The "Porting and Polishing" Myth

[MandarinaRacing]

The "Porting and Polishing" Myth

from Wikipedia:

It is popularly held that enlarging the ports to the maximum possible size and applying a mirror finish is what porting is. However that is not so. Some ports may be enlarged to their maximum possible size (in keeping with the highest level of aerodynamic efficiency) but those engines are highly developed very high speed units where the actual size of the ports has become a restriction. Often the size of the port is reduced to increase power. A mirror finish of the port (intake) does not provide the increase that intuition would suggest. In fact, within intake systems, the surface is usually deliberately textured to a degree of uniform roughness to encourage fuel deposited on the port walls to evaporate quickly. A rough surface on selected areas of the port may also alter flow by energizing the boundary layer, which can alter the flow path noticeably, possibly increasing flow. This is similar to what the dimples on a golf ball do. Flow bench testing shows that the difference between a mirror finished port and a rough textured port is typically less than 1%. The difference between a smooth to the touch port and an optically mirrored surface is not measurable by ordinary means. Exhaust ports may be smooth finished because of the dry gas flow but an optical finish is wasted effort and money.

The reason that polished ports are not advantageous from a flow standpoint is that at the interface between the metal wall and the air, the air speed is ZERO. This is due to the wetting action of the air and indeed all fluids. The first layer of molecules adheres to the wall and does not move significantly. The rest of the flow field must shear past which develops a velocity profile (or gradient) across the duct. In order for surface roughness to impact flow appreciably, the high spots must be high enough to protrude into the faster moving air toward the center. Only a very rough surface does this.

A developed velocity profile in a duct that shows why polished surfaces have little effect on flow. The air speed at the wall interface is zero regardless of how smooth it is.

What do you guys make of this? What happens then when a head is CNC ported? What about "particle suspension"? Waste of money?

Alex

 

[SuperMag]

It's pretty much accepted as true that a slightly rough surface will flow better than a mirror smooth one, but I don't think the reason behind it is as convoluted as it's made out to be in the article.

I'm thinking the increase in flow can be explained by simple friction-- with a fairly consistent, slightly rough surface, the fluid only has to slide over the "high" points of the surface. On a smooth surface, it has to smear across all of the surface...

The same phenomenon exists in the barrel of a rifle. If the inside finish of the barrel is too smooth, the surface contact with the bullet is 100%. Knock the "glaze" off the finish of the barrel with some hand lapping, your internal gas pressures will decrease, and the velocity of the bullet will increase-- there is not as much barrel surface in contact with the bullet to slow it down...

But I can't speak to the surface characteristics of a CNC port job; I've never seen one.

 

[addo]

CNC porting is still "rough" compared to a compounded surface. Maybe the article needs quantification with some Reynolds numbers as guidelines.

 

[Lazy JW]

Deano cited evidence that removing the 'as-cast' surface finish inside the intake manifold would definitely hinder fuel economy. When I did the pocket porting on my 300 head I deliberately left the runners alone and also left the surfaces that I ported fairly rough. I dunno, but it sure pulls strong
Joe

 

[Anonymous]

That article doesn't make the explanation very clear, but the conclusions are correct.


Air is a fluid and fluids behave differently (much!) than solids so while the friction idea seems good at first it doesn't quite apply to fluid flow. Gasses will completely occupy any volume they are contained in so contact will be made with the entire surface area of the container reguardless of surface quality. (If you want to see this.... grab a ribbed or studded condom or such and inflate it. All the irregularies are filled unlike if you just stick a solid object in it. Sorry, best example off the top of my head)

It's been awhile since I took fluid flow classes but here's the basic scoop.

What the article is trying to get into and what the picture somewhat dictates is fully developed laminar flow. And it is true that there is a boundry layer is formed at the edges of the flow and this boundry layer does "stick" to the wall and has a 0 velocity. The somewhat tricky part about all this is actually having laminar flow, because laminar vs turbulent flow is a function of many factors!

