Designing a single carb manifold. Need flange dimensions.

[slabghost]

I'm certainly not an expert but I have been studying manifold design and theory. In the longer runners the fuel charge contained is 3-4 times the amount that will actually get into the cylinder at one valve opening. This charge changes directions several times bouncing back and forth until the next opening of the valve. If the charge is bouncing back toward the carb at valve opening the charge will be weak. If the charge is bouncing toward the valve just as the valve opens it can multiply the amount of charge entering the cylinder. Inside diameter size is the main option in adjusting the timing of the charge. Runners of course can be lengthened by adding curves but that is problematic. Timing of the "bounce" can only be done at one rpm and above or below that rpm the charge will arrive later. So the real power is noticed differently with different manifolds. Some may feel best power at lower rpm while others feel the big kick at higher rpm. Since lengths are pretty much set. It has to be due to the inside diameter of the runners.

 

[Steve83]

I had a '90 Taurus SHO, 3.0 V-6, with one of the most interesting intake manifolds I've ever seen. It had 6 long primaries that were tuned for lower RPM, and 6 short, vacuum-controlled secondaries, tuned for high revs. That was one sweet running car!

Taurus SHO Intake

• Steve83
• May 6, 2014

 

[chuck c]

Steve83 » Tue May 06, 2014 6:04 pm[/url]":2l8nbzqh]
I had a '90 Taurus SHO, 3.0 V-6, with one of the most interesting intake manifolds I've ever seen. It had 6 long primaries that were tuned for lower RPM, and 6 short, vacuum-controlled secondaries, tuned for high revs. That was one sweet running car!

Taurus SHO Intake

• Steve83
• May 6, 2014

That's really cool. Interesting design.

 

[chuck c]

Think of a trombone. The length changes it's pitch. There's one frequency where it resonates. The same physics works for intakes and exhaust systems. Work out so they are both resonant with the timing of the intake and exhaust valves and you can get a good power boost at one RPM as you said. That resonance can help pull the exhaust out and hit the intake valve with a pressure wave just as it's opening to push the next charge in. Notice how headers are all the same length?

I've never studied the topic (it might be a good time to start) to see the effect of different runner diameters. My gut says the smaller the diameter, the narrower the power band. That means there's a trade-off between tuning and flow.

Yeah, the old cast iron manifolds on an inline 6 might have a factor of 3 in the difference between the shortest and longest runners and yup, they worked well enough. They were usually nice smooth graceful curves though. I have to wonder if the runners had slightly different IDs to help even out the flow. Get a flammable mix to the combustion chamber and it will go boom.

The big problem with some of the monocarb manifold designs looks like bad and uneven flow, making some cylinders run lean. I think a roomy plenum box helps counter that, it will hold a volume of mix large enough for each cylinder to get an even amount, but it's a solution with its own problems.

I'm having second thoughts about the expanded ends on my design to fit the Honda intake runners. When a gas expands it cools and a vaporized liquid like water or gasoline in it starts to condense out and form droplets which is exactly the opposite of what I want and at the worst place! If anything the runner's ID should exactly match the intake ID. I will have to find another way to do that.

 

[chuck c]

I just found this. Can anybody here run this for a GL1100?
Is this right? I tried it and got an Id of .001. I think I messed up.
square root of: 1503x4x1x115CFM(solex 34pict3) / .8x4x67547x3.14

 

[dan filipi]

Chuck I think your doing great and I completely agree with you about maintaining even flows in all the runners. This flat 4 depends on it for that sewing machine smooth.
Keep going on the path your going and thinking everything through carefully.

 

[chuck c]

Also this:
The runner length is inversly proportional to the engine speed, higher the engine speed, the shorter the runner length. There is an euation to calculate resonant tuning speed for a naturally asperated engine:

Nt(rev/min)=(955/K)*a*(A/lVeff)1/2

Nt = engine speed at resonant tunning
K = a constant equal to about 2 for most engines
a = the sound speed (m/s)
A = effective cross-sectional area of the inet system(cm2)
l = effective runner length from inlet to valve (cm)
Veff = Vd(rc+1)/[2(rc-1)] (cm3)
Vd = volume of one cylinder (cm3)
rc = compression ratio

The above is for a single cylinder engine but should be pretty close. If you want to get crazy, do a search on the web for Helmholtz theory for multicylinder engines which takes into acout the intake runners of the other cylinders.

Another important thing to know about intake runners is that they should tapper down to a smaller area as they reach the valve. 2 to 3 degrees of tapper per wall is a good start. This will keep the up the mommentum of the charge as it travels to the cylinder.

That supports the claim that some have made for better power at low speed at the sacrifice of top end by going to a monocarb. The OEMs are designed for good top end with almost no runners. Guys say by going to a single they loose 10 or 15 mph off the OEM top speed, which is still faster than I want to ride a 34 year old 675lb motorcycle.

 

[chuck c]

Then I found this tidbit in an article about working on high performance intake manifolds:

Rocker-arm ratio —This can be a very effective way of aiding poor-flowing intake manifold runners. Slight increases in rocker ratio can boost volumetric efficiency, leading to improved net port flow for runners incapable of matching the flow provided by other and more efficient runners. In such cases, increasing the ratio on the intake side is often the most effective area in which to work.

Wow. To really get all cylinders the same charge they sometimes tweak the rocker arms on individual cylinders for a bit of extra lift to compensate for a manifold that doesn't get it right by itself. I think that backs up my instinct for the importance of equal flow. It's also a heck of an argument for adjusting your valves!

 

[dan filipi]

I've been meaning to mention to you to check your carb positioning between the frame rails before you start building this.
Possibly may need to adjust somewhere to fit the carb you go with.

 

[chuck c]

Thanks for the warning. I hope both the Solex and Weber will fit in the frame without serious redesign of the manifold. My Solex should be here monday so I can start measuring it.

 

[mcgovern61]

Just an observation (I really know absolutely nothing about this stuff), but the Ford manifold and the Suburu manifold have something in common:

Each runner on the long runners on the Ford manifold arch up before turning down to the intake, same as the Suburu manifold. Any possibility that is needed?

 

[Joep]

That may be their way to make the intakes equidistant from the flow... "tuned" runners..

 

[Ansimp]

A lot of Subarus' changing of manifold length and design was to reduce intake noise

 

[chuck c]

I'm thinking getting a smooth path with minimal sharp turns into the head was high priority. I also think doing that would decrease noise as a bonus.

 

[chuck c]

I must remember to put a piece of screen into the plenum to prevent hardware from making a trip into the combustion chamber.

 

[chuck c]

The aluminum manifold is likely a few weeks away but I'm getting impatient. I am sorely tempted to make a trip to the Lowe's for PVC pipe, hose clamps, and RTV to piece something together to try out the carb.

 

[Ansimp]

Don't do it Chuck, it may cause you to doubt your new alloy manifold and you don't want that to happen after all your hard work! :smilie_happy: