NGWClub®, Inc.

Here is a dumb answer - dunno.
This is off the top of my head. Maybe it's real and maybe it's fiction.
With dual carbs (or at least dual throats ie. the Weber sidedrafts) there may not be an issue with unequal runners. Air flow is an issue for any manifold. Ideally you want the air to spiral into the ports. Next best is a laminar airflow. Worst case is turbulence. (I'm 61. Fluid dynamics classes were a long time ago.) I wanted to have at least some semblance of smooth airflow. If the front tube just right-angled down to the port the air would run across an edge. Not good. Air on the outside of a radius has to travel a lot faster to get to the same point as air on the inside of a radius. The larger the radius, the closer the speed differential. Back in the day it was determined that Harley exhaust ports were much more efficient when the ports were "D" shaped. That brought up the floor of the radius so the speed differential between the air on the outside of the radius and air on the floor of the "D" was much closer. That results in less turbulence.

So I wanted at least some sort of radius instead of the right angle. After that, making them the same length was fo free.

Here is todays progress. The one on the right has 3 coats of glass, the one on the left has two coats of gel coat, and the long tube in the foreground has one gel coat. Final OD is to be 1 3/4" and the ID matches the head ports. The carb end will get opened up a bit to match the back end of the carbs. Here is a new use for the fishing rod lathe. No drips around here (except maybe me).

Actually you do want a little turbulence. It helps break up the fuel droplets and atomizes the mixture. You will also want a little texture to the inside of the intake tract. This too helps atomize the fuel.

For an example. Suzuki used to have what they called the TSCC. Total Swirl Combustion Chamber.

One other thing/trick. Make the intake runner a little smaller as it enters the port than it is where it attatches to the carb. This will increase velocity. This can be done in the head too.

Fun idea but I see some real challenges. First you have to have some resistance to air flow (to create the vacuum) on the intake side for the CV to work. Without the proper resistance to airflow provided by the right filter and intake assembly the CV's will eventually work but no where near the correct profile. Getting fuel to the carbs is another and tuning could be an adventure all of it's own. But it does look like a fun project

I love the design. I wish you luck on your project.

AndyB

Go for it. It's gonna look awesome!!!

Yes, yes, and yes.
Some turbulence will be there I'm sure. I didn't over-sand the mandrels.
There may indeed need to be a restrictive air cleaner. I'm thinking K&N cones modified in some unknown magical way.
I have no idea if this will actually work. At this point I'm hoping it will idle.
The linkage is going to take a bit of head scratching. Gotta incorporate the left-to-right synchronizer in there some way. Even the choke control is going to have to be split.

just my thoughts (and echoing some other comments-much of this is based on the work of gordon jennings, some on the work of the naca regarding laminar flow, and some on basic fluid dynamics gleaned from years of fiddling with stuff like this): on the inside radius you want a little texture to help maintain laminar flow around the bend (think of airplane wing spoilers: activated at high angles of attack to maintain flow, preventing stall) to prevent serious turbulence that will effectively block the passage-so the tubes should be a bit rough on the inside of the bend, but as smooth as possible on the outside of the bend.
The tubes should taper from the head out to the carb to create a bit of a funnel shape to enhance laminar flow and, perhaps, to introduce a bit of a swirl (though that will actually reduce overall flow velocity to the head, it will reduce the thickness of the boundary layer, therefore increasing the effective bore of the tube a bit-I wouldn't make the swirl a priority-to test this, take a funnel, pour a large amount of water through it, time the rate it flows with as little swirl as you can manage, and with as much as you can manage by moving it in a circle. The difference is major!).
Fluid flow through a tube creates resistance (back pressure). The longer the tube, of a given size, the more resistance. Putting a taper in it reduces this resistance so that you basically have the resistance created by the narrowest portion of the tube. It doesn't take much taper to acheive this effect.
Getting fuel to the carbs will be a challenge, but is something I wouldn't get too hung up on until the final fit/mock-up as a set of fittings, some "T's" or other can solve it.
The jetting will change, as the runner length and bore diameter and the efficiency of fuel/air delivery will change, but it should start and run well enough to test with stock jetting, and be dialed in from there. The vaccuum you need to be concerned with is that after the carb-the filter should be as free flowing as possible. The slide and butterfly creates the velocity at the throat which draws the fuel, the runner has less effect, the filter no effect.
(Edit)need to rephrase that: the filter has a huge effect, but for maximum efficiency, it needs to flow with as little restriction as possible.

