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

[fixeruperer]

my head hurts.

[SeptemberSquall]

conversation moved on

[ed]

exhaust is polished to prevent carbon deposition and build up. same as combustion chambers

[SeptemberSquall]

exhaust is polished to prevent carbon deposition and build up. same as combustion chambers

yes ten characters

[30psi 4agte]

I thought a polished port flows best . But the reason for having a semi rough finish on the intake ports was to allow the air /fuel to atomise and mix better.
A polished port can lead to fuel pooling.

The reason for a polished exhaust port is because it indeed flows better and as a bonus prevents carbon from sticking.

[Mr Revhead]

do some googling on "Riblet technique"

and re the wave pulses "heimholtz"

and yeah.. i was of the opinion its due to fuel suspension in the air rather thna over all flow

[oldcorollas]

some interesting thoughts in this thread, but i'm with Ed and Tricky.

port flow is turbulaent, and you would want nothing less. why decrease flow by maintaining laminar flow?

as for boundary layer thickness.. calculate it. then enlarge ports by that small amount if you are really worried
porting is totally empricial anyway, so the "best" port size and "best" surface finish is interrelated.

i don't believe that surface finish promotes mixing of air and fuel. the boudary layer is far away from the centre of a port, and unless the boundary layer is directly inducing massive turbulence, how can it promote fuel/air mixing? it doesn't make sense.

in carby cars, fuel drop out is an issue.
in fuel injected cars, the pressure the fuel is injectd under may mean the fuel hits the opposite port wall, or the intake valve anyway! (and that contact with a hot surface helps the creation of smaller fuel drops... since you don't want the fuel to actually vapourise before the cylinder anyway )

[30psi 4agte]

I was lucky enough to get a look at some ports on a v8 supercar head some years ago.

Thet were quite different to anything i had ever seen. They were basicly corrugated like a ribbed condom.

What do you reckon stu ?

Premote mixing or to do with the boundary layer ??

[MS-75]

Haven't read the whole thread so maybe already said but my understanding is that a non polished surface is better for the reason of fuel atomisation, ie air fuel mixing.

In a nutshell-yep. The rough port practice is basically a left over from when cars ran carbies and a perfect mirror finish runner/port tended to increase fuel condensation.

Now-in relation to porting/polishing, there is one thing I want to get off my chest-I'm sick and tired of seeing knife-edge finishes on the casting where ports split into the individual valve bowls. DON'T DO THIS-IS GIVES AN UNSTABLE STAGNATION POINT AND CREATES UNNESSECARY TURBULENCE-PLEASE!!!!!!!!!!!111111 When was the last time you saw an aircraft with a knife edge on the leading edge?????????

There. I feel better now.

I saw this the other day at an engine builders who is doing an awesome 2JZ head and had to bite my tongue.

[Mr Revhead]

why not?
an aircraft wing is a different situation.

yeah when i think about it, a sharp edge isnt common on fast things.... but why?

[ed]

why not?.... but why?

a sharp edge splits air and generate vortices behind the leading edge. a round edge promotes continuity and reduces turbulence

[tricky]

I'm sick and tired of seeing knife-edge finishes on the casting where ports split into the individual valve bowls. DON'T DO THIS-IS GIVES AN UNSTABLE STAGNATION POINT AND CREATES UNNESSECARY TURBULENCE

In this whole thread, this is by far the comment most relevant to volumetric flow loss!

EDIT: For a practical example, next time you are down at a fast flowing stream with rocks in it, look at what happens behind the object when the water goes over a rock with a shallow radius. The try to find a sharp leading edge and compare the flows.

Onto the golf ball... No! The evenly and regularly textured surface on a golf ball promotes the spatial longevity (with respect to the circumference of a cross section) of a laminar boundary layer, which means with the correct spin, it can maintain lift (ie stay off the ground).

The reynolds number which everyone has been referring to, for those who don't know, is the ratio of inertial to viscous forces. That is, a high reynolds number will have a high inertia relative to the viscosity of the fluid. So a high reynolds number means high shear.

A port flow has a high Reynolds number. This implies that the 'golf ball' boundary layer which could theoretically be incident in the port would shear away close to the port wall and become insignificant. Here's a piccy of the velocity profile I mentioned earlier:

a) is the laminar profile and b) is a high reynolds number flow.



So the turbulent scenario profile is closer to what you would expect in a non-viscous flow (in profile, but not magnitude). The friction factor due to a rough port wall contributes a poofteenth to the flow volume in real terms...

Sorry not very relevant to the thread, but an attempt to debunk an old wives tale even less relevant to the thread!

[ed]

excellent post by tricky

my image of the same concept


taken from discussion here:
http://www.v-eight.com/tech_forum/viewtopic.php?t=148

[MS-75]

nerds......

[RAd28]

alrighty... i'm starting to get a little lost, seems to be a lot of talk about laminar flow, reynolds number etc but what is the conclusion? what port finish are we suppose to achieve, good smooth finish? mirror finish? rough as shit?