What effect would there be on intake if there was one opening into the manifold with an area equal in size to the area of the normal two openiongs?

That is, other than an obviously necessary physical transition between carb and manifold, what would be the effect on feeding the engine? Or what is the difference between a two hole feed and a one hole feed - effectively?

Would the break between two holes cause better fuel mixture?

Guessing that you're asking about the 2bbl intakes, wanting to know why they have two round holes instead of one oblong one. Both types have been made. The '67-up export-only 2bbl setups were mostly "one oblong hole" type. The '76-'82 North American market 2bbl setups were "two round hole" types. I've run both kinds of manifold; they both work fine when the carb is set up correctly. I'm thinking there are bigger determinants of driveability/economy/performance than this one.

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Welp, how about "moneyability?" According to my calculations 2 BBD throats are equal to one hole of a diameter of 2.034 inches. Given a distance between single barrel intake studs of 2.625 inches, one could bore the single barrel intake manifold to 2.034 (a good two inch hole saw would give all of that) and have more than 1/4 (.295) inch left on each side, short of the studs.

Given all that, a fabricated adapter (made from a couple pieces of 1/2 inch aluminum?) should make it possible to put a BBD on a bored single manifold with no ill effects. This is why I was asking "why two holes?" Where did I go wrong?

See here: click me

Thanks. Yeah I've seen parts of that somewhere. Looks like a lot of work, which is what my calculations were intended to bypass - bore one hole.

There was the comment about having two holes instead of one, My idea has both, seems mathematically sound and Dan seemed to say one was as good as two. In the end, in my approach the two separate venturi are still there.

Do you have a comment on it? And my question, "what did I get wrong?" I guess I mean that mathematically as well as mechanically. Thanks.

Well, as long as you can get the edges of the 2 inch hole that will stick out on the front and back of the carb to seal, why not?

That's what the adapter is for, right? Thanks.

Yep. That is a good idea and far simpler than mine. It would be much easier on an aluminium intake too.

The two hole thing doesn't really make any difference. The intake isn't a dual plane intake so there really isn't any point in having two separate holes.

Two small holes as compared to one large hole is supposed to keep the velocity of the air flow up.

I can't see the pictures here at work, so bare with me.
Could you also do this manifold mod orienting the carb 90°, depending on the type of carb of course?

Right, but the two small holes are about 3/8 of an inch thick, then it goes to a big fat hole before the runners. I just don't see how such a short "run" of two-holedness would do anything for the air velocity. But I could be wrong.

And the real air speed is up in the venturis.

If your adapter is relatively short (like your 1/2 inch plate of aluminum), the airflow will need to make a couple of very hard turns to go from the BBD venturis to a single circular hole. You'd probably need a plate a couple inches tall to get a smooth transition, and that will affect plenum volume and hood clearance. And getting that adapter to have smooth transitions is going to be very tricky.

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"Mad Scientist" Matt Cramer
'66 Dart - turbocharged 225
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I don't follow that at all. The one 2 inch hole is directly below the two small holes. I don't see any hard turns. And momentarily it expands into the non-descript space of the manifold plenum. Plenty of work but not that tricky or complicated. Other than plain effort, it's harder to describe than to do. Here's what I had in mind, using 2 plates which makes a 1 inch transition.

Put simply it is this. Cut carb holes in one plate and a manifold hole in the other - bolt them together and grind out the differences between them.

Put less simply, I see 2 possible routes. I would not recommend the second one. The boring can be done with a slow drill, top quality (Milwaukee?) hole saws and plenty of milling lube and patience.

1. Bore the 2 inch hole in the manifold flange.
2. Line two appropriately sized (3 x 5?) plates together and drill them for the flange studs.
3. Between the stud holes, clearly scribe a 2 inch circle matching the flange hole in one plate, but do not bore it.
4. Make 2 one inch holes in that plate, oriented in the same direction as the 2 holes in the carb. These holes must fall exactly within the 2 inch circle you scribed earlier.
5. Cut out the web between the 2 holes, leaving one oblong hole.
6. Clearly mark 2 hole positions in the other plate, oriented the same way, matching those in the bottom of the carb, but do not bore them.
7. Make 2 (carb sized) holes in that plate, but not at the marked positions. These 2 holes would be centered closer together than those in the bottom of the carb, by a specific amount: The outside measurement of the two holes together minus the 2 inches.
8. Cut out the web between the 2 holes, leaving one oblong hole.
9. Bolt the two plates together. This will leave overhanging material on both plates, some from the ends and some from the sides.
10. With a tooling burr (or a good round rasp) work from the manifold side (with the shorter oblong) to extend the opening out to the 2 inch circle you scribed and on a slope to the width of the oblong hole on the opposite face.
11. From the carb side, work to extend the hole out to the circles you scribed and sloped to the diameter of the 2 inch circle on the opposite face.

As a separate step (or steps), you can do whatever I left out.

This method should leave a completely smooth transition. A possible easier way to do this would be:

1. Bore the two plates to exactly match the carb and the manifold. This would leave larger holes.
2. Remove the web in the one and grind out any overhangs - a lot less than the previous way.
3. Fill in any gaps with plastic aluminum - a move that might end up dumping pieces of the plastic aluminum into your cylinders.