Greg, if you PM or e-mail me a really good side shot of the profile of just the pipes. I may be able to put them in a CFD program I have (if I remember how to use it) just for grins and giggles to see what the flow would look like. Granted it wouldn't be 100% accurate, it would be fun though. Give me the same for the stock one and I can compare the two. Anyone know the CFM these would see at WOT?

I was wondering the same thing. On sport bikes, designers specifically design velocity stacks in different lengths from factory. This reduces peak Hp, but increases power under the curve. Racers will sometimes replace longer stacks with the short ones to increase peak high rpm power.
Edit: I didn't realize I was on the first page when I tried to answer the question about different length intakes. Looks like its been covered

I'll put together a couple of dimensioned drawings and send them.
My main concern (one of them anyway) is the difference in flow characteristic (the delta) between the front "S" tube and the much straighter rear tube.
The design for the Webers started at 40mm and tapered down to 36mm at the port. This design is a constant 36mm with just a chamfer to match up to the carbs. At least this design can be built.

A good side shot or picture from the drawing with a single color background would be perfect. This program is pretty simple, and will just give an idea.

I am very interested in this due to some ideas that I have for my up-coming project.
One more thing that I thought of, and I am wondering if it will impact this is that the ethinol they are putting in gasoline now damages the plastic used with fiberglass. I remember boat owners with fiberglass tanks having major issues with leaks for this reason. Will this be a non-issue since the manifolds are not soaking in gas, or will a coating be required?

gregforesi wrote:There may indeed need to be a restrictive air cleaner. I'm thinking K&N cones modified in some unknown magical way.
I have no idea if this will actually work. At this point I'm hoping it will idle.

Oh, you can get it to run, and pretty decent at that, but it will teach you some new, not so nice words before you are done. The 900F ran pretty good with CV carbs and K&N pod filters, but everything effects everything with a factor of five. If I cleaned and re-oiled the filters, I had to tweak the settings for idle and then again as the filters settled in.

AODRN wrote:I was wondering the same thing. On sport bikes, designers specifically design velocity stacks in different lengths from factory. This reduces peak Hp, but increases power under the curve. Racers will sometimes replace longer stacks with the short ones to increase peak high rpm power.
Edit: I didn't realize I was on the first page when I tried to answer the question about different length intakes. Looks like its been covered

True enough.

On the tuned Caterham K series engines we run velocity stacks of different lengths ....the two centre trumpets are 80mm long on one I've just designed an airbox for, and the outers are 50mm. This means two cylinders have enhanced mid range torque (the long trumpets) and the outer two are more top endy. It gives both mid range AND top end, a very satisfactory compromise.
This setup is running Titan Phantom fuel injection rather than carbs.








Some sports bikes actually use a servo motor to alter the trumpet length on command from the ECU, so they are longer at midrange and shorten as the revs rise. This gives optimum resonance.

Greg, before I forget, be very careful with paint stripper on gelcoat! I read something about stripping paint off the inside of the pipes?

I've seen paint stripper I used here causing the gelcoat to literally pop off the GRP, and it was years old!
It totally ruined the piece, a lower tail section on a CBR racer.

Dangeruss wrote:

gregforesi wrote:There may indeed need to be a restrictive air cleaner. I'm thinking K&N cones modified in some unknown magical way.
I have no idea if this will actually work. At this point I'm hoping it will idle.

Oh, you can get it to run, and pretty decent at that, but it will teach you some new, not so nice words before you are done. The 900F ran pretty good with CV carbs and K&N pod filters, but everything effects everything with a factor of five. If I cleaned and re-oiled the filters, I had to tweak the settings for idle and then again as the filters settled in.

On my old 1100F I had to be very careful oiling those filters, the amount of oil affected everything.

Gord