Intake Port Extreme Narrowing Before Bowl ?

[C_Stock_409Chevy]

NOW we ARE freaking confused !
As I stated in my first post... the heads are "FULLY CNC' ported".
Because of an unrelated issue that developed with them, I am exchanging them for a set that are described as having a "street port".
Again... if this replacement set are CNC'd or not..... I have NO IDEA. As mentioned... CNC'd stuff doesn't automatically impress me !
The main thing is that they won't have the "issue" that these have... and I'll be able to do my own work on them.

[user-23911]

The term CNC is another one that's over used by the marketing dept, just like "billet".

All it means is that it was made with a computer and it might be just as good as a polished turd.
It doesn't mean it's any good and it doesn't mean that the person who made it actually knows what they're doing.

[mag2555]

Well in getting back to your first post that started this string and to bring things down to a simple basic level here your are!
Above a certain velocity a given mass of air will not follow around anything greater than 15 degree bend, and the higher the velocity is then all bets are off if it will make it aorund even that 15 degree bend! To get air to do that you need to slow it down and the only way to do that is to allow it to pass into a port area of greater size to drop off that velocity just enough to make for a small enough flow loss that you can deal with!
Sometimes the shape that you really need for example on a short turn to maintain control of air trying to make its way around it is not what you would first off think of!

[Erland Cox]

But if you go up in area through a turn the pressure rises, the boundary layer thickens and it is much easier for the air to separate from the surface.
If you slow the <air down enough before the bend and accelerate it through the bend pressure goes down the boundary layer gets thinner
and the flow has a chance to stay attached.
This is a gas with gas properties and it does not have the physics of a race car taking a turn.

Erland

[jim_ss409]

This is a gas with gas properties and it does not have the physics of a race car taking a turn.

Erland, I'm certainly not qualified to argue one way or another about this but I have often seen the "race car slowing to make a corner" analogy used.
The air does have weight, and the fuel would add more weight, so you'd think the race car idea would be somewhat true???
I have noticed that on the exhaust side the paint will burn off of the outside of the first bend which would seem to indicate that the fast moving exhaust is being pushed to the outside of the corner, like a race car.


You also said... "If you slow the air down enough before the bend and accelerate it through the bend pressure goes down the boundary layer gets thinner
and the flow has a chance to stay attached."

If I break that down so I can understand it better, the idea of slowing the air down BEFORE the bend seems to make sense. After all, even if you use the race car idea it only makes sense that you would slow the car BEFORE you hit the corner. So, I'm guessing that you would slowly expand the port from somewhere near the pushrod pinch area through to the start of the turn?

The part I'm not sure about is what happens in the turn and bowl area. I've always thought that the port would continue to grow through that area, like in this picture...


But I gather that you're saying it might be better to shrink the port down a little through the turn. That seems contrary to what I've read, assuming I understood it properly.
I can easily see how widening the port at the turn could work and maybe even pinching the floor and roof closer together in order not to allow the air to overshoot the turn as badly. Then again, I'm seeing the race car idea.
CFD pictures seem to suggest that the air does seem to follow the same line around a corner as a race car...


At the same time we know that turning vanes in a duct will help air make it around a tight corner.
Could it be that pinching the turn down a little might have a somewhat similar effect?


Anyway, I keep seeing the race car but I've been totally wrong about how things like this work before.

[Erland Cox]

The second picture with the port expanding just before the seat is right but the expansion must be after the turn.
On a valve size limited engine like most bigger 2 valve engines the expansion is after the throat.
You can see that in picture 3, the CFD test of a port, constantly tapering down to the throat.
A almost straight port like a modern motorcycle has its pinch just before the valve guide and expands through the bend.
This works as you flow both ports simultaneously but not when you flow only one of them at a time.
A trick on that type of head that is worth at least 5 hp on a 1 liter engine is to fill the backside of the bowl from the guide to the seat.
This way you lessen the expansion thriugh the bend and the engine likes it.





If you want to get more insight on how air behaves watch these movies: http://web.mit.edu/hml/ncfmf.html
It was a while ago I watched but I will go through them and find one that relates to this subject.

Erland

[Erland Cox]

Erland

[jim_ss409]

Thanks Erland. I think I might have seen those videos a while back but I'm going to watch them again.

[C_Stock_409Chevy]

Jim... there you go, thinking again !

Thanks, gentlemen. This is VERY informative.