INLET MANIFOLD MODIFICATION

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The following article was submitted by

ALAN OSBORN ; www.aoservices.co.uk

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INLET MANIFOLD MODIFICATION ;

Written Article about Modifying Inlet Manifold submitted by Alan Osborn

Manifold (again)

In an attempt to get it all together herewith a nice correct drawing of the modifications I have done to the Reliant Inlet manifold (thank you Chris T). For those who have been ‘looking in’ for some time a brief overlook of past articles. Back in Mewsletter 88 I spoke of my disappointment, in what has been named as the Rebel Racing modifications. That is the manifold with the cut away between 1-2 and 3-4 at the head face (some details by Brian in Mews 91-Pg30) I find that nobody can explain how this works, while I am of the opinion that it doesn’t! I came to this opinion by replacing the RR one with A) a standard manifold and B) the one with my ideas. But my few improvements can be confirmed by improvements in pick up and sparkle. To complete the story we have Clive Angel in Mews 92 Pg23 and Mews 93 with words and pictures by me.

A repeat of the facts in case you can’t find old Mews. First if we look at the inlet manifold channels you will see that 1-4 are 50% longer than 2-3. If you ever remove a head after a few miles then you should see that 2-3 is richer as opposed to 1-4. The difference is only slight and it just shows on the plugs, provided every thing else is in good order. SO to balance this mixture 1-4 needs to be richer while 2-3 needs to be a touch weaker. So see the 4 or 5 mm hole between 1-2 and 3-4. This steals from 2 and 3 and gives to 1 and 4. In itself this doesn’t give any massive improvements but it does balance the mixture so it must be a step in the right direction.

Second idea is that mixture (air) when it is moving does not like going around a sharp corner, it will turbulate hence effectively reduce the diameter of the pipe it is travelling down. So we now look at the point where the swan neck drops into the manifold and we find a very sharp corner! Remove it. Help the gas get around the corner quicker etc. You then should have a circle from the swan neck drop and an ‘egg’ at the manifold, these two need blending, this is easiest done within a ¼” gasket between the two parts.

Once you have done these modifications and refitted the manifold you should find that the increased flow makes it draw better hence the mixture goes a touch rich-so you weaken it off a touch.

This must be tuning the right way, as the engine is now pulling more efficiently. Some persons have then questioned the overall MPG, fine, but IF you can modify/improve so that the engine pulls on the carb better and/or runs richer { due to tuning before the carb main jet } then it MUST be running MORE efficient . SO provided you don’t ‘woosh’ everywhere then you must use less petrol to do what ever you do with your car/engine .

I haven’t done a before and after MPG but the mixture change and sparkle tells me it works and costs nothing. Again this is a something for nothing mod. Have fun.

Alan Osborn Mid Norfolk.

Modified Inlet Manifold Picture submitted by Alan Osborn

Modified Inlet Manifold Drawing submitted by Alan Osborn

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ALAN OSBORN ; www.aoservices.co.uk

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