1/06 at 12:40 PM |
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Carbon Fibre has the whole bling factor. GRP is probably OK.
The shape of the wall of the trumpet is to do with smooth air flow. I would go for a parabolic shape. Print out an oval drawn in, say, MS word and
cut out 1/4 of it so that the outside is perpendicular to the air flow and the outlet side is parallel, if you see what I mean.
In loudspeaker design, the diameter of the throat is related to the highest frequency of air that is required to flow through it and the diameter of
the bell is related to the lowest frequency. 1/4 of a wave length I think. The length of the horn is to do with the ratio of throat to bell
diameters [no sniggering at the back]. A look on google might dig up some formulas.
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David Jenkins
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| posted on 16/1/06 at 12:43 PM |
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I can't see any harm in trying, although you would probably be better off using epoxy resin, rather than the usual stuff that may not like
continual exposure to petrol fumes.
Down-side of epoxy resin is that it's not as easy to use as the regular resin (guess how I found that out...  ). It's not as easy to
wet-out the grp mat, and I think you have to use special mat or cloth that hasn't got the 'primer' on it intended to help the
initial wetting with polyester resins. The epoxy is a lot more expensive than polyester, and can cause allergic reactions for some people.
Once it's hardened off though, it's fairly impervious to run-of-the-mill liquids found on a car.
David
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CairB
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| posted on 16/1/06 at 12:46 PM |
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The benefit of air horns comes from the shape to ensure that the air flow remains attached and changes the direction without causing eddies etc.
A plain taper could be worse airflow wise than it not being there.
Sorry I can't remember the equation of the profile that is required but a search should yield it. This could then be used to cut a wooden former
that could be used as an internal mould. Alternatively could you borrow one to make a mould from.
Take a look at this for the general shape of some lengthy ones.
Cheers,
Colin
[Edited on 16/1/06 by CairB]
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