So what does roughening up the surface do? Well, it disturbs the broundry layer and introduces turbulence. Now normally this might not be the best thing, but we are not dealing with just gas or liquid flow. We have liquid suspended in a gas and want things to stay nicely atomized for good combustion. Liquid doesn't stay in suspension well during laminar flow, especially at low velocity around the edges. However, turbulent flow keeps things nicely mixed up and suspended!

If you want I can dig up my books and work out a couple of examples for flowrates of laminar vs turbulent flow but the moral of the story is that it's really all about keeping the mixture atomized better.

Now if you notice, the article does mention that all this only matters on the intake side. Why? Because we don't care about suspension on the exhaust! In fact, a nice polish on the exhaust can be good to reduce carbon adhesion... but really it's not worth your time. Polishing the combusion chamber on the other hand can be worthwhile, bto reduce hotspots that can cause detonation as well as redue carbon buildup.

The reason golf balls are dimpled is similiar, but not really the same cup of tea. Basically if you draw the profile of airflow over a smooth ball vs a dimpled ball. The smooth ball has the boundry layer that wants to stick the the ball. This causes the airflow to "cling" to the ball for a much longer time and wrap itself a good ways around the back of the ball. The dimples ( actually the peaks surrounding the dimples) break up this boundry layer and cause turbulence. So the airflow seperates from the ball much sooner and reduces drag on the ball. I know I have a nice picture of this in one of my books or notes somewhere, but I'm sure it can be found online if desired.

 

[SuperMag]

Liquid doesn't stay in suspension well during laminar flow, especially at low velocity around the edges. However, turbulent flow keeps things nicely mixed up and suspended!

I've got no problem with that; we can all see how more turbulence keeps the gasoline suspended. But how the increased turbulence enhances flow is a bit of a puzzler, and the best I could come up with was the friction thing.

The reason golf balls are dimpled is similiar, but not really the same cup of tea.

Correct, the bit about the golfball isn't really applicable. The reason a golf ball is dimpled is NOT to make it slice through the air better-- indeed, a dimpled ball has more drag. The reason a golf ball is dimpled has everything to do with the back spin imparted to the ball... When adequately struck, a golf ball can back-spin 4,000 RPM or more. Combine this with the roughened surface of the ball (the dimples), and the forward motion of the ball itself, you create higher pressure under the ball (and lower pressure above it), which holds it in the air longer . More time in the air = greater distance.

So other than the laws of fluid dynamics that apply to both scenarios, there's not a lot of commonality between golf ball flight and air/fuel flow through a manifold...

 

[rbohm]

8) as an example of proof that a rough finish port works better than a mirror finish port, david vizard found that two heads ported the same, on the same mini engine, tested on the same dyno back to back, found that the rough port head made 5hp more than the mirror port head did. denny wykopf also found the same thing as well many years ago(for both guys we are talking the late 60's to early 70's).

 

[Anonymous]

Liquid doesn't stay in suspension well during laminar flow, especially at low velocity around the edges. However, turbulent flow keeps things nicely mixed up and suspended!

I've got no problem with that; we can all see how more turbulence keeps the gasoline suspended. But how the increased turbulence enhances flow is a bit of a puzzler, and the best I could come up with was the friction thing.

The reason golf balls are dimpled is similiar, but not really the same cup of tea.

Correct, the bit about the golfball isn't really applicable. The reason a golf ball is dimpled is NOT to make it slice through the air better-- indeed, a dimpled ball has more drag. The reason a golf ball is dimpled has everything to do with the back spin imparted to the ball... When adequately struck, a golf ball can back-spin 4,000 RPM or more. Combine this with the roughened surface of the ball (the dimples), and the forward motion of the ball itself, you create higher pressure under the ball (and lower pressure above it), which holds it in the air longer . More time in the air = greater distance.

So other than the laws of fluid dynamics that apply to both scenarios, there's not a lot of commonality between golf ball flight and air/fuel flow through a manifold...

