Fabricating my own inlet manifold.
Turbo Mini Forums

I replied to another topic concerning inlet manifolds.
My theory concerning forced induction and cylinders not going lean turns out to be completely stupid..

Doing some more reading about inlet manifolds for forced induction revealed that indeed the inner two cylinders do go leaner during boost.

Now I am in the process of building my own blow through, intercooled, supercharged 1293cc engine.
Currently I am working on the inlet manifold and reading about inner cylinders going lean made me realise I might be able to reduce this effect because I am making my own manifold.

My questions:
-What is causing the inner cylinders to go lean? Is this because flow direction through inlet manifold goes from central point (carb) outwards to the head directing the flow to the ports of cylinder 1 and 4?
-Will this be better when the manifold has two pipes that go straight into the head?
-I am planning to make a 90 degrees turn directly under the paravan from the right side down and then another 90 degrees turn to the front towards the head. I am afraid this combination of turns will affect the distribution of charge flow to left and right side of the cylinder head. I will see if I have a picture to clarify.

I realise that most of the people here have turbo engines and have limited possibilities when it concerns intake manifolds but basically I would like to know if anyone ever made their own manifold and how the above issues where approached.

Thanks in advance for your help.
Han

Dazed and Confused....

This is the thread covering my inlet manifold design:



I'm not sure how you have determined that the inner cylinders go lean under boost. My dual wideband testing with a NA MG Metro inlet showed that the inners ran rich through most the rev range.

Data in this thread:

Edited by Paul S on 27th Aug, 2008.

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

My experience with superchargers may be a bit out of date now,but it used to be recognised practice to use what is called a"log" manifold i.e.two tubes straight in to the inlet ports connected by another tube across their outer ends.The theory was that the sharp turns help to cause turbulence in the airflow and keep the fuel vapourised and the charge more homogeneous (Thats a fancy word for evenly distributed)This probably applies more to suck through systems as they have been more common.Try reading a book by Allard on forced induction or spend an hour or two on research on this site and the net generally -its all out there somewhere

Thanks for answering you my questions guys!

Paul,
I did already read your post about making your manifold.
Actually I did a search on "manifold" "inlet" etc, a lot of info but not what I was looking for.
Anyway, I have nowhere near the space you have as my supercharger will be in that place.
I guess tadge answered my initial question, isn't the way the inlet runners are placed going to reduce flow dramatically?

Tadge,
I did a search (also on the net), I even have read a book by Allard.. :)
I just can't find (or am not smart enough to understand) what is important in making a inlet manifold work.. Homogeneous charge vs flow?

I understand what tadge is saying, you want to have a homogeneous charge.
I looked at the space I have and think I will be able to use a log type manifold design.
My plan is to make a cross tube perpendicular to inlet tube; both inlet and 'cross-tube' will be 48mm ID.
On both ends I will attach 39mm tubes to the inlets.

My final question before I start sawing.
Should I weld the 90 degree turns I have to each end of the cross tube to improve flow towards the head or should I weld the inlet tubes straight (perpendicular) to the cross tube because of reasons tadge mentioned?

Cheers,
Han

Dazed and Confused....

Hi,
I think you're going to have problems due to the siamesed ports on the A series. Where you have a smooth bend the charge will favour the outside of the bend, not a problem if it's only got to feed one cylinder.
From the posts on here I've read it seems there's still no concensus on what causes distribution problems with straight runners.
Cheers,
Gavin :)

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Hi Gavin,

Thanks for sharing your thoughts.
I understand what you are saying but I'm not sure if this will be a problem; there would be about 130mm length of staight tube from the 90 dgree turns to the head.

One other thing came to mind, will the cross tube/plenum be large enough if I use the same size as that of the inlet tube (48mm)? Not that I have space for a substantially bigger plenum but still want to know.

Cheers,
Han

Dazed and Confused....

Maybe I shouldn't call it a plenum, it's an inlet manifold.
Carb is a fair bit upstream.

Han

Dazed and Confused....