Polish and port, take 2

[Talyn]

I know that the step up to the larger exhaust port helps stop flow reversal on the exhaust ports. I would think that hitting an edge like that would cause a lot of turbulence if larger.

 

[alex22]

I was listening to an Speedtalk.com interview with Phil Martin (15 seasons with NASCAR) where he was talking about intake manifold and cylinder head porting. Keep in mind that the majority of his experience is with Bush series carborated V8's.
He had dyno tested different mouth openings of the intake manifold. To get a base line he took a final ported intake manifold and matched the mouth exactly to the port, ran it on the dyno and it made the power they wanted. They removed the manifold and opened each wall .050 and it made about 4HP more, removed it again and opened it another .050, reinstalled and it made 7HP more. This lead him to believe that there is such a thing as a boundary layer on the walls of the intake manifold. One of the other interviews goes more into depth about the boundary layer, but I forgot exactly what he said about it.

That is comeing from a NASCAR builder, will we see 7HP on our jeeps, no, but it might be worth 1 or 2HP, but the little gain is better than a little loss that would come from a smaller intake mouth than the port opening.

~Alex

 

[wolfpackjeeper]

it is a theory that there is a more turbulant, slower moving stream that moves along the walles of the manifold, and that a faster smoother stream runs along the interior. by opening up the manifold a shade, they are making the assumption that the faster moving air will flow strait into the head without the slower moving turbulent wall

 

[Flash]

Hmmm Vary interesting thoughts, Thanks guys!

My thought, after i posted that question was that the air hitting the shelf would cause the air to tumble, causing a turbulence, that my have actually been good. It may cause the injection spray in that area to mix the air with the fuel better:dunno: This idea wouldn't hold water in a carb eng as the intake is carrying fuel as well as air thru the manifold.

Flash

 

[corbinafly]

Hmmm Vary interesting thoughts, Thanks guys!

My thought, after i posted that question was that the air hitting the shelf would cause the air to tumble, causing a turbulence, that my have actually been good. It may cause the injection spray in that area to mix the air with the fuel better:dunno: This idea wouldn't hold water in a carb eng as the intake is carrying fuel as well as air thru the manifold.

Flash

And/or that the air speed would increase as it enters the smaller opening in the head........

 

[Smittty9785]

purdy motor. i can only imagine what mine looks like on the inside.. YUCK

 

[Talyn]

Hmmm Vary interesting thoughts, Thanks guys!

My thought, after i posted that question was that the air hitting the shelf would cause the air to tumble, causing a turbulence, that my have actually been good. It may cause the injection spray in that area to mix the air with the fuel better:dunno: This idea wouldn't hold water in a carb eng as the intake is carrying fuel as well as air thru the manifold.

Flash

That is the thought I was trying to get across in #21... I didn't think I did that good of a job.. lol.

 

[Talyn]

purdy motor. i can only imagine what mine looks like on the inside.. YUCK

Maybe now, but it will get dirty.. that and it won't be seen again for a while.

 

[alex22]

And/or that the air speed would increase as it enters the smaller opening in the head........

The air traveling through the intake manifold will have its boundary layer "scraped/sheared off" by the cylinder head which will mean that the same amount of air has to pass through a smaller hole. That will increase the velocity, increaced velocity in the port will improve the low lift flow numbers. This is also why gasket matching does not work, creating a low velocity area in the middle of the air stream.

~Alex

 

[Talyn]

So, making the intake manifold ports slightly larger than the head ports is the best way to go then?

 

[Slo-Sho]

That is the not so smart way to go. Having a rough surface area lets a 'layer' of air adhere to the surface. Since air can travel more easily over air this is beneficial. The same principal applies to the hull of a boat, if a layer of water can adhere to the rough surface, then water can flow more easily over water instead of a smooth surface.

 

[hubs97xj]

So it's better to have the gasket protruding into the port openings on either side, than it is to have the gaskets matched to the size of either opening? And port matching the head and manifold, with a gasket to match precisely, is going to provide lesser performance (albeit not really a significant amount in the case of 4.0s)?

 

[Slo-Sho]

Gasket match is really all you need, although you can enlargen the gasket as well if you care to do so. Just don't make a ledge from the intake to the head...On the exhaust however it is beneficial to have the exhaust ports slightly smaller than the manifold to discourage 'reversion' at mid to high RPMs.

 

[alex22]

By gasket matching you just let the designer of the gasket (an engineer who may know nothing about airflow) determine the final size and placement of the port. You should not look at the intake port, gasket and intake manifold as one tube, not 3 pieces. By gasket matching you make the cross sectional area larger at the gasket than it is before and after. Ask yourself what will flow better a straight piece of box tubeing (close in shape to our port) or a piece of box tubeing that was put in a press and now bulges out in all directions in one spot. I said that makeing the opening in the intake manifold .050 larger on each side because the gain has been proven by a NASCAR builder on an engine dyno. The gain in performance would have to do with how the boundary layer reacts to the step, i'm not sure what happens there.

One thing that is often overlooked is how well the header lines up with the port, many times the header is shifted to one side, you should check and then port accordingly.

