I think it was Rehr Morrison (sp) that said its unwise to port/polish the intake runners on a carbed motor. I think his reasoning was the rough runners help atomize the air and fuel, whereas fuel injection its shot into it and doesnt really need to atomize as much (??). Can anyone help me out with this?

Im gettin some vortec heads for my 383 and I know Im gonna polish the hell out of the exhaust runners, but Im still not sure about the physics on fuel/air atomization. Plus it might be a while till I get the whole motor together, and I got plenty of downtime at the shop. I figured since vortecs are onlyk 170 runners, if I polish them I could really get the low end power up. Top end dont matter much to me, 6500rpms max. Any physics professors on here??

 

check out David Vizards book, how to build and modify chevy small block cylinder heads. I just bought it a few days ago....

anyway, that info is in there, the rough walls on the intake (carbide cutter, without sanding and polishing) will help with mixing at low rpms. If it's really slick you have more chance of the fuel puddling out at low rpms. Higher rpms will be helped by polishing it, since it's more flow area, and easier flow. (IIRC)

I might be able to scan that page or something, but i'd suggest you buy the book, it'll be invaluable once you start porting.

In your case I think you'd want to leave it rough for low RPM...

 

I was just going to quote some David Vizard myself, but I was beat to it...

Vizard talks about the Vortecs in his books, and in the 'Building a Max Horsepower SBC on a Budget' he talks about porting the Vortecs a bit. He suggests polishing the chambers to help reduce detonation, but you're right about leaving the finish rough on the intakes.

 

I'll have to pick up that book, sounds right up my alley!

hey, Edmonton! I'll have to keep an eye out for that car during the summer road trips

 

with carbed motors they are a wet system another words the fuel goes through the whole intake tract generally in larger droplets while a fuel injection system puts the injector right near the end of the intake and injects the fuel right there aimed straight at the intake port in a very nice fine mist.
so on a carb motor if everything is nice and smooth there isn't any turbulance on the side of the walls to help keep the fuel off of them and instead it can pool up or stick to the walls of the intake manifold or head.
another thing I heard about though with polishing is some ppl don't really like doing it on the intake tract because they look at it like a golf ball I think was the term they used. when the air flows along the walls on the head/intake the little bumpies create a little boundery layer so the air doesn't just stall out when it hits the sides where when the walls are smooth it can possibly slow down but don't ask me I don't know haven't tried either way just throwing ideas out there

 

thanks guys. I took your advice and got the Vizard book. hell, hes got a whole section on portin L31s! Ill give this a good read and see what I can come up with.

 

Correct me if I'm wrong, but you should concenrate your porting or cleaning up efforts to the areas behind each valve. That is where the biggest gains can be made. Leaving a rough finish on the intake is a good thing, roughly(hahah bad pun) an 80 grit finish is plenty. Like rx7speed was refering to is that it will create a boundary layer along the surface of wall. This is a thin layer of turbulent air currents that sort of creates a tunnel (I have a feeling my explanation is going to be bad) that the air/fuel rides on as it is sucked into the chamber. Now if the surface was smooth, the flow is now laminar and when the air is trying to be pulled into the chamber the layer near the wall creates currents behind the flow. These small currents act against the pull of the chamber creating more drag than with a small tubulent layer.
Sorry, if that was bad, I tried...

 

That's dead on. As much as i'd love to attach a picture showing a laminar boundary layer, and turbulent compressible air flow from my fluid mechanics class.... Well I hated that course, it's irrelavant, and I got a D+.... but yes, you are correct.

He'll get to that chapter soon enough though, i'm sure Vizard mentions that most. I like how he bench flows after each mod and shows gains on everything. 7 mods, with 7 flow tests... Excellent.

oh yea, port matching (as vizard sez) is pretty useless.... And the ports themselves are not the flow restrictor until after you've ported the crap out of the "bowl" area. *This is for the few people that will come across this post in the future, and haven't bought the book*

 

Sonix, thanks for the encouragement, but I think there's much more to it. I haven't read his book about cylinder heads yet, but it sounds pretty good. Flow bench testing is really the only way to go, if you really want to dial in a head.
For the most part the intake tract surface finish is irrelevant compared to the shape of the "bowl." The biggest gains come from qaulity shaping of that area.

 

The issue at hand isn't "flow", so flow bench numbers aren't the indicator of what he's looking for.

A rough surface finish helps keep the fuel droplets in suspension in the air stream. The fuel tends to condense or settle out; having the roughness makes like swirls and eddies right along the surface of the port, to keep the fuel mixed in.

The measurement that shows the effect most clearly, is the BSFC of a finished motor. It will always be higher (more fuel to produce the same HP) in a motor with smooth intake port surfaces, than the same ports left slightly rough.

 

question though is on a dry intake system porting the area above where the fuel gets injected how much of a difference would that make?

 

From my basic physics knowledge, Id say that on FI motors, since the fuel is sprayed into the end of the runners in a fine mist, it wouldnt hurt much, if anything, to port and polish the runners before the fuel is added. But Im only reffering to fuel atomization. As far as flow, Id guess that you would reach peak velocity sooner. Novice answer I know

I heard somewhere that when it comes to Vortecs, once you start messin around with the bowls and unshrouding the valves, you actually hurt performance. From the brief section I was able to read last night in Vizards vortec section, he said that Vortecs are so good out of the box, theres really not a whole lot you can do to make them better. Ill read some more tonight and see if I can come up with something better.

