I need an intake manifold for my 20B. I have a 750CFM Edelbrock carb to use. It should be large enough CFM-wise. NA of course (for now).



I have two main choices right now. The first choice is to use an Atkins 9" supercharger intake manifold (no SC at this time due to lack of funds). The other choice is to modify my stock 20B LIM.



The Atkins manifold:

I could build a carb spacer to mate the carb to the manifold. The manifold opening is 7" long by 2 5/8" wide. The carb square bore is 3 9/16" by 3 5/8" (essentially a square).



Do you guys think I'd have a lean-running condition in the front and rear rotors if I were to build a square-to-rectangle carb spacer at least 6" tall? The height of the spacer will mimick the height of the supercharger. Maybe it would work better if I added a flow director/deflector inside to help guide the AF mixture fore and aft a little better. Thoughts?



Modified LIM:

I'd find an aluminum pipe anywhere from 3" to 4" diameter by around 14" long. Then I'd slice it lengthwise, cut six holes lengthwise in one piece, shorten the LIM, weld the piece with the six holes to the LIM and clean up the runners a bit, then weld the other piece back on with a carb adaptor on top. Two end caps, and it's done.



This idea has an open plenum beneath the carb, with stock (shortened) runners leading into the ports. Is this idea any better than the other one for preventing a lean running condition? Mind you, the carb will have less area beneath it for the AF mixture to sort of 'spread out' than with the SC manifold.



I obviously know very little about intake manifold theory and design, so building my own from scratch is not an option. Please give me your input. Thanks. https://www.nopistons.com/forums/pub...IR#>/smile.png

yeah efi is easy casue you can do anything you want, the ideal setup for the 20b migh be like dual carbs or something.

Remember that baffle idea we were talking about a while back? It seems like two the baffle plates stacked one inch apart will work in either manifold. The idea is to drill many randomly sized holes all over the plates. It apearantly makes an 'homogenous' mixture.



In the aluminum tube idea, they would have the effect of helping to spread out the AF mixture so it won't all go into the middle rotor. That's the theory anyway. I have no idea if it would work like that, or at all.



In the Atkins manifold idea, two baffle plates could be installed in the carb spacer. Again, I have no idea if it would even work, and what's more, if it's even necessary in this particular idea.



Thinking about it some more, the baffles in the aluminum tube would have more surface area than in the carb spacer on the Atkins manifold.



The aluminum tube idea definetly needs some baffles in it. Otherwise, the front and rear rotors will go lean.



The Atkins manifold carb spacer probably doesn't need any baffles because the AF mixture will go into the manifold and expand from 7" out to 17 1/4". Also, the carb will be pointing the correct direction for a rotary (like a Racing Beat Holley with the primaries toward the engine), so the primaries will mix with the secondaries as well as can be expected.



Ok, let's assume the 7" by 2 5/8" opening at the top of the Atkins manifold is centered front to back. I measured across my stock LIM and found the distance to be 17 1/4" from front secondary to rear secondary. Then I found the middle of this (8 5/8" from each end) and marked it on the gasket surface. Measuring 3 1/2" forward and back from this center point, I noticed that four out of the six 20B ports were within 7" of each other. Interesting.



The secondaries are a lot larger than the primary ports of course, but having three primaries and one secondary already within the 7" range, is quite promising. The primaries will have an equal shot at getting a good amount of fuel out of the AF mixture. Which leaves the problem with the secondaries.



How about if I installed a partition inside the manifold around the middle rotor's secondary port runner so it would't get the same direct shot as the primaries? What if I made the opening of the partition a little further away from the primaries so when the momentary reversion of the middle rotor happens (you know, when the ports close and it pushes the AF mixture the wrong direction for a microsecond), it doesn't screw things up for the primary ports of the other two rotors? Maybe that part doesn't make sense, but what if the opening of the partition was more near the other secondaries? It might work. Right?



I'll draw up an idea for the partition inside of the manifold.

Maybe 'partition' isn't the right thing to call it, but this should give you the basic idea. The green represents a tube inside the manifold. The red rectangle is the 7" by 2 5/8" opening at the top of the manifold. The blue circles are the ports (in case you couldn't figure it out https://www.nopistons.com/forums/pub...IR ).



What I'm trying to do is make the secondary of the middle rotor less accesible to the incoming AF mixture. As long as the tube opens just beyond the rectangular hole at the top of the manifold, it should effectively be outside of the main incoming AF mixture.



Do you guys think this is too extreme?

i always had a problem with the e6k on the 20b with it fouling #1 rotor because it fired all the primary injectors at once, so #1 was rich, and #3 was prolly lean.



i think with a setup like yours the center rotor will be the richer one. so i like your idea of partitioning the manifold, it should help?

I hope it will.



What if I added a tube to each secondary? Hmm, that's probably unnecessary.



You know how the GSL-SE's cold air intake tube goes from in front of the radiator to the air filter box? The cold air tube is perpendicular to the incoming air, yet it still works fine (for stock power anyway). That was my inspiration for the tube in the atkins manifold.

Can anyone else here help me with flow dynamics? I need to know if the small tube inside the manifold will do what I think it'll do.

Here is a third idea. What if I were to actually build myself a manifold? Imagine that! https://www.nopistons.com/forums/pub...IR#>/smile.png



Mazdatrix sells steel and aluminum intake flanges. I know how to work with steel, so I'd get the steel one even though it's a little more expensive.



What do you think about this drawing? I didn't include primary runners here for simplicity. I think the middle pipe needs to be longer for equal length runners.

i was thinking about a cut down stock intake with like a tubular upper. um the problem with the stock intake is that all of the holes are in a straight line. i'm thinking you mod it so that it looks more like a 12a intake? or at least comes to a square?

Yes, that would work very well! I need to put a plenum directly between the the carb and the LIM because the carb is designed for a plenum type manifold on a V8.



Maybe I should look more closely at a V8 highrise manifold to get a few ideas?