Custom ITB Intake Manifold Design

[cockerstar]

I've been wanting to make a custom intake manifold (along with a tubular exhaust manifold/header, but that's another thread!) for my turbo swap this summer. I acquired some 46mm mikuni ITBs from S130Z awhile ago, and now that I have them here in front of me up at school I'm working on the design.

 

Here are a few pics of what I have to work with:

 

 

 

 

My basic shape has been a hybrid of a few different designs I've seen (Monzter, PrOxLaMuS, Kaihai, a few Japanese manifolds I don't know the origins of, stock rb25dett, etc.). I've tried to pick out the best aspects from each design, and add them into mine.

 

To me, it seems like the easiest method of distributing airflow to all of the cylinders equally is to have the inlet on the side of the plenum (think rb25 manifold) rather than at the front like a stock l-series is placed. MONZTER has obviously shown us that this is quite possible, but I'm attracted to routing the intake over the valve cover. Not only does this give a "straight" shot at the runners, but it also minimizes intake piping clutter from the intercooler.

All of my drawings are nowhere near to scale, and only intended to convey the concept of shape! Sorry in advance for the sub-par MS Paint 'renderings'

 

Here is a surge tank for a set of triple webers that comes over the top with two different intake ports: Please let me know if you know where whose this is so I can give credit!

 

Here is a honda intake that I am really fond of. It was originally posted up by mootori:

 

I've been doing a lot of reading on intake manifold theory online, both here on hybridZ and through google, and I have some clarifying questions as well as my preliminary thoughts and drawings to share.

 

 

1)

Through my reading on here, for some reason I remember the "ideal" volume for an intake manifold being 1.5 times the effective displacement (including 'displacement' added by boost) of the engine, but in reading online, I've found articles claiming that 50-60% and 50-70% is the "ideal".

 

- Which of these numbers is accurate?

- Where is the volume measured? Just the plenum itself, or the plenum and the runners?

I'm assuming this is just the plenum, as there are separate calculations for both runner length and diameter (and subsequently runner volume).

 

2)

Would my intake design be better off with the triangulated inlet into a plenum that breaks into runners as seen in the honda intake (Design #1), or would it be better to have an inlet with a constant thickness that fed straight into runners via velocity stacks(design #2)? In the latter of the two I would incorporate some kind of flow diverters to maximize even distribution. Design #3 is a direct hybridization of the first two manifold examples I posted in the thread. Is retains the diverters of #2

 

Design 1:

 

Design 2:

 

Design 3:

 

3)

Would it be more logical to place the ITBs before (location A) or after (location the bend in the runners?

Intuatively, I am leaning towards location A, but I am unsure as to how this will effect fuel atomization with the injectors spraying directly into a 180 degree bend. I guess it would also be an option to place weld in injector bungs in location B, and keep the ITBs in location A.

 

 

 

4)

How does the 180 degree bend effect calculating runner length?

I plan on trying to do some acoustic tuning of the runners with some of the online calculators even though the manifold is supposed to be under boost most of the time.

 

As for runner diameter I plan on listening to what RTz had to say in PrOxLaMuS's design thread

 

Welcome to the art of intake manifolds

 

The cross-sectional area of the runner is the single biggest influence on where torque peak falls...

 

1.500" ID puts the natural torque peak (of the runner) around 5540 RPM on an L28. Pretty steep for most folk.

1.450" ID... 5200 RPM

1.400" ID... 4850 RPM

1.350" ID... 4500 RPM

 

I wouldn't run any less than 6" length and I'd like to see more like 7"+.

Don't discount taper. General consensus is about 2.5% (in area) per inch.

 

 

Here are a few goals:

- maximize throttle response

- lower boost threshold

- ~300hp out of my l24 before I blow it up and move everything over to an l28. Everything will be designed with an l28 @ 15-18psi, but I have a fresh engine that is begging for some boost, even if it has 15% less displacement. The 300whp out of an l24 translates to around 355 out of an l28.

 

 

Any input I can get from you guys would be fantastic! I know there are a ton of amazingly smart people on these boards, and I'm constantly impressed by what gets contributed

 

Feel free to share any ideas/thoughts/pictures you may have!

Also, any suggested reading on the topic is greatly appreciated!

 

-Ryan

[olie05]

I don't have any input on your manifold ideas, but I'm interested in your throttle bodies. They look like normal motorcycle TB's except for the flange mount and the two mounting holes (perpendicular to the flange).

 

Do you have any more information on them? Like what car they are off of, or if they are completely custom ordered?

 

Also, is the bolt pattern the same as the DCOE style carb? In other words, will this bolt up to a cannon manifold- or similar?

[19762802+2]

I don't have any input on your manifold ideas, but I'm interested in your throttle bodies. They look like normal motorcycle TB's except for the flange mount and the two mounting holes (perpendicular to the flange).

 

Do you have any more information on them? Like what car they are off of, or if they are completely custom ordered?

 

Also, is the bolt pattern the same as the DCOE style carb? In other words, will this bolt up to a cannon manifold- or similar?

 

They are off a jet ski there is a thread about them by S130Z, they were a recall at his work place and they were going to throw them away so he sold them.

 

Edit: here is the link to that thread http://forums.hybridz.org/showthread.php?t=156998&highlight=S130Z

[cockerstar]

olie05, Nathan hit it right on the head.

They're from a wave runner that Adam (S130Z) was able to get through a recall at his work. Apparently there was concern that they may corrode of used extensively, so after I agreed to not drive my Z into the ocean Adam was willing to sell them to me

 

As you can see in the pictures, they unbolt from each other which will really help in spacing them however I want to (for a straight shot down each runner). I'm not sure what the standard triple carb manifold bolt spacing is, but I can measure the mounting holes on the flange this afternoon.

