Just fuel shear ?

[Tom68]

and should we all run intake manifolds bigger than the ports in the head ?

There's enough anecdotal evidence (decades of it) that BBCs rip with Rec port manifolds on oval port heads.

It may be that they only rip compared to the (most likely) OEM manifold that came off, so it may all be in a better manifold and carb.

But maybe it's the fuel that's fallen out of suspension sitting in the lip at the bottom of the manifold to cylinder head joint (where velocity jumps up) getting atomised by the sudden rush each time the port gets flow from the opening intake valve, rather than dribbling around the port walls in the boundary layer.

[juuhanaa]

Right now the things i do in garage for myself are a combination of flow, wet or dry flow and hole process. Therefore manifolds i do are cylinder head specific and i cant imagine how the BBC would be any different from this.

Screenshot_20221227-144216.jpg

and should we all run intake manifolds bigger than the ports in the head ?

Here are the 5 inputs i would choose to start comparing to the needs of the example engine.

Average intake port CSA
Port lenght
Engine size (one cylinder)
Static CR
Time the intake valve remains closed in crank degrees. (720 - seat duration)



-juhana

[BOOT]

Some say larger into smaller and some say smaller into larger, others say same size both perfectly lined up. All 3 will run fine and some combos will like one best But I'm no dyno/track tester so do what makes you feel Best. I've run all at some point, just using what parts I had.

Best comment I've read bout this in the multiple many past discussions, is most the flow is in the center of the port, not the walls But then texture blah blah layer blah blah and so on. Well again do what makes sense to you. I like the idea of same size perfectly matched up and out of the other two options I'll take the smaller into larger for a reversion step if I can.

Almost forgot some say to step the bottom only

[juuhanaa]

When i say i would like to have little bit more below 8000, guess where the engine peaks?

Renault F7R rallycar harmonics.png

-juhana

[ClassAct]

and should we all run intake manifolds bigger than the ports in the head ?

There's enough anecdotal evidence (decades of it) that BBCs rip with Rec port manifolds on oval port heads.

It may be that they only rip compared to the (most likely) OEM manifold that came off, so it may all be in a better manifold and carb.

But maybe it's the fuel that's fallen out of suspension sitting in the lip at the bottom of the manifold to cylinder head joint (where velocity jumps up) getting atomised by the sudden rush each time the port gets flow from the opening intake valve, rather than dribbling around the port walls in the boundary layer.

That one particular combination is the outlier for running the intake smaller than the head.

In the early 2000’s I tested this so much it was stupid.

The best is that the intake and the head match. If you have to have a mismatch, the intake can be SLIGHTLY bigger than the head.

On everything I tested this on, using a smaller intake on a bigger head was a flow and a power killer.

IIRC, Darin Morgan said otherwise but until my testing proves otherwise I will not make the intake port smaller than the head.

[juuhanaa]

Some say larger into smaller and some say smaller into larger, others say same size both perfectly lined up. All 3 will run fine and some combos will like one best But I'm no dyno/track tester so do what makes you feel Best. I've run all at some point, just using what parts I had.

Best comment I've read bout this in the multiple many past discussions, is most the flow is in the center of the port, not the walls But then texture blah blah layer blah blah and so on. Well again do what makes sense to you. I like the idea of same size perfectly matched up and out of the other two options I'll take the smaller into larger for a reversion step if I can.

Almost forgot some say to step the bottom only

Theres nothing wrong to use the parts we have, but it would be great if we could even match them better.. Heres what i mean, when im watching in-car afterward about "combo" i used for one weekend: Headers dropped up to 30 horsepower from 7000 rpm, tuning was shit, induction was ALL OVER THE PLACE and the list goes on.

Renault F7R rallycar harmonics all over the place.png

https://www.youtube.com/watch?v=0LsjkfhfrQk

There are different ways to do a dyno/track, or whatever testing.



-juhana

[Tom68]

That one particular combination is the outlier for running the intake smaller than the head.

In the early 2000’s I tested this so much it was stupid.

The best is that the intake and the head match. If you have to have a mismatch, the intake can be SLIGHTLY bigger than the head.

On everything I tested this on, using a smaller intake on a bigger head was a flow and a power killer.

IIRC, Darin Morgan said otherwise but until my testing proves otherwise I will not make the intake port smaller than the head.

Cheers, thanks for mentioning Darin, found where he at least comes up with a possible explanation so he doesn't completely discount it in all cases.


viewtopic.php?f=1&t=4264&start=15

I've always thought you'd get less turbulent flow climbing a step than you would dropping off it, but all the literature says I'm wrong, so I have to accept the increase in air speed picks up and atomises the puddling fuel.

[juuhanaa]

That one particular combination is the outlier for running the intake smaller than the head.