 

The reason you want a rough finish has very little to do with whether it is a wet or dry system. The thing you want to avoid by leaving the surface rough is air and fuel sticking to the polished surface and slowing the flow down.

Yeah everbody is doing the big UHHH what are you talking about I know but try this.

take a sheet of sandpaper let say 80 grit and put a drop of water on one end of it make it about 1/4 inch dia. Then put a piece of glass like an old window pane next to it and put a 1/4 inch drop of water on it. Now pick up the glass and sand paper and hold them vertical.

What happened???

The water on the sand paper fell straight off and the water on the glass s l o w l y r a n d o w n the glass until it finally started to drip off the glass.

That is surface adheision that is what you don't want in your intake runners.

glass and sandpaper demo compliments of Smokey Unick

 

Wow dude, thats a very good comparison, seriously. i never thought of it like that. But I guess it makes sense though. But What about rx7speeds question. Polishing the runners before the fuel is entered into the equation.

 

I can see both sides being helpfull as far as a smooth or slightly rough set of intake runners before the fuel.


the fuel tends to stick a little more and is a little heavier so will have a greater chance of falling out of the air flow


but air is another story

 

I believe Air can be considered a fluid, like gasoline, in this case.

Just like golfballs go further with the dimples, and baseballs go faster with the threads...

 

Golf ***** don't go further because of the dimples; rather, their trajectory is either higher or flatter (depending on what club you use, and which direction of spin it produces) because of the interference with the air that the dimples create. Different designs of dimples work to a greater or lesser extent than others, with either a greater or lesser "efficiency" (amount of drag per amount of "loft" effect).

Same for the baseball seams: they make the baseball "grip" the air more, which puts more curve on curve *****, etc.

I think you'll find that the effect of polishing the intake runners is, at best, far less than the effect of polishing the exhausts. The exhaust is at higher pressure, which means that the WHOLE cross-section is used for flow, compared to the intake; exhausts benefit from greater heat retention in the gas which polishing helps, where in intake runners that doesn't matter; etc.

On the whole, polishing the intakes produces a net loss to overall engine operation; regardless of whether it produces a slight increase in flow-bench numbers or not.

 

Air is a fluid in the fluid mechanics sense. Anything that isn't a solid is a "fluid".

Normally:
Gas
Liquid
Solid

but for fluid mechanics:
"fluid"
solid

but "liquid" fluids are usually incompressible. Whereas gaseous "fluids" are compressible. (Think pneumatic compared to hydraulic... or air in brake lines etc...)


but this really doesn't have any bearing on head porting anymore...? just don't polish your intake ports. period.

 

RB, have you been a Presidential Adviser or something? Your just too Analytically Correct!

Beatin a dead-horse, and off subject, I reckon...but from people I know with lifetime of baseball experience say ***** have higher speeds with the threads, too. Maybe the threads "raise" the air off the majority of the baseball's surface, creating less friction...and localizing fricton on certain areas for better motion characteristics?

 

funny how you say "maybe", then follow it with a very intellectual statement....
That's like einstein starting his relativity theory with: "maybe"....

haha, just cracked me up, that's all

 

No, can't say that I have; I probably wouldn't make a very good one anyway, because no one ever pays any attention to anything I have to say. Just look at people on here. I'm getting used to it though, I've got a wife and 2 teenage kids, so I have now officially become the stupidest blob of protoplasm to ever waste the resources of this planet. Nice thought though.

Incidentally, I was a math and physics double-major in college, but that didn't lead to a marketable skill, so I am incapable of holding a real job. I'm permanently stuck in broadcasting nowadays.

 

Hey...I think I'm right, but when RB is on a thread...he'll jump your mistakes in a hurry. A little gunshy, I reckon.

I'll have my own Engineering company, soon. Maybe I could hire you and throw you in the shop working on my trucks and cars Just for maintenance of course...

 

Golf ball dimples/baseball seams increase drag, and if it wasn’t for the fact that the ball isn’t really a streamlined shape they would hurt the distance that it could travel (I’m not sure that baseball seams don’t anyway, I would suspect that it depends on how it’s spinning in the air). Without the extra turbulence the back side of the ball curves in too fast and you’d get boundry layer separation which in general is worse then trying to push something brick shaped through the air. You’ll never see a Bonneville streamliner with dimples on it unless they’ve got a packaging problem…

Compressible fluids (air) follow some different rules then basically non compressible (most liquids), as does to some extent air flow over a rough vs a smooth surface. To some extent, you can get away with more extreme transitions with a rough surface, before the boundary layer separates resulting in turbulence and drag. The surface tension example (sandpaper vs glass) is almost exactly the opposite of what his happening here. Basically, the better the profile the smoother the surface that you can run with less drag.

Real world, port designs are compromises for packaging and other reasons, so you can’t have a perfect profile, so the recommendation of “keep the intake rough” is to compensate for any deficiencies (if you had all the time and testing equipment in the world and were still forced to stay with imperfect port shapes you’d probably make the port as smooth as possible and just dimple or even scratch across the direction of air flow where ever you get flow separation), and the best exhaust designs actually rely on flow separation and the most important thing there when actually running is to minimize heat transfer, so a shiny/polished port is recommended.

While I’m saying something about this, am I the only one out there that seems to get a smoother port finish with a light going over with a carbide rather then 80 grit?