pollished ports (supposedly not all their cracked up to be?)

[MitchE]

well i was having a chat to the car restorer/mechanic (responsible for my celica being comlitley blasted and on a roticary) about my engine, when the topic of ports came up my original plan wast match and pollish them but he raised the point that pollishing is far from the best way to get the most air flow.
Befor he started restoring cars he was a reputable aircraft painter and he was saying that air will stick to a polished or verry smooth surface creating friction. He then proceded to tell me that a consistant matt or rougher surface will hold a thin layer of air that will allow the air to flow more freely tham pollishing the same prinsiple is used on high speed aircraft aparently.
my question is has anyone herad of this theory or knows a little more about it?

[RAd28]

best example i know if is in pipe, fluid dynamics works the same (or very similar) with liquids as it does with air. when you have a surface, any surface, fluid closest to the boundary will flow slowest, the closer to the center of the pipe (or port in this case) the faster the air can flow due to less drag.

when you DON'T polish your ports, and leave the standard casting finish, you will definatly create the thin layer of air that your friend suggests, however this is just moving the low friction region closer to the center of the port, hence restricting the flow, when you polish the ports, the same thin layer is created, although the thin layer itself is moving faster then what it would be if you didn't polish it...

so basically, yes there is a thin layer, but it's there in both cases, but when you polish the ports, the thin layer is moving faster then it would be otherwise, hence you get better air flow...


just look at thorpy's swim suit... if the thin layer of water around his suit was in anyway beneficial, they'd dress him in a bundy bear outfit.

[MitchE]

alright thanks mate ill get them polished then
the way it was explaind to me was that air stuck to polished surfaces somehow, cant remember the smaller details but it made sense when it was explaind fully.

[Hen]

I'm not sure if that explanation given is 100% correct. I too have heard that a slightly rough finish gives better flow than mirror finished ports, though have no sources or experience to back that up.

All I can say though is beware simple explanations of fluid flow/dynamics. It is incredibly involved science and often counter-intuitive (for example in a well built NA engine you can get cylinder pressures higher than atmospheric pressure during the intake stroke).

Hen

[shelldrake]

Two experienced sources in head reconditioning/modding told me that a polish/mirror finish in the ports is not desirable. From factory the casting is a) Too rough and b) has imperfections at vital points where good airflow is beneficial. When you get a head ported properly, it will have an evenly textured finish, which will have taken out the common imperfections produced from factory and have increased your flow. I don't know the proper science of it, but I'm sure someone here will be able to go into further detail.

[The Real Roadrunner]

i dont recommend actually polishing ports as yes you can get decreased airflow, but i would definatly have them smoothed right out with something like 2000 grit w/d with the grain running in the same direction as the air flow .

polished being a mirror finish in theory should flow better but flowbenches and dyno charts dont lie.

my 2c

[andurils_sheath]

Hey guys

From what ive been told and seen on the intake side its actually more beneficial to have a slight roughness due to mirror surfaces reacting badly with the fuel vapour. Ive been told it is possible for the fuel vapour to condense on a mirror surface. Im sure im not quite right in my explanation however from experience when i got my head race ported they left it rough however it almost looked like the roughness was designed into a rifle type swirl. Looked cool and does the job. But apparently on the exhaust side gains are there to be had for the most mirror surface possible.

Cheers

[wilbo666]

What about the fuel being more likely to stick to a polished port surface? (hence coming out of the air...)

Cheers
Wilbo

[celicapain]

Porting is complicated stuff-all about best possible flow for least possible volume.
what you want a slightly rough finish because that creates a boundary layer of turbulent air, which reduces drag meaning you get better air velocity which equals more tourqe. what your friend told you is absoloutly true, keep the mirror stuff for your crank(helps fling oil of it) and rocker covers

[MitchE]

so i shoud be looking for evenly textured surface.
what i was told that the surface would slow the air down enough to let the rest of it flow over this verry thin layer of air.
its hard to explain it properly because he used too many big words.

