albertz
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| posted on 24/6/05 at 07:10 AM |
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Fabricating an inlet manifold - advice?
I am currently considering making an inlet manifold and plenum for my 4AGE. Due to a mismatch of newer and older version engines I am estimating that
I am losing about 20bhp with my current set-up compared with the standard factory engine. Therefore urgent mods are required!
I plan to make a simple plenum from round tube (I have read that 200% of the engine capacity is about right - PPC June issue). I am planning to use
tube for the runners and flatten it out to match the no circular ports on the engine. The runners will be straight and be as long as possible,
probably about 150mm.
My question is, in the PPC (Practical Performance Car) magazine there are a series of photos showing a manifold being made and they appear to show the
runners protruding into the plenum by about 25-30mm. Is that correct? I would have thought that they should have been flush and smoothed of to aid
airflow.
Any words of wisdom on this subject appreciated. This is not a burning issue right now for me and will probably not be done until after the summer (if
summer ever happens up here).
Cheers
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NS Dev
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| posted on 24/6/05 at 07:28 AM |
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I thought exactly the same re. that article!!
The best way to do it would be to flare out the ends of the runners to form bellmouths, then cut the plenham wall with holes to suit the bellmouths,
and weld these in flush with the wall of the plenham.
If you have access to a lathe it shouldn't be too tricky to turn up a former to make the bellmouths, and heat the tube up and press/hammer it
over the former. (Make the tubes too long to you can cut the hammer damage off if doing it that way!)
Cheers
Nat
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nick205
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| posted on 24/6/05 at 07:29 AM |
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I would have thought the inlet runners would be better ending flush with the plenum as well (I'm no expert though).
Is it worth having a look at some factory fitted cast inlet manifolds? They might offer some pointers on how to design your own.
Keep us posted on your progress.
Nick
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MikeR
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| posted on 24/6/05 at 09:20 AM |
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isn't there some theory that by making it stick out a little you create a low pressure type thing around the edges of hole and slightly more air
is sucked in from the sides of the tube......
I can remember the image but can't think of the technical terms.
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bimbleuk
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| posted on 24/6/05 at 09:33 AM |
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I'm on the same thinking as MikeR
The bell mouth shape accelerates air into the runner and the effect extends behind the bell mouth edge. Plus you want the flat baffle behind the
trumpet.
I can't remember which book I read this in, either 21st Century Peformance or one of Corkey bell's books.
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paulf
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| posted on 24/6/05 at 09:40 AM |
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I was looking at some BMW bike throttle bodys complete with airbox and they were also like this, the trumpets were in the centre of the box. I think
it is to do with the reflection from the airbox sides creating a pulse effect.
Paul.
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bigandy
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| posted on 24/6/05 at 09:49 AM |
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I'm going to be looking at doing somthing with the 4a-ge that is going into my car soon too. I've got to either cut down the stock inlet
manifold to fit under the engine cover, or make up a custom one.
I was hoping to take inspiration from some of the more expensive stuff available off the shelf to fit. I'm still not sure about throttle
bodies, or a single butterfly on the inlet though yet.
I think the best thing you can do is look at what is about and working well already. It should give you a better idea as to what can be done...
Cheers
Andy
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Jeffers_S13
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| posted on 24/6/05 at 09:58 AM |
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I made my headers/bellmouths flush with the inside of the plenum as per the factory one. I found a lot of people making plenums with the bellmouths
in the middle of the plenum, but I guessed this was mor critical for NA applications. I also have very short headers, as there is very little room in
the engine bay of a 7 its quite limiting on what can be done regards the header lengths even though most inofo I found on this recomended relatively
long headers (I was using 'Forced Induction Performance Tuning' by Bell as a guide). After speaking to a fairly knowledgable friend he
said forget the header lengths its not *that* critical and nor is the bellmouth position, my engine runs great so far, Im not after squeezing the
ultimate power out of the engine or getting fantastic efficiency etc which I guess is affected by the header lengths and bellmouth positions. Pics of
my effort are in my archive...I think. I think header lengths and bellmouth positioning in the plenum is very important for NA power outputs.
[Edited on 24/6/05 by Jeffers_S13]
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NS Dev
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| posted on 24/6/05 at 11:06 AM |
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quote:
Originally posted by MikeR
isn't there some theory that by making it stick out a little you create a low pressure type thing around the edges of hole and slightly more air
is sucked in from the sides of the tube......
I can remember the image but can't think of the technical terms.
That's right Mike, but if you flare/bell the ends of the tubes, with a radius at the end, you will get much the same effect with the tubes
mounted with the "back of the radius" welded to the airbox. Pushing them slightly further in could help, but then you would need to think
of a way of welding it up.............................(somehow getting the flared end through the hole that is the size of the unflared bit!!)
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MikeR
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| posted on 24/6/05 at 11:42 AM |
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my way is easier - no need to make a flare!
(well thats my story and i'm sticking to it as i'd what you said didn't occur to me)
As for welding ...... couldn't you just weld the other side ?
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NS Dev
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| posted on 24/6/05 at 11:50 AM |
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If you are saying that you can use pipes with unradiused ends then no, that is incorrect. Some time ago Dave Walker (of Emerald) did some flowbench
tests in CCC mag on various ram pipe designs, and discovered that the taper/bell didn'r really matter, but the end needed to be radiused. If it
was just a parallel tube end, the flow dropped off dramatically.
You can't weld the other side cos it's inside the manifold chamber!! (unless you have long thin arms and a very small welding torch)
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dmottaway
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| posted on 24/6/05 at 11:56 AM |
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you might want to look at
http://www.sdsefi.com/techinta.htm
the whole FAQ is an interesting read.
dave
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garage19
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| posted on 24/6/05 at 11:58 AM |
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quote:
Originally posted by Jeffers_S13
I made my headers/bellmouths flush with the inside of the plenum as per the factory one. I found a lot of people making plenums with the bellmouths
in the middle of the plenum, but I guessed this was mor critical for NA applications. I also have very short headers, as there is very little room in
the engine bay of a 7 its quite limiting on what can be done regards the header lengths even though most inofo I found on this recomended relatively
long headers (I was using 'Forced Induction Performance Tuning' by Bell as a guide). After speaking to a fairly knowledgable friend he
said forget the header lengths its not *that* critical and nor is the bellmouth position, my engine runs great so far, Im not after squeezing the
ultimate power out of the engine or getting fantastic efficiency etc which I guess is affected by the header lengths and bellmouth positions. Pics of
my effort are in my archive...I think. I think header lengths and bellmouth positioning in the plenum is very important for NA power outputs.
[Edited on 24/6/05 by Jeffers_S13]
I too made an aluminium plenum chamber with internal bell mouths. But to be honest with you as we are forcing the air in with a turbocharger i think
it was probably over the top!
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bimbleuk
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| posted on 24/6/05 at 07:02 PM |
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Yep with forced induction just make the flow path smooth.
I'm also just a bout to play with my 4AGE 20V. From the factory theres a large surge tank with curved inlet runners.
At the mo I have just the factory plastic inlet runners and get a decent 150BHP.
First thing will be to try alloy straight inlet trumpets from 50-120mm to see if theirs anything to gain.
I'm really after a better induction noise but they may become TOO load so then I'll think about an airbox.
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