Polishing myth?

[shinn497]

So I'm studying aerodynamics and I discovered that air has zero velocity on the surface of materials. If this is true, what purpose is there to polishing? I can understand porting but not polishing I've checked other sources and they say that hp gains from a polished intake manifold are a myth. Some say a rough surfaces might be beneficial as they interrupt the boundry layer ( the golf ball dimple argument). Has anyone had any experience with this and would like to comment. I've tried looking up SAE papers on this but I don't have access. This seems to be something that is poorly studied.

I'm wondering if it is possible to treat a surface and make it chemically/physically repulsive to air. Any idea if this is possible?

[David Hodgkins]

...I'm wondering if it is possible to treat a surface and make it chemically/physically repulsive to air. Any idea if this is possible?

I wash my skin every day with soap and my wife finds it physically repulsive.

So maybe I'm more aerodynamic?

:D

[68GT500MAN]

O, Great - an aerodynamic wookie!

[Ray]

I would think that if polishing didn't do anything then the major air-lines and jet builders wouldn't take the time to polish/paint aircraft. They would leave the surface rough. In the case of intakes-my thought is that you want the air flow to be as smooth and clean as possible in order to get the air-fuel mixture into the combustion chamber. Any rough spots and you could, potentially, leave a molecule or two of hydro-carbons behind.

Mythbusters did a show a few seasons back where they tested this and found that a car got better mileage with a "golf ball dimple treatment" though. I don't remember why but if might be worth a search so here it is: http://dsc.discovery.com/tv-shows/mythbusters/videos/dimpled-car-minimyth.htm

Ray

PS: David, you haven't looked like a Wookie since mullets went of of style.

[Jacob McCrea]

For what it's worth, I'd take a look at the surface of a Zipp deep dish carbon fiber road bike wheel, like a 404 or an 808, particularly the dimpled surface. The website (www.zipp.com) has a decent discussion of the aerodynamic effect of the dimples.

I have never studied this but I do recall an article in a motorcycle magazine saying that a highly polished surface was undesireable. My recollection is that the fuel was more likely to stick to the polished walls, but that is just going on memory.

[2FAST4U]

I'm not sure what you're referring to in terms of air over a smooth surface. If you're referring to the intake manifold runners, a smooth surface allows the air and fuel mixture to flow smoothly without interruptions in the flow path. A rough surface as seen from the casting surface interferes with the flow path. The molecules of air and fuel bounce of these interruptions causing the flow to slow down.

[PhyrraM]

I seem to recall reading something many years ago (Corky Bell?) that the polishing had more to do with keeping fuel suspended and not pooling in the intake manifold/ports. IF this is true, then it would be less important on port fuel injection and no longer apply at all to the new direct injection motors.


Edit: With OEMs now doing mostly plastic manifolds it is very easy to cast surface roughness, dimples, ridges, or whatever other shape the computers tell them is best. Has anybody ever cracked one open to see what they are doing on the inside?

[68GT500MAN]

I just read an interesting article on why/how to polish an intake and cylinder head and found out that you can get too smooth of a surface finish. It seems that the recommendation is to use nothing finer than 80 grit emery when polishing. This will allow you to get rid of any sharp edges and rough spots to promote better efficiency of the air/fuel charge into the cylinder (remember - more air/fuel in = more power out). Anything finer and 80 would be too smooth and not keep the fuel in the air mixture.

[Bob Cowan]

Air will always form a boundary layer over the surface. This boundary layer is either moving significantly slower than the air outside of it, or it is not moving at all - being somewhat bound to the surface. This is actually desirable in terms of aerodynamics. Take a moving surface - say, your hood. as it moves through the air, that boundary layer forms over the surface. The boundray layer is composed of only gases, and is therefore very slippery. Offering very little resistance to movement.

A course surface interruptions like NACA ducts, hood scoops, etc, disrupt the boundary layer, and cause turbulence. Turbulence creates drag, which slows you down. Or decreases fuel mileage. Have you ever seen those little winglets usually placed in front of the front wheel openings? They are not there to create down force. They are there to expand the boundary layer along the sides of the car, and decrease turbulence from things like tires, and flares.

The smoother a surface is, the tighter the boundary layer "adheres". When it's close to the surface, it's very smooth and very slippery.