I'm gonna have to disagree about the drag on the golf ball. A dimpled ball has less drag for the reasons I stated before. I'll try to find the nice example picture/problem

As far as the flow rates of turbulent vs laminar flow, I'll dig out my books and work a few problems cause now I'm trying to remember as well! Materials and structures were always my thing more than fluids and thermo.

 

[SuperMag]

I'll try to find the nice example picture/problem

This one?

Link: http://helix.gatech.edu/Classes/ME3760/ ... ts/Martin/

A dimpled ball has less drag for the reasons I stated before.

You are right; I stand corrected...

Test results show that a smooth face golf ball has twice as much drag as a dimpled golf ball. (article)

But...

So what has more effects on lift and distance: dimples or spin? It is definitely the spin rate... Dimples can change the amount of lift, especially at low velocities, but the spin is what makes a golf ball act like a wing.

More later...

 

[Anonymous]

Actually this is more along the lines of what I was thinking off http://www.aerospaceweb.org/question/ae ... 0215.shtml

I wasn't considering spin rates, lift ect... just pure fluid flow and drag.


I did get things reversed in my previous post though, I appologize. Dimples reduce seperation ( not increase as I said earlier.... brain fart, sorry) which leaves a smaller wake behind the ball.

 

[MandarinaRacing]

CNC porting is still "rough" compared to a compounded surface. Maybe the article needs quantification with some Reynolds numbers as guidelines.

I guess, it's still a helluva lot smoother than "as cast".

Here's a pic:

As cast vs CNC'ed

as an example of proof that a rough finish port works better than a mirror finish port, david vizard found that two heads ported the same, on the same mini engine, tested on the same dyno back to back, found that the rough port head made 5hp more than the mirror port head did. denny wykopf also found the same thing as well many years ago(for both guys we are talking the late 60's to early 70's).

Cool! When I buy one of the FSPP heads I won't have to spend a lotta time on it before I bolt it on!!!!! LOL!

Alex

 

[aussie7mains]

Forget laminar flow altogether, the ports in almost all internal combustion engines are far too small to get that, try air conditioning dusts to get an idea of that.
The surface finish in my view makes almost NO difference to overall restriction, the shape however makes all the difference, go check out some Cosworth heads and you will see well developed ports, almost straight right through. I dont even go with the theory that ports need to be rough to maintain mixture suspension, if that is needed then your manifold carby/injection needs some more thought.
CNC porting is only a high tech method of cutting costs, hand port is just as good when done well, the CNC ports are modelled from hand ported jobs anyway.
A7M

 

[Anonymous]

so going along with the golf ball and rough surface theorys, air flows better across itself then a solid object. like if the little pockets in the casting of a head the air gets trapped in them and other air flows over the trapped air better. right? or am i way off of the idea?

 

[xctasy]

Your onto it. A7M is the man. Lamina flow only exists in laboratory tests with glass ducts and low pressure. In real life, all flow is turbulent or super critical.

Since air is always crashing into golf balls and intkae port walls, the smart race enginers have found that CNC maching sorts out the port shape first. Shape is the most important thing for flow, but surface texture is the most impoortant for fuel economy.

CNC machining leaves enough surface texture to stop gasoline particles falling out an washing the cylinder walls. The particles are not much more than 25 thou in diameter, and if the port is smooth as a babies bottom, the fuel forms a wet port which hurts air flow. If the sufrace is rougher than about 25 Ra, the fuel never puddles, and the rought port ensures air fuel mixture is always in suspension.

The suface air speed goes to a condition called 'super critical' when the port is rough. In English, that means the air flow at the surface is turbulent enough to create a standing wave. That is always good for fuel economy but hurts top end power. Generally, peak power at maximum rpm goes up when the port is glass smooth, but fuel econmy and power down in the rev range is drastically hurt.

What most of us don't realise is that most engine tuners have to pay for gasoline when testing engines. When the engine tester get the good power numbers and use less gasoline, they then try an figure out why. 9 times out of 10 it is becasue they have removed moderate amounts of metal at the long radius of every bendl, and then kept the port rough.

