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Old 08-27-2007, 02:26 PM
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Dual Carb

I'm just looking at dual carb manifolds. I was wondering if you had dual 750cfm carbs would that almost double your flow or at least greatly increase it? If not, whats the point of them other than looking cool?
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Old 08-27-2007, 03:03 PM
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If I remember correctly, there are only two reasons for going dual carb:

1) Looks
2) A need for massive amounts of air where no single carb is large enough.

If your engine requires 800 CFM of air, you'll get better performance out of a single 800 CFM carb than you would if you used two 400 CFM carbs in a dual setup. Not to mention that one carb is easier to use in terms of tuning and linkage.

This is not from experience, only from what I can remember reading and researching. Somebody please correct me if I'm wrong.
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Old 08-27-2007, 03:04 PM
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Thats what I thought and they sure look pretty.
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Old 08-27-2007, 04:10 PM
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Dual carb setups do look cool but the secret to getting them to run right on the street is to choose small CFM carbs so you don't over carburate your engine. If your small block only requires 800 CFM then a couple SMALL Holleys or AFB's will do the trick.
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Old 08-27-2007, 05:20 PM
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I see, I don't actually plan on using one but I was wondering about why somebody might.
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Old 08-27-2007, 05:27 PM
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Dual Carbs

Take a look under the hood of a 64 Impala / 409 dual four car some cruise-nite at the drive-in. You'll get it right away! Thump factor.........
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Old 08-27-2007, 08:15 PM
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Dual quads originally came into production because engine outputs increased faster than carb flow. Back in the day, the biggest 4-barrel you could get was about 400 cfm, so when they started building "muscle" motors the only solution was to put two carbs on it.

Today there are only really two reasons for double carbs. Either you want to look cool, or you have an engine that is so powerful that it can't be supplied with a 1200 cfm single carb. A single carb will almost always make more power, in fact I've never seen a case where doubles made more than a good single.

More flow isn't better. Proper flow is what you need. If your engine needs 750 cfm and you have a 750 carb, it flows 750. If you put two 750s on it, it doesn't suddenly flow 1500, it still pulls 750. What you have done is cut the air velocity in half which is not a good thing.
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Old 08-27-2007, 08:53 PM
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4-barrel carburetors are flow rated at 1.5" of Hg (That's 20.4" of H2O) pressure drop across the carburetor with the throttles held wide open. If you have an engine that needs 750CFM say at 6500RPM, and you have a 750CFM carburetor on the engine your manifold vacuum at 6500 would be 1.5". Assuming atmospheric pressure is 30.00" of Hg, your manifold pressure would be 28.5" (30.00" - 1.5"= 28.5"). To make the most power we want to run as close to 0.0 manifold vacuum (30.0" of manifold pressure) as possible. Superchargers actually put positive pressure in the intake to make their power.

Now if you installed 2, 750's on the same engine, the engine will still need 750CFM at 6500. That is all the airflow you will get through the carburetors (carbs are not superchargers and don’t automatically fill the engine with more air!). Each carburetor will be passing 375CFM of air. However the manifold vacuum will only be .375” of Hg under the wide open throttle plates. (There is a fairly straightforward formula for this, but it is not easily written in a manner that can be posted here.)

If the manifold vacuum is .375” of Hg, then the manifold pressure is 30.00”-.375”= 29.625” of Hg. The difference between the manifold pressure at WOT between the dual-quad application (29.625”) and the single quad application (28.5”) is 1.125” of Hg. If you divide this difference (1.125”) by the manifold pressure of the single quad (28.5”) you get an answer of 0.039 that in percentage is 3.9%.

What this all means is the manifold charge density is 3.9% greater with a dual quad system as with a single quad system in this example. If the single 750 equipped engine made 450 HP, the dual quad engine would make 3.9% (or 17.5HP) more..467.5HP. This proves out very closely on engine dynamometer tests. Now the difference is not always 3.9%, it varies with the relative size comparisons. The bigger the engine and the higher the RPM, the more the difference becomes.

One very important point is that there is a minimum manifold vacuum at WOT at which the booster venturii will start flowing the proper ratio of fuel. For this reason, it is always better to use 2, vacuum secondary carburetors (I prefer Holleys because it is easy to run an equalizing vacuum line between their secondary throttle diaphragms) than 2 mechanical secondary carburetors on the street. With the vacuum secondaries, only the primary throttles open up when you go to WOT. The secondary throttles open only as airflow through the primaries reaches a point where the WOT vacuum will create enough airflow to cause a good booster signal on the secondary throttle side.

Two 600’s work great on an application like this (especially with the vacuum secondary equalizing kit that Holley sells). Many people buy the 2, 465CFM carb set-up from Summit. This is a terrible set-up as you are getting mechanical secondaries without a secondary accelerator pump. For a customer who insisted on it, I have made 2, 600CFM Holleys work on a 283 Chevy in a 63 Impala with a tunnel ram and a Powerglide. The engine had headers and the cam was a 204/214 @.050” with 420”/443” on a 112LSA. It drove like a dream and got plenty of looks. It didn’t smoke the tires on a WOT stab from a dead stop but it didn’t bog either. It ran just like it had a single 4-barrel on it. I admit I have an advantage; I was a development engineer at Holley back in the ‘70’s so I know a few little tricks, but it is not difficult. Jetting was box stock. I agree that a single four-barrel is easier to work with, and with all the different sizes there are a lot of choices, but if you need that last bit of power, 2,4’s can get it for you and they look impressive. Just keep in mind that the power increase going to dual quads is pretty minimal unless you are over 500CID. Hope this helps!
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Old 08-27-2007, 09:01 PM
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2 4's Have To Be Set Up Right

for 2 4's to work right on the street you need progressive linckage and proper accelerator pump set up.. you cruise on the front 2 of the rear carb, then the front 2 of the front carb kicks in then the rear 2 on the rear carb, then the rear 2 on the front carb.and everything is open at wot... the smaller venturi's on a small carb giver better atomization than one big 4 barrel....I set up 2- 4's on a 57 bird motor, it got 22 mpg on the highway and lots of power when he wanted it ... The Ford dealer brought his customers 63 427's Ford to me to tune. I stil like multi carbs. 2- 4's on on of my T buckets and 3-2's on another.....retired ford engineer...
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