Matching the inlet manifold...smooth or not?

[Tommy Boy]

Whilst I have the top end off I am matching the inlet manifold to the port on a stage 4 (indian) barrel.

Think I heard/read somewhere that it is not advisable to polish the inlet port or inside of the manifold and it is better to have a 'rougher' finish... something to do with affecting fuel flow???... or did I just dream it?

If you could put down your dremels for 5 mins and offer your opinions it would be greatly appreciated.

Cheers,

Tommy

[Doom Patrol]

I don't think it can do any harm to match it personally. It improves the flow. On the other hand some say if it's rough it stirs up the mixture. You pays your money you takes your choice.

[nelson pk]

In my opinion matching the manifold is a good idea. Super smoothness is not considered essential but removing obstructions to general flow is good.

[Tommy Boy]

Thanks...

Def matching the manifold, (and gasket) to the port to improve flow... it was more the finish on the inside I was considering.

"On the other hand some say if it's rough it stirs up the mixture"... this is what I had heard/read somewhere back in time... wanted to see if it was an urban myth or if the clever people on here could offer their opinions.

[Doom Patrol]

My feeling is that the difference is likely to be negligible anyway.

[RICSPEED]

cant say i have heard any one say polished ports are a good thing

[Doom Patrol]

Polished ports are a good thing.

[warts]

I think it was Jerry Branch, the premier head man for US bike racing and big on measuring stuff, found that to polish or not didn't affect peak flow, but did hurt HP. The reasons given were that a polished surface encouraged fuel to "drop out" causing inconsistant mixture strength. A surface as dremelled, was about right, discouraging pooling of the fuel, while optimsing flow numbers.
The big thing in the US V8 racing world is CNC'd ports. These are left a machined ie quite rough to the eye, but polishing them rarley improves numbers.
When I had contacts in the Dept of Auto Eng at Lufbra uni, research was being done into what were effectively grooves across the piston crown to improve flame propagation. Apparently Ferrari showed some interest. There has been a flurry of interest recently into the Singh grooves to do likewise.
The question wether to remove or leave steps is somewhat more fraught. Mention of anti reversion steps is always a good way to start a heated controversy.
At first glance, less restriction mustbe better, but as flow is reversing - aka spit back or fuel stand off, a small loss of peak flow (in one direction)may result in higher output by optimising control of fueling. Remember, spit back has been through a carb at least twice, picking up fuel each time it passes the venturi in or out the carb don't care, so it is little wonder jetting can be such a drag.
Does anyone actually put lambretta ports on a flow bench?
ps. I know all the above is from four strokes and there will be differences, but two stokes are heat engines too and must obey the laws of thermodynamics.

[coaster]

So are we saying that if the manifold opening is bigger than the port you should match the port to the manifold to restrict flow in. But if the manifold opening is smaller it should be left so as to leave a 'step' to impede reverse flow (spitback) a bit like a pseudo reed valve?

[warts]

It's fairly obvious that you can flow more air through a bigger hole. The neatest trick is to flow the most air through the smallest hole, which means air flow has to be faster. Higher velocity means fuel is less likely to drop out and continue to flow longer due to inertia effects after BDC. This should aid running at smaller throttle openings, and as the throttle is rolled on.
As far as coasters question - this is the debate. AS two strokes get less fundamental research nowadays, there is less published work on which to base a judgement, but if the 4t reversion idea IS applicable then yes. smaller into bigger. Within reason, not humungous steps to disrupt flow and cause turbulence.
I'm on holiday in france atm and the connection is crap, but if this gets through, be assured I am having a great time and any grammatical and spelling mistakes are due to google translator and the free availability of alcohol, yup my bro in law the wine maker is here again.