In the early 2000’s I tested this so much it was stupid.

The best is that the intake and the head match. If you have to have a mismatch, the intake can be SLIGHTLY bigger than the head.

On everything I tested this on, using a smaller intake on a bigger head was a flow and a power killer.

IIRC, Darin Morgan said otherwise but until my testing proves otherwise I will not make the intake port smaller than the head.

Cheers, thanks for mentioning Darin, found where he at least comes up with a possible explanation so he doesn't completely discount it in all cases.


viewtopic.php?f=1&t=4264&start=15

I've always thought you'd get less turbulent flow climbing a step than you would dropping off it, but all the literature says I'm wrong, so I have to accept the increase in air speed picks up and atomises the puddling fuel.

A quote from the thread linked above:

The reason it doesn't hurt the top end is because there is just enough volume and area to support the engines needs at those rpms. Put a bigger cam in them and they would more than likely choke.

I found it interesting that a higher lift cam could be more likely to cause choking, because i dont remember thinking that before. Thats cool, so one extra vote for, but wait a minute now we could choose which way to put the step or area, if we want..? I may have used odd shapes around the valves, but either way im aware of choking that might occur in the engine im working on too!

Shoot me no two without a third, so heres one more This 122cid 4 cylinder worked just fine on upper revs above 7000rpm with a 23degree "dryflow" intake and 26cm long taper before the 1,78in2 minimum cross section at the manifold flange.. Now lets consider what the taper do to velocity, mach index, and what it does to so called hole process with all the port volume. Had the same bottom end than incar i posted above, same exhaust, same cams with 260 @ seat, 180 @ 1mm lift, valves open 1mm at TDC and i didnt realize to retard the cam..

My take is when port volume is in the right place, it doest hurt the bottom end, nor the top end, because it is food for the wave.

But a mild cam the port size dont matter.

You think nothing happen at BTC? Watch this and listen.. A change of one increment in CR input is a small difference, but we should get some idea. The cylinder head had probably adequate airflow on intake. Exhaust was restricted by the manifold big time.. And i havent been able to fix that with a port work only, even i have gotten it better, anyway off topic at two in the morning.

but dyno.png

https://www.youtube.com/watch?v=qHJGluWV550



-juhana

[juuhanaa]

I've always thought you'd get less turbulent flow climbing a step than you would dropping off it, but all the literature says I'm wrong, so I have to accept the increase in air speed picks up and atomises the puddling fuel.

Dropping off a step can cause a turbulence so well, that it is used for example to reduce control surface forces on a aircraft trailing edges. Some French aircraft at least.. And my kitcar spoiler which is made in France, but errr those places work in different enviroment than NA intake under vacuum



-juhana

[Tom68]

Not the same thing but it's obviously more than fuel shear.

Go to 6 min 50 sec.

https://youtu.be/ilBZW3K1lP0

Although he also goes on to improve the flow of a small hole on a bigger pipe, so it's more about the pipe, but stepping down looks like it could be better than stepping up.

[juuhanaa]

Not the same thing but it's obviously more than fuel shear.

Go to 6 min 50 sec.

https://youtu.be/ilBZW3K1lP0

Although he also goes on to improve the flow of a small hole on a bigger pipe, so it's more about the pipe, but stepping down looks like it could be better than stepping up.

This

Screenshot_20220629005845__01.jpg

-juhana

[BOOT]

Engines don't work in constant flow and sometimes not in one direction.

[Bigchief632]

I've corrected terrible port match jobs in the past, one example was where there was a spacer involved, to use a standard deck intake on a tall deck sbc. This was a 800hp deal too, so you'd think there would be more to gain. He had the spacer mounted to the intake, and wasn't very careful, it didn't line up for crap. I mounted the spacers to the heads, lined everything up really nice, pretty much perfect, we figured it was going to gain a bunch. It gained zero.

The intake port on the head, is sucking through the manifold. The big mismatch doesn't really hurt. If the air was being blown into the head, from the manifold side, there is some turbulence created, but then usually that's on a blower or turbo deal and it just pounds the air in anyhow.

As far as the rectangle port manifold on an oval port, making big gains, or some do it the opposite way, a oval port manifold on a big rectangle port heads, is to band aid a velocity issue one way or the other, and works, but the correct sized intake and head combo is still the best.

[Tom68]

https://youtu.be/2sbICMpLiBI

So the 1275 is an excellent 550 to 600 Hp upper ci SBC manifold.
Yet again we see big runners feeding smaller ports making good power.
If you think about it, if it was the other way you'd have a small runner feeding a big head ( effectively reverse inlet runner taper) that'll always be crap.

[BLSTIC]

Wasn't there some Mercruiser big block with mismatched manifold/head combo that would lose an absolute crapton of low and mid range if you "fixed" the step?