blakep82
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| posted on 26/4/09 at 04:41 PM |
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effects of the intake manifold not being smooth inside?
made an intake manifold last year, but the angle the tubes came out meant the carbs hit the chassis, i need to introduce a 45 degree bend, which is
fine.
I've got some 45 degree exhaust bends which i'm going to weld in, but the inside diameter of the tube is slightly bigger than the outside
diameter of the original tube. (the inlets of my new carbs are also slightly bigger than the old carbs)
what sort of effect might this have on my performance?
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brianthemagical
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| posted on 26/4/09 at 04:43 PM |
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any chance of a quick diagram. changes in diameter can heve effects, good and bad, deoending on where they are in the tract.
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blakep82
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| posted on 26/4/09 at 04:51 PM |
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very rough diagram 
 
Description
the bigger tube isn't a tight fit over the old one, its as close as i could get. the blue blobs are weld, the gap between the tubes isn't to
scale, but not far off really.
i think the narrow tube is 48mm, the bigger tube is 52mm i think. can't remember
ideally i want to cut it down so theres no gap round the tube,
 
Description
but a step from the weld, but sleeving it would help for lining up and welding. would having a gap all round cause any problems? puddles of fuel
collecting in a hot manifold etc?
[Edited on 26/4/09 by blakep82]
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don't write OT on a new thread title, you're creating the topic, everything you write is very much ON topic!
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Mark G
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| posted on 26/4/09 at 05:00 PM |
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If you think of an engine as a pump, the idea is to pump as much air as possible so therefore to be efficient you need the inlet and exhaust manifolds
to be as smooth and efficient as possible. By using the pipes you intend to you will induce a disturbance in the inlet manifold which will in turn
reduce the amount of air you can get into the engine.
Reducing the efficiency of the air pump is just reducing the power of your engine.
Hope this makes sense but if I were in your situation I'd be looking at making some new manifolds from scratch.
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jacko
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| posted on 26/4/09 at 05:04 PM |
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Not the best but you could braze weld the inside and then smooth it down
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clairetoo
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| posted on 26/4/09 at 05:43 PM |
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Looks like a right bodge to me 
Why not just make a complete new manifold and do the job right ? I'm gussing the manifold will be a larger diameter than your carbs , if so not
only will the flow be rubbish you will effectively have a plenum between the carbs and head , so thrittle response will not be good .
Its cuz I is blond , innit
Claire xx
Will weld for food......
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blakep82
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| posted on 26/4/09 at 06:15 PM |
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the manifold is actually the same diameter (although the inlet ports on the head are slightly smaller, which now has me thinking thats problems too
)
i'm not sure its a bodge (yeah i know it is really...) but if i just did it again from scratch, i think i'd get it even worse.
anyone wat to make it for me?
________________________
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don't write OT on a new thread title, you're creating the topic, everything you write is very much ON topic!
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brianthemagical
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| posted on 26/4/09 at 06:40 PM |
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Are the injectors at the end of large dia tube then? If so, the gap will be a poor design. Eithe way it's still poor, although it is changing
dia. in the right direction. Turbulance will be dire, which might be an advantage. Personally, unless you can provide a smooth and gap free run, even
if there is a taper, it'll be poor. With the gap i'd so not a chnace.
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ncoll
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| posted on 26/4/09 at 07:06 PM |
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I would be more worried about the 45 degree bend more than anything, the only engine that i know with a 45 degree bend in it is a lotus sunbeam. With
the head modified and big valves fitted you get about 165 cubic foot of air per minute, fit the manifold with a 45 degree bend in it, you lose 20 % of
this flow, where as with a zetec, with a straight inlet manifold for webbers, you would only loose 3% of the flow. This may not help you, but it does
go to show what putting a bend in an inlet manifold can do.
Neil Collingwood
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DIY Si
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| posted on 27/4/09 at 01:47 PM |
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As said, bin it and start again. Yes, it's a bit more work, but you'll get the mainfold you want/need rather than bodging a bad together.
4 bits of mandrel bent pipe of the correct OD/ID, a flange (you may be able to re-use the old one), a hacksaw and a welder and you'll be done.
“Let your plans be dark and as impenetratable as night, and when you move, fall like a thunderbolt.”
Sun Tzu, The Art of War
My new blog: http://spritecave.blogspot.co.uk/
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coozer
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| posted on 27/4/09 at 01:52 PM |
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You can always cut some slots in the bug tube and nip them in till its a tight fit on the old one.
1972 V8 Jago
1980 Z750
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Fred W B
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| posted on 27/4/09 at 03:51 PM |
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Or an exaust shop may be able to flare the cut down stubs on a pipe expander to match your bigger elbows?
Cheers
Fred W B
You can do it quickly. You can do it cheap. You can do it right. – Pick any two.
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