15 replies to this topic

[Dusky]

Hey!

On the pictures you can see my inlet manifold has lots of 'bumps' and stuff inside it.

Isn't this very restrictive for the gasflow?? Should these be sanded away?

 

 

 

 

 

 

 

grtz

[Cooperman]

A 'Flapwheel' on a flexible shaft is one way to smooth it all out.

[carbon]

A slightly rough surface won't make a big difference to the gasflow, but can help to stop fuel 'puddling' in the inlet manifold.

 

Unless you're planning to significantly uprate the engine I would suggest leaving as is, by all means look to blend in any radii and avoid any mismatches between the carb / inlet / head as this will give you much more flow improvement for less effort.

[Carlos W]

There are several schools of thought on this.

 

 

 

The rough surface may cause friction between the fluid travelling through the pipe and the particles in contact with the pipe, this then causes friction between particles next to them slowing down the whole flow.

 

People then talk about creating turbulent flow ensuring the fuel vapour is thoroughly mixed in the air!

 

I like the idea of smooth and shiny!

[Tamworthbay]

Having been bored by my mate who used to run Lola's wind tunnel on MANY occasions, he was sure that a rough surface in the region of 20-50 microns was the best. Or in the real world, just a normal 'unpolished' surface is fine. Very rough (like the pic) and very smooth are equally bad for different reasons. As said above you probably won't notice any difference whether or not you do it.

[Turbo Phil]

Remove the large lumps & bumps, a decent ground finish will be fine, I'd use an 80 grit flap wheel personally. It does NOT want to be polished.

Phil.

[Dusky]

okay, so if I read all the comments I should remove the big lumps but not polish it.

Imma get hol dof some 80 grit flapweels ^^

[Carlos W]

Yeah, I just like shiny things, take the rough off but don't mirror polish sounds like the way forward!

[timmy850]

What I was told when we used to do a lot of small boat sailing and prepping the bottom of the hull was that if the surface is mirror smooth it created more resistance. I think the theory was that with a slightly matte finish a layer of water would stick to the hull and the friction going through the water would be water/water friction, which was apparently less than water/hull friction.

The same could apply here I guess, it might also prevent droplets forming?

[Turbo Phil]

The inside of the manifold does not want to be finely polished, a nicely ground finish will help keep the fuel atomised by causing a very small amount of turbulence & stop the fuel puddling & running down the walls.

 

Phil.

[minilee94]

If its so bad polishing it whats the whole point in polishing and porting heads that works well so how come its no good on an inlet

Also look at the maniflow manifold best manifolds out there for mini etc an all there manifold are smoothed out due to being made out of steel

Edited by minilee94, 22 December 2013 - 10:52 AM.

[Ethel]

There isn't one, well not the polishing bit anyway. The commercial reason is shiny sells.

The fuel puddling argument is the best I've heard, the increased surface area will also affect heat transfer. Cylinder head ports are likely to be a fair bit warmer than the manifold, so better at evaporating fuel regardless of finish. The manifold will have been matched to the engine flow with the surface finish it has, I doubt it will be a restriction unless you increase the airflow demands put on it. The fastest moving air will be round the outside of the curve. Inertia will tend to slap the fuel sprayed from the carb against the inside of the curve. So a rougher inner curve will do less to slow the air and more to return the fuel to the air stream

[Fast Ivan]

If its so bad polishing it whats the whole point in polishing and porting heads that works well so how come its no good on an inlet

Also look at the maniflow manifold best manifolds out there for mini etc an all there manifold are smoothed out due to being made out of steel

the term porting and polishing a head is a bit misleading as the same advice given above is true for the head as well, they should not be polished and smooth.
I'm not sure about the puddleing etc. as I just don't know about these things but I do know that a slightly rough surface can improve flow.
Google Reynolds number, boundary layer affect for some in depth reading or Google how a golf ball flies through the air for a simple example

[Turbo Phil]

If its so bad polishing it whats the whole point in polishing and porting heads that works well so how come its no good on an inlet

Also look at the maniflow manifold best manifolds out there for mini etc an all there manifold are smoothed out due to being made out of steel

 

It isn't a good idea to polish the inlet port on a head. The term "port n polish" is still banded about, but a polished finish is not what you want.

Phil.

[tiger99]

Yes, smooth it, but don't polish. I WOULD polish the combustion chambers in the head, and valve faces, because gas flow directly against those surfaces is not so important, and a polished surface will help discourage carbon buildup, but I would NOT polish the ports, where there is high gas flow. I don't do many heads these days, but generally just used coarse grinding stones and small flap wheels, whatever would get to where I wanted to clean up.

 

It is far more important to match the manifold and head ports, and gasket, and find a way of keeping them aligned, and I am not convinced that anything I, or anyone else, has done in that area is entirely successful. A good trick, used on certain engines, but applicable only to the inlet ports on the Mini, which are circular, is to conterbore the manifold and head, and insert a sleeve, which will maintain alignment, and just make the exhaust ports on the manifold larger than those of the head. Someone is bound to disagree.....