CELICAPAIN yeah that sound something like what my mate was saying i think i can put this one to bed now thanks again everyone!
Mitch

[RAd28]

okay, if your talking mirror finished polish, then no.. don't go there, a) waste of time, b) doesn't allow the fuel/air to mix as effectively...

but the rough surface DOES NOT INCREASE AIR FLOW!!! this theory about a "thin layer of air creating better air flow" is crap! so long as there is a surface, there will DEFINATLY be a thin layer, no matter how rough the surface is. any thin layer of air, especially a turbulent one, will create drag, resulting in loss of air flow...

the way fuel air mixes is the same as when you boil (boiling meaning bubbling) water, if you have a totally polished (perfect surface) vessel with water, the water will not boil at 100degC, and probably not for a long time after that, however if you introduce something to this water once it's passed 100degC (say a powder like sugar) then the water will boil instantly since it's now got all these places for air bubbles to neucleate.

the principal is the same with fuel/air mixing, if they have a pitted surface, then it'll mix much better, if it's polished smooth, then it won't mix well, (larger drops of fuel).

SO, a polished smooth surface is DEFINATLY better for air flow, but not so good for fuel/air mix, your best bet would be to match up the size of your intake ports with your manifold gasket, then match up the manifold gasket with your intake manifold, smooth out the surface of the ports somewhat (but not polished). and your laughing...

have a read of this site for more info


http://www.torquecars.com/tuning/Porting-polishing.php

[mic*]

AFAIK mirror-finish on intakes being bad is fuel related - as said.

Other than that you WOULD have it mirror finish. The idea is laminar flow as it is the most efficient way for air particles to move. ANY surface that air moves over/past has a "boundary layer". This layer is effectively where the particles closest to the surface are affected by friction with the surface. The smoother the surface, the less that particles in this layer "bump" and "swirl" over surface imperfections. Effectively the boundary layer is thinner the smoother the surface, and is less likely to cause transistion to turbulence.

Eg: P-51 Mustangs were found to lose up to 200hp because of the wing rivets. Similarly even a painted wing vs bare aluminium suffered about 90hp.

In saying all this, i expect that flow in engine headers is not exactly laminar at the best of times. Controlling unwanted exaggerated turbulence at certain points in the header and port is the aim i believe. And laminar-turbulent transistion can be manipulated by "flow control" finishes. Ie a directional grain.

Hen, the "greater than atmo" intake pressure in tuned NA's is to do with sound refelction tuning. The sound of the valve hitting its seat creates pressure pulses up and down the header. If the pulse reflecting back up the header (towards the valve) coincides with the valve opening for the next cycle, significatly more air gets in.

[RAd28]

exactly the point i'm trying to make...

well said.

one point though... it's not the sound of the valve hitting it's seat that creats the pressure pulse, it's the wave of air bouncing back off something (the valve) that create a reversion pulse.

[andurils_sheath]

okay, if your talking mirror finished polish, then no.. don't go there, a) waste of time, b) doesn't allow the fuel/air to mix as effectively...

but the rough surface DOES NOT INCREASE AIR FLOW!!! this theory about a "thin layer of air creating better air flow" is crap! so long as there is a surface, there will DEFINATLY be a thin layer, no matter how rough the surface is. any thin layer of air, especially a turbulent one, will create drag, resulting in loss of air flow...

the way fuel air mixes is the same as when you boil (boiling meaning bubbling) water, if you have a totally polished (perfect surface) vessel with water, the water will not boil at 100degC, and probably not for a long time after that, however if you introduce something to this water once it's passed 100degC (say a powder like sugar) then the water will boil instantly since it's now got all these places for air bubbles to neucleate.

the principal is the same with fuel/air mixing, if they have a pitted surface, then it'll mix much better, if it's polished smooth, then it won't mix well, (larger drops of fuel).

SO, a polished smooth surface is DEFINATLY better for air flow, but not so good for fuel/air mix, your best bet would be to match up the size of your intake ports with your manifold gasket, then match up the manifold gasket with your intake manifold, smooth out the surface of the ports somewhat (but not polished). and your laughing...

have a read of this site for more info


http://www.torquecars.com/tuning/Porting-polishing.php

Thats what i was trying to say,
only better.
Cheers

[mic*]

exactly the point i'm trying to make...

well said.

one point though... it's not the sound of the valve hitting it's seat that creats the pressure pulse, it's the wave of air bouncing back off something (the valve) that create a reversion pulse.

Changing the inertia of the air does create pressure, but is probably more important for the exhaust. Tuning NA intakes is primarily to do with resonance between the valve and the runner end. The sound (compression pulses in air) created by the valve slamming the seat. Most like a pipe organ, or slamming a thong over open PVC for example...

Google; helmholtz resonator. I think thats spelt right...

Once you soak that up, you'll see why intakes have a plenum and individual runners rather than a one-into-two-into-four splitting pipe, like an exhaust...