Inside a wet intake manifold - carburater - is a whole different matter. Aerodynamics are not the primary concern. You're not moving a pure gas, you're moving a gas with a suspended liquid. To keep the liquid in suspension, you need a little turbulence. It's like dumping sugar in your ice tea. Unless you stir it, it all sinks to the bottom.

I remember a while back some one was experimenting with sanding the head ports by hand longitudinally with 80 grit paper. They wanted to see if it made a difference. I don't think the answer was ever found.

A dry EFI manifold does move strictly air. And there you do want a nice mirror surface for everything before the injectors. Smooth surfaces and gentle curves keep that air moving in a nice straight and smooth line, and it will pick up a lot of velocity. Once the fuel is injected, then you need a little turbulence to maintain the suspension. But if you've designed the intake tract correctly, you'll have enough velocity in that short distance to maintain the suspension. I would think it would be extremely difficult to get the EFI tract so smooth that the fuel would fall out of suspension before it crosses the valve face.

[myjones]

Air will always form a boundary layer over the surface. This boundary layer is either moving significantly slower than the air outside of it, or it is not moving at all - being somewhat bound to the surface.
Inside a wet intake manifold - carburater - is a whole different matter. Aerodynamics are not the primary concern. You're not moving a pure gas, you're moving a gas with a suspended liquid. To keep the liquid in suspension, you need a little turbulence. It's like dumping sugar in your ice tea. Unless you stir it, it all sinks to the bottom.

A dry EFI manifold does move strictly air. .

This portion of his comments is very important to saving your engines. The boundary layer is crucial to suspension and abrupt turns in a wet manifold will cause a lot of changes to occur. Even more important is to remember that dry manifolds EFI port style will really separate any supplemental fuel that is added to the top/front of a dry manifold. A lot of the new Hemi's were ruined by that approach when aftermarket blowers were added and rings and pistons were blamed when it was really poor fuel distribution issues.
DB

[tirod]

X2 on Bob's post. The point is that air does develop a boundary layer near the surface. It's when none exists that air is in direct contact, and that actually has MORE friction than when air is passing over another thin layer of trapped air. Air to air has much less friction.

Externally, race car body designers look to keep air connected to the body shape, therefore major changes in slope are much more important. The Daytona Coupe has a gradually sloping rear roofline which works with the overall body shape. Compare that to the Audi TT - which was recalled due to the air becoming detached, changing the center of air pressure, and causing it to swap ends. Not good.

Carroll Smith has a good look at aero in his book "Tune to Win." Even F1 cars in the '70s didn't get it right, and the science is still kept pretty close to the vest of aerodynamic engineers. The financial loss of a proprietary edge over the competition has been an obstacle in applied mechanics since the age of blacksmiths.

[Mike N]

Keeping the boundary layer attached to the surface sometimes requires a little deviation from the ideal. It is not unusual to see vortex generators on aero surfaces to 'trip' the air flow, increasing the boundary layer thickness and keeping the air flow from going turbulent. Think of the Mitsu Evo roof edge strip 16591 This picture gives a good idea of the intent 16592

[myjones]

X2 on Bob's post. The point is that air does develop a boundary layer near the surface. It's when none exists that air is in direct contact, and that actually has MORE friction than when air is passing over another thin layer of trapped air. Air to air has much less friction.

Externally, race car body designers look to keep air connected to the body shape, therefore major changes in slope are much more important. The Daytona Coupe has a gradually sloping rear roofline which works with the overall body shape. Compare that to the Audi TT - which was recalled due to the air becoming detached, changing the center of air pressure, and causing it to swap ends. Not good.

Carroll Smith has a good look at aero in his book "Tune to Win." Even F1 cars in the '70s didn't get it right, and the science is still kept pretty close to the vest of aerodynamic engineers. The financial loss of a proprietary edge over the competition has been an obstacle in applied mechanics since the age of blacksmiths.
I talked a friend into coming to the HB show and his degree is in computational flow dynamics IIR the proper term. I talk to him now and then about his work on rockets, land speed record cars and even wave generators for pools etc. but mostly about airflow in intake manifolds because I can almost understand that part of his research. Funny thing is that just last week I asked about golf balling a car body when I heard it mentioned elsewhere and his answer was negative but interesting and as always I was lost by the second paragraph. Did I mention "He has the Knack"
DB