It works. Polished ports will work if you are at 100% of the engines throttle all the timne, but we never drive engines like that, even in race cars. And races are won on less fuel stops for the same power. So even racers tend to favour rougher intake ports.

Last thing. If your are Kenneth Duckworth or have a Cosworth or Yamaha or Ferrari, every 4 or 5 valve per cylinder engine has ports almost mirror smooth ports. Mainly because the intake ports are perfectly shaped, and state of the art. There is no gain in having the surface rough because there is no chance fuel will ever pool.

 

[mikeyo]

Xstacy said
" It works. Polished ports will work if you are at 100% of the engines throttle all the timne, but we never drive engines like that, even in race cars. And races are won on less fuel stops for the same power. So even racers tend to favour rougher intake ports."

This is why drag racing engines need full polished ports...they do work at max flow and drag racing is full throttle, full rpm the whole race.

 

[Stubby]

Yes, but if you are bracket racing, you will see more consistent ETs and easier tuning from slightly rougher ports. At 4.00 a gallon for race gas the better milage has some appeal.

Check with the class racers and you will find some who have different jets in some corners to help with distribution problems. It's not always distribution, the firing order causes some problems in V8s.

I have, and will, always port for quality instead of quanity. It just makes life easier.

 

[ludwig]

I think the worst part of the original article is the use of 'impact' as a transitive verb when the author clearly means 'affect'.

...this will affect the flow characteristics.

Now back to our regularly scheduled programming.

 

[Anonymous]

This is a totally contritedictory to everything I have been taught. Nonetheless all of the hairs on the back of my neck are standing up. They are telling me you are right. So here is the question, do I shell out an extra 350 dollers on a pro rebuild to port and polish a daily driver or is that simply wasted money.

 

[Lazy JW]

This is a totally contritedictory to everything I have been taught. Nonetheless all of the hairs on the back of my neck are standing up. They are telling me you are right. So here is the question, do I shell out an extra 350 dollers on a pro rebuild to port and polish a daily driver or is that simply wasted money.

Having never built a racing engine my experience is limited to mild "semi-stock" rebuilds.

However, one only needs to take a quick look at the stock ports on a 300 to realize that they are in dire need of some help. I did some basic bowl porting on my 300 head to eliminate that horrific ridge left just above the valve seats from the manufacturing process. Just blend it in to the port. I left the main portion of the ports as-cast.

I believe it was worthwhile. For a daily driver I would leave the intake as-cast, the exhaust wouldn't hurt to smooth up a bit.
Joe

 

[jamyers]

The consensus among Buick engine builders/testers is that for anything short of a full-race drag engine, basic porting is good, polishing is bad.
Porting meaning basic stuff - cleaning up casting lines, the bowl area, reducing the enormous valve guide boss, making sure the port floor makes a nice smooth curving transition, and blending the valve cuts. Leave the intake walls rough and the exhaust reasonably smooth (100-120 grit).

Polishing anything but the exhaust side is seen as a waste of time/money, unless you're building a 7000-rpm screamer. Then you go nuts reshaping and polishing while your wallet gets significantly lighter.

In my limited street-engine-only experience, there's a really steep return-on-investment curve...basic port and bowl cleanup will make a real difference, but beyond that the gains go down pretty fast while the cost goes up as fast or faster.
Key areas to look at:
#1 is where the intake floor transitions down to the valve, it needs to be a nice smooth curve without bumps or lips to spoil flow. Most times you can lay this short side back and it'll help (but be careful you don't hit water)
#2 is the valve guide boss, most of these are enormous, and right in the middle of the way (especially on the exhaust side).
#3 is the bowl area where the valve seat cuts are. Make sure there's a smooth transition.
#4 Make sure the entrance to the intake ports aren't bigger than the intake manifold exits (not a problem on Ford integrated-manifold heads), and make sure that the exhaust manifold/heater passages aren't smaller than the heads exhaust passages. From carb to tailpipe everything can get bigger or stay equal, but not smaller.

I also like a good 3-angle valve job with back-cut valves.