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MG Midget and Sprite Technical - 1500 inlet manifolds

good evening folks...i am getting a new head soon with improved/polished ports. Is there any need to do the same with inlet manifolds as the are usually cast alluminium and very rough inside. your thoughts appreciated
a boyle

You do not actually want to polish ports, a rough ground finish is better, or leave the cast surface, removing only big lumps.
It is not recommended to reshape ports until you understand it all, which would mean you are not asking. The single important point is that there is no step for the incoming charge to hit as it flows - pretend you are a fuel molecule running into a subway tunnel. A similar step in the opposite direction may even be useful, and it makes the previous step one impossible - the manifold port should be slightly smaller than the head port for amateur/street tuning.
It is not desirable to "match" ports, especially if it causes a bulge at the mating surface.

FRM
FR Millmore

I have a feeling that polishing the inlet tracts to a mirror finish is actually detrimental to flow. Does anyone know if this is correct?
BH Harvey

hi there BH
I seem to think i had read that somewhere, hence this topic..Thanks your comment, lets see if there is a reply
rgds tony
a boyle

Already told you - read!

Too smooth causes fuel to drop out of the flow and condense on the walls. The is even some info to the effect that this applies even in combustion chambers, such that for a while, race engine builders were using a punch to put dimples in the chambers; undesirable part here is carbon build up. Ground with a coarse stone is the norm for ports.

FRM
FR Millmore

BH, AB,

As FRM states reading (and understanding) is key to getting the most out of your new head. There are books by Vizard, Burgess and others which will give you most of the info you need. Postings here just won't be able to cover the field in the required detail.

But to answer your specific question... Yes, there will be some work to do on the manifold to get the very best out of the head. But you need to ask yourself "what type of driving am I doing and are the improvements really required".
BTW, polished is bad, too rough is bad, too big is bad, too small is bad. Now work out whats right if you can ;0)

MGmike
PS: 60 grit emery paper as a single strand flap disc is commonly accepted as the way to go.
M McAndrew

I think what FH is saying is that he told you the answer is that on the inlet, the air flow needs to be slightly agitated by the rough metal. . . . the exhaust manifold needs to be as smooth as possible



(In simple terms. . . this plus high flow filters and exhaust constitute the first stage of tuning)
d j kirk

This thread was discussed between 01/08/2012 and 12/08/2012

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