 

I do know that every other one would have to be flipped, alternating the opening direction of the butterfly, so even if they do bolt up it would be interesting to get the throttle linkage worked out!

[olie05]

Spacing is an issue that we can discuss here since it applies to your manifold design. (the manifold from the head to the tb's. What were you planning? Even spacing, or something else?

 

when I was planning my ITB setup I wanted to do 2 sets of 3 evenly spaced throttles with a space in between the two sets. I think this would have a cool three barrel weber look to it.

 

Why do you have to alternate the throttles the way you describe?

[cockerstar]

I'm planning on an uneven spacing as seen in the lonewolf intakes. Sunbelt also followed this general design.

 

lonewolf:

 

sunbelt:

 

I'm thinking this will offer the best atomization as well as a 'straight shot' into each cylinder. I'm assuming this will come at the penalty of trying to maximizer the evenness in flow between the cylinders because of the asymmetrical design between each runner. I'm hoping to remedy this with the flow diverters as seen in design #2

 

To achieve the spacing I was planning on simply using some aluminum tubing with the same id ad the space for the threaded rods that connect all of the ITBs together.

 

I would need to alternate the throttles due to the bolt spacing in all of them being parallel. In the traditional triples configuration the bolt holes slant inward to make a " /" shape. I guess I could weld them shut and drill new mounting holes in the flange, but I would rather retain them how they are since I'm making a whole manifold.

 

Here's a pic for reference:

[S130Z]

Looks great man! Glad you got them and have started playing around with them. Plans look great by the way.

[olie05]

You probably will not be able to run the runners parallel on the closer spaced ports. The reason is that your throttle bodies most likely have a wider spacing than the ports. This is what I had designed for a manifold, using the minimum spacing allowed by the throttle bodies.

 

 

edit: I forgot to mention that in the above drawings, the throttle body spacing is 80mm. This is the standard spacing on the 2003 gsx-r 750 throttle bodies that I was designing for.

[cockerstar]

That's something I was afraid would be an issue, nut if it comes down to it I can run them how you have pictured. Unfortunately, almost all of my Z stuff if across the state so I don't even have a stock intake manifold to look at for reference.

Do you have the spacing on the intake ports on the head, as well as the stock, unported diameter? If the info proves to be hard to find I can always go pick up an intake/exhaust gasket

 

Right now it looks like I'm working with 46mm diameter throttles with a minimum of 36mm between openings. That puts centers 59mm apart when they're bolted together and all of the throttle linkages work like stock, so I'm guessing that I'm either going to have to make that 36mm gap less, or space them out a little bit.

[Xnke]

Ok, stock N36 carb manifold, head side:

 

ports 1-2, 50mm, ports 2-3, 96mm, ports 3-4 don't know, ports 4-5 96mm, ports 5-6 50mm.

 

These are all measured center to center.

 

Stock port diameter on my N42 head was 33-34mm. I'll try to get the distance from 4-5 tomarrow, off one of my spare heads.

 

If it was me, I'd situate the throttle plates horizontal, placing the throttle bodies so that if you got a wild hair to run N/A, you could run big vertical horns through the hood, or bolt the top half of the intake on to run forced induction.

 

Something like this:

 

 

It also simplifies the mounting considerably, since you don't have to worry quite so much about hood clearance.

[cockerstar]

The more I'm looking at things, the more I am favoring my second design, simply due to ease of fabrication and production costs. The 180 degree bends in aluminum are $35 each, and needing 6 of them really starts to add up.

Being able to avoid that cost would definitely be beneficial on the wallet. Additionally, I would be able to weld on injector bungs directly opposite the velocity stacks to run secondary injectors or as a location to spray water/methanol.

[Xnke]

Problem with that is the runner length is maxed out at about 7", once you account for the bend you need. Plus, that really sharp turn the air has to make to get into the throttle bodies is going to completely negate any kind of gain, unless you smooth the turn out some. (Or, I'm just totally not seeing how this goes together) EDIT: I see how it works now. Disregard that bit about negation, and all that crap. Forgot about the turbo...

 

Use what is in your hands to minimize the amount of complexity. A good fabricated part is as simple as possible, but no simpler.

 

I'd start out by setting up the intake flange and runner tubing to the throttle bodies, and get that worked up first; and make it flat. Then you could have the plenum mount on that, then the top of the plenum has the "upper plenum" (your triangular thing) coming out of the top of it, eliminating the need for any bent tubing.

 

The whole upper plenum could be made from formed sheet aluminum, fairly easily. cut out the upper and lower plates, roll them to fit the curve, then the diverters and sideplates could be cut from sheet to fit the already formed plenum curves. A bit of hammertime, and some aluminum tubing to match your intercooler tubing, and BAM! upper plenum that bolts/welds onto whatever size main plenum you want.

[Xnke]

 

 

Might think about the design of this intake as well, it's much more compact and probably has about the same performace, at the power levels in question.

[cockerstar]

Anyone have any opinions on the flow differences between the manifolds?

It seems like flow distribution is really negated with boost and that any of the designs are going to perform similarly, but at the same time better flow should yield a lower boost threshold, right?

That really is my main concern here

[260DET]

Concerning plenum volume, there is no right formula. The primary concern is to design the plenum so as to achieve even flow to all cylinders, everything else follows. All extra volume does is to provide a buffer which may help even out flow and delay throttle response. A plenum could be made using a reverse collector design, it would work fine with very little volume if correctly done.

 

Anyway the important thing is to understand the characteristics of air flow, what I did may help http://forums.hybridz.org/showthread.php?t=157316