~Alex

 

[hubs97xj]

I was just having a bit of trouble trying to figure out exactly what folks were getting at. Some of the wording is pretty muddled.

 

[Talyn]

What part are you having trouble with?

 

[hubs97xj]

So, making the intake manifold ports slightly larger than the head ports is the best way to go then?

That is the not so smart way to go. Having a rough surface area lets a 'layer' of air adhere to the surface. Since air can travel more easily over air this is beneficial.

The first line threw me off, but I think Slo is saying exactly what Talyn was getting at. Then, I started thinking the gasket was actually smaller than either the head or manifold ports, hence my next remark (I have absolutely no idea why I thought this, in retrospect- gasket matching can't work with a gasket opening smaller than either piece)

Gasket match is really all you need, although you can enlargen the gasket as well if you care to do so. Just don't make a ledge from the intake to the head...

I thought I had it figured out up to this point; Slo and Alex are taking contrary positions. Okay, no problems.

You should not look at the intake port, gasket and intake manifold as one tube, not 3 pieces.
~Alex

Just an extra "not" in there? It seems to make sense again, but we're getting two conflicting perspectives. The first is that the lip created by having a smaller port on the head than on the manifold shears the boundary layer, and the second that a match of the ports on both sides (I believe this is what is commonly referred to as gasket matching- line the gasket up on both pieces, trace, grind until they all align) results in cleaner, unrestricted flow (unless I'm reading this wrong), but then, this.

The gain in performance would have to do with how the boundary layer reacts to the step, i'm not sure what happens there.

One thing that is often overlooked is how well the header lines up with the port, many times the header is shifted to one side, you should check and then port accordingly.

~Alex

he mentions misalignment of the manifold and head, which would create some sort of lip which would obstruct the airflow. Why would this be worse than intentionally creating a lip?

So there's 3 situations now- smaller head port, larger manifold port; equally sized and aligned ports; and then slightly offset/misaligned ports. That's what I'm not understanding fully. I hadn't given much thought to any of this, but Alex's comments on the bulge created by gasket-matching (sort of the reverse of a Venturi), and the theory about the boundary layer and the Nascar testing all seems reasonable.

Part of my misunderstanding was due to some really odd thinking on my part. Having realized my fundamental mistake, I'm still trying to figure out if the difference in manifold and head port sizes is good or bad. Maybe it's just me, but it's not readily apparent which is the better situation. Would a smoother intake mean a thinner boundary layer? If misaligned ports create an obstruction or restrict airflow, isn't intentionally restricting the air uniformly worse? Is any of this even (the percentage of power gained /lost) relevant to our (or at least my, nearly stock) 4.0s? I think I'll just crack a beer.

 

[alex22]

You would not notice the difference in power resulting from how you choose to port the area where the cylinder head meets the intake manifold. Port matching vs. gasket matching, and same size openings in the cylinder head and intake manifold vs. a slightly larger opening in the intake manifold.

Alot of builders will spend hours to get the intake manifold to match perfectly, and then just bolt the headers on and never check if they are alligned. Some of the SBC headers can be off by a 1/4 of an inch.
The header (exhaust) primary should be larger than the exhaust port in the cylinder head. When I mentioned the header allignment what I was trying to say was that the holes in the flange of the header are not always centered on the ports on the head. You should check how well the ports line up (exhaust into header) and if they do not line up, make adjustments where you can. Adjustments could be slotting the bolt holes in the flange to shift the entire header, grinding some of the header flange or grinding more off of one of the exhaust port walls than the rest.

~Alex

 

[hubs97xj]

Okay, I think I've got it now. An intake manifold should have a slightly larger port, so that the boundary layer is clipped, and only the faster air enters the head. The exhaust should be the reverse, so there's no shear of the exhaust air, and potentially a venturi effect as the gas flows into the larger opening of the primary? And the alignment only really comes into play when the ports are completely out of whack- a slight lip isn't a huge deal, but surely a quarter inch is? And gasket matching can be bad, in that it creates a larger volume where there's no need, as long as things align decently?

Am I on the right track here?

 

[Talyn]

Okay, I think I've got it now. An intake manifold should have a slightly larger port, so that the boundary layer is clipped, and only the faster air enters the head. The exhaust should be the reverse, so there's no shear of the exhaust air, and potentially a venturi effect as the gas flows into the larger opening of the primary? And the alignment only really comes into play when the ports are completely out of whack- a slight lip isn't a huge deal, but surely a quarter inch is? And gasket matching can be bad, in that it creates a larger volume where there's no need, as long as things align decently?

Am I on the right track here?

Seems so.. this is what I have learned/know.

Intake:

You want it to either match the gasket, both intake and head(either the way it came or manipulated) or the intake to be .050 larger

exhaust:
you want the ports in the head to be smaller than the manifold and the manifold port to be centered as much as possible.

What I have found is that a) the intake gasket that I was using wasn't consistent in the port sizes.. i changed them to be uniform b) the gasket that I used on the head didn't require that much modification on the gasket nor the head, c) the intake manifold is significantly smaller in stock form that the head to begin with.