Would it be better to run a larger cross section intake into a smaller one into the head or would it be best to keep the cross section consistant across the whole runner? Would changing the intake runner length have any effect on this?

 

most race heads flow air from large to small SC, but its nothing radical like radiator hose to fuel injector "hole"

 

Thoughts:

The engine shouldn't know where the intake ends and the head port begin; it's all just one continuous runner to the (dumb) engine.

A tapering intake runner (smaller near the head) can help flow. This taper is on the order of 1/2 to 1 degree per side, so it's not huge. Taper smoothly increase the intake charge velocity from the plenum to the valve area.

If you have a mismatch at the intake/head junction, it's better to have a smaller intake manifold section so that it creates an anti-reversion step. That's just the opposite of a header-to-exhaust port mismatch, but it achieves the same effect because of the direction of the flow. The air will also flow across a smaller-to-larger step MUCH better than across a larger-to-smaller step.

Any significant changes in cross section (CS) along the intake tract have an effect on the flow. If the CS momentarily gets larger then smaller again, say at the manifold flange where head and manifold were "gasket matched" to a larger CS, intake velocity slows as the CS increases then tries to speed up again which usually decreases total flow.

Runner length is more critical for intake tuning. Overly large intake runners could cause low rpm problems due to low velocities.

 

agreed. you can really see it with long runners, but on short runners the angle is so slight you cant even make it out

 

What about running a larger Xsection at the intake and port match it at the head, smaller Xsection? Would it give up a lot on the lowend?

 

Why? I can't see the advantage to a significant CS change in a short transition lenght. 1/2 degree per side is about 2-1/2% increase in CS per inch of runner length, more or less. For a 5-1/2 inch runner (SuperVic for SBC) that's about 3/32 inches wider and deeper at the plenum than at the head, and about 15% more CS. How much CS change are you looking at, and in what length?

I can't recall ever seeing a manifold with a large taper on the intake runners. If the runner were a lot larger than the port in the head with a short transition, the engine would tend to treat the intake runner as plenum, and not an extension of the port in the head. The tuning lenght would then be VERY short; probably shorter than an F1 19k engine!

Well ported LT1 heads often have a larger CS than the stock short manifold runners. Opening up the manifold to the head size helps. The downside is there's not enough material in the LT1/4 manifold so welding is needed.

 

I am just building a 393, yes a Ford, and am trying to decided between the Vic 5.8 intake and the TFS Box R. All running into AFR 185s, I would run a larger head, but the budget doesn't permit it. I was planning on a XE282HR for the cam.

I know the head is small, but this is a daily driver and I'm not looking to shift much past 6500.

 

Run a single plane intake even if it has to be converted to EFI for the flow and the runner length.

Bret

 

I think I am going to run some ported Vic Jrs instead of AFR 185s, can get them about the same price. I talked to someone that had some AFR 225s on a 410 with the same Comp cam I want and a Vic Jr carb intake. He made 450 rwhp@5700 rpm,445 rwtq@4800 rpm on a Mustang Dyno.

Would there be any advantage to running a Vic 5.8 over a Vic Jr carb intake? The reason I want to run the Vic 5.8 is the longer runners will help the lowend on the street.

Edit: Forgot to ask how accurate your guys think EA 3.0 is?

 

I've attempted in the past, in my own little way, to convey that the accepted approach used to port the LT4 manifold, to match LT4 heads, done by high profile/popular members here, is ignoring the rule of larger CS tapering DOWN to a smaller CS, prior to head ports.

 

What you talking about willis?

Getting any tapper into a properly done LT intake would be really hard since that would require the intake to have even more weld on the roof of the ports.

So are you saying that you should leave the injector bump in the intake or what?

just confused to what you mean here arnie?

Bret

 

No! Who's Willis?

Getting any taper into a properly done LT intake would be really hard since that would require the intake to have even more weld on the roof of the ports.

Agreed! Starting the the oem intake provides for limited options, regards to ending up with a package that resides within the 'rules' of porting/port fabrication.

So are you saying that you should leave the injector bump in the intake or what? Just confused as to what you mean here arnie.

No! It is the lesser of two evils, compared to doing nothing. You end up with a net gain, but AFAIC, damned it ya do, damned if ya don't, with result being a poor approach/line of flow from manifold to heads. A major plus however, is the relative ease, at which this approach can be implemented. Just stating it gets more blind acceptance and applause than the decision deserves, that's all. If GM engineers had their way (in lieu of bean counters), different casting would have been the choice, IMO. Although that would have allowed less clearance under hood of F and Y bodies.

 

Eh hell with the rules. If you can't weld the outside of those intakes then you just have to learn how to hide it better! That or run a single plane.

It's not to hard to get a constant cross section and parallel walls in a LT intake just leaves you with a injector sealing problem since the heads that need a intake like this most have the highest roof on them.

Bret

 

I'll let ya have the product 'plug'.

It's not to hard to get a constant cross section and parallel walls in a LT intake, just leaves you with a injector sealing problem, since the heads that need a intake like this, must have the highest roof on them.

It's not that easy then. Personally, I don't want a constant cross section port, in the manifold. Rule I had in mind, that you are familiar with, is having a slight taper to the port, prior to entering head. I'll admit, easier to obtain, having this in mind when designing manifold, such as sheetmetal intakes, but attainable, nontheless.

 

Depends on the lenght of the port too. I'd rather have some tapper to the runner but when you get stuck with a short runner the only way to make it think like it's a longer runner is to keep the tapper out of it.

Bret