Porting - rough or smooth?

[Chancer]

I had an interesting chat about porting with the mechanic that's currently sorting my brakes. He recommended that rather than actually polishing the metal smooth within the ports, one should actually roughen them up. He's a very competent mechanic and has tried both methods, and felt that you mixed the fuel and air better than by having completely smooth ports. I'd never heard of this before, so I'd be interested to see what other people thought.

My initial reaction was that it seemed to defeat the object, but then again, the logic seems sound.

 

[Sarge]

For the most part smooth porting is for high rpm operation and rougher is for low rpm stuff. Different engines do respond differently tho so it's possible the 4.0 might like smooth porting. Matching the ports is good for either. Basically use an exhaust gasket to figure out what needs to be removed to get rid of any lips.

Sarge

 

[brokexj]

I always heard that the exhaust side should be as smooth as possible and the intake side should have a smoothed texture to it. That's what I did on the current engine which will be going in next week. I'll let you know how it goes.

 

[mikeyl]

just a thought here. i would imagine a rough port might work better if you had a carb or a throttle body injection as the fuel/air would be mixing before it enters the combustion chamber. but for our port injection engines,one would feel that a smoother laminar flow would allow the air to enter the chamber better from the intake mainfold without any pressure changing disturbances.
just my 2 cents and it makes sense to me , so it's probably incorrect! :laugh3:

 

[NCSUcherokee]

On the head I am working on, I am putting a near mirror finish on the CC and exhaust and a rough to the touch on the intake. I wouldn’t call myself an expert in this area, but all my research has said something of the like of this procedure.

-smooth the CC to prevent hot spots and detonation
-smooth the exhaust runner to prevent carbon build-up
-leave the intake rough to the touch for reasons I can't remember

 

[Sarge]

-leave the intake rough to the touch for reasons I can't remember

Believe it's to help create the swirling which helps mix the air/fuel mixture.

Sarge

 

[NCSUcherokee]

yeah, i think you are right sarge

 

[XJ Dreamin']

Believe it's to help create the swirling which helps mix the air/fuel mixture.

Sarge

There is also the issue of "surface boundary layer." Air is sticky. It likes to stick to itself and it also sticks to other materials. With a smooth surface that creates little turbulance the air flowing along the surface of the tube tends to stick to it and slow down . It also sticks to other air flowing by and slows it down creating a gradual transition from the slowest air next to the surface toward the fastest air in the center of the tube. A rough surface can create a thin layer of turbulance just above the surface of the tube. This tends to create a sharp transition between the totally chaotic air next to the surface and the rest of the air that flows through the tube relatively unhindered. The effect is to increase the overall average velocity of air that does flow through the tube. However, the boundary layer reduces the effective cross-section of the tube and, thereby, the total effective volume.

So, it comes down to a balance between the average velocity vs. the total volume. Then you throw in other factors like desired turblance for mixing, or polishing to prevent build-up and hot-spots. The bottom line is, fluid flow and aerodynamics is in the realm of the butterly effect. In practical applications it's hit-and -miss and then back to the drawing board.

Most important are the transitions from TB-to-manifold, manifold-to-head, and head-to-header. Get those ported as has already been advised. For my own, when I get around to it, I would port and polish the exhaust (no need for mixing and less build-up), and port the intake but leave the surface just slightly rough. Not so much for mixing as for a smoother, slightly faster flow. Fuel enters the system so late that you are primarily dependent on dynamics within the combustion chamber for mixing.

My in-expert and very humble opinion. Let us know what you decide and how it works out.

XJ Dreamin'
stock 2WD '93 XJ, 4.0L HO, AW4, LT 235/75R15 A/T's

 

[MudDawg]

Very well put Dreamin'...you have hit the proverbial nail on the head.

 

[5-90]

I'd leave it slightly rough (enough to feel with a fingertip,) for reasons of "boundary layer" flow already discussed. It is important that the shape of the runner within the head is NOT changed, and that very little to no additional port volume is created, since both of these will actually DECREASE airflow into the cylinder due to a decrease in the Venturi Effect (as I recall.)

Since fuel is sprayed right on the back of the intake valve, keeping the air and fuel mixed within the intake stream is a non-issue, but the boundary flow characteristics cited earlier are correct.

The idea of smooth polishing the chamber is also correct, for the reasons cited. The exhaust runner should also have a slight texture to it - same as the intake. Since exhaust gas is flowing faster (higher heat - less density,) it's not quite as critical, but should still be done.

Port-matching the intake is always a good idea, but the exhaust should NOT be port-matched. The slight "step" outward from the head runner to the exhaust manifold works as an "anti-reversion" device, which helps keep exhaust flow from running back into the head and chamber. While this is minimal for NA engines (it's worst in turbocharged applications, and sometimes an "Anti-reversion baffle" is installed) it's still a real issue, and the step can actually help to increase VE (and therefore power output.)

I'm still trying to get my hands on a couple warped heads (prefereably 7120's - anyone have a couple?) so I can section them in a bandsaw and do some porting experiments.

5-90

 

[CW]

You can still open the exhaust ports up quite a ways and still be safe though. My head was horrible on the exhaust ports so I opened them up a bit. I polished the exhaust and cc but left the intake ports fairly rough. Porting the head is basically unnoticable until 3000 rpms, my stroker pulls hard from idle up but you can definetely feel the extra bump after 3k to the redline and a rev limiter is important.

 

[mikeyl]

standing corrected on the quality of the surface finish on the intake manifold. i know there was a common practice of extrusion honing (oatmeal anyone?),and this definatly makes a slightly rough surface to reduce the air slowing from sidewall adhesion. i wasn't remembering why golfballs are dimpled either. guess i shoulda payed more attention during my engineering classes!

 

[Dr. Dyno]

Extrude honing gives the inside of the intake manifold a smooth, almost mirror-like finish. That's exactly what you want for an EFI manifold, but a carb manifold should have a slightly rough internal surface to promote turbulence and keep the fuel finely atomized.
The intake ports of the heads should also have a slightly rough surface for the same reason. Since the injectors of a multi-point EFI system spray fuel straight into the intake ports, the fuel doesn't pass through the intake manifold and therefore the manifold surface doesn't need to be rough like that of a carb manifold. Only air passes through the manifold so a smooth surface keeps airflow at a higher velocity and enhances torque production.

 

[FUNKYTEE5]

I had an interesting chat about porting with the mechanic that's currently sorting my brakes. He recommended that rather than actually polishing the metal smooth within the ports, one should actually roughen them up. He's a very competent mechanic and has tried both methods, and felt that you mixed the fuel and air better than by having completely smooth ports. I'd never heard of this before, so I'd be interested to see what other people thought.

My initial reaction was that it seemed to defeat the object, but then again, the logic seems sound.[/QUOT

In the days before fuel injectors it was found that slightly rough textured intake runners would help fuel atomize better than shiny polished ones. Guys found out that while carbs drooled fuel into the intake manifold the fuel droplets would collect/stick to the walls of polished intake runners, providing less than ideal fuel delivery. Conversly, when some texture was left on the walls it was found to create turbulence and help the fuel stay suspended in the runner instead of forming droplets on the walls. Since you probably have an injector atomizing the fuel for you, mixing the air/fuel is no longer the issue as with carburetors. The goal is in keeping the fuel from adhering to the walls and getting it into the chamber efficiently. Most sources say no finer than 80/100grit in the intake ports.
Good Luck

FUNKYTEE

 

[Chancer]

I'm very glad I asked. Thanks all!