View Full Version : Dual-Quad, Big Carbs?

02-10-2007, 12:46 PM
for an average 440 with a single carb most people recomends a 750 cfm carb.
for more of a strip use a 850 is generally suggested.

why for a dual-quad (or tunnel ram) a pair of 600-650 cfm carbs usually are indicated?

for the CH-28 Dual-Quad intake manifold (for a 440 engine) Edelbrock recommends a pair of 600 cfm carbs for street use.
in this combo with a progressive linkage while cruising only the primaries of the rear carb are in use but at full throtle comes a total of 1200 cfm.

in the 440 Six-Pack cars a total of 900-950 cfm (in 4bbl numbers) are available at wot but while in cruise only the center carb works with a cfm rating of 230-250 (4bbl equivalence).

so, in a dual-quad application why not just use a pair of 390-450 cfm carbs?
with a non-progressive linkage the primaries of the two carbs can flow more than the center carb of a Six-Pack i think, and at wot a total of 780-900 cfm is at hand matching the ratings of the suggested single carb combos...

Jack_440 Savoy
02-12-2007, 01:26 AM
I believe the 6-pack set up is 1200 cfm. The centre carb is 500 cfm and the outboard carbs are 350 cfm. ea. I think Edelbrock is just matching the factory cfm. I think you are right in thinking smaller carbs would work. I am building a car with the CH-28 manifold and two 500 cfm Carter Comp. carbs, No chokes and progressive linkage. I plan on driving the car on the street but only in the summer. No choke works on the street ok, but you have to play with it until it gets warmed up. I had a hopped up 69 VW with a Webber 2 Barrel on it with No chock and I drove it year round. I think you would have better throttle response on the street with smaller carbs too. Good Luck with your carb choice.

02-12-2007, 10:47 AM
well you have to understand that those 600's or larger are vacuum operated secondaries and if the engine can't pull the full 1200cfm it doesn't, the engine is going to only consume only what it requires which is based on how it's built (compression, cubic inches, head design, valve size, cam design & lift, etc.) What you don't want to do is under carb a combination, (been there done that and have the tee shirt) They studder, flat spots, warmup bad, low power,fall on their face, can damage valves from running to lean and on it goes.Those 600's that they recommend have all the proper jets setup and the all the homework has already been done for most of the engine setups, I have one and love it.That's kinda the short answer Sundrop

02-13-2007, 05:06 PM
thanks for the replies!

02-13-2007, 06:14 PM
The six pack is 1350 CFM when measured as 2bbls. It is about 900 cfm when measured as a 4bbl.

Jack_440 Savoy
02-14-2007, 12:06 AM
E could you explain further, I always thought the 6 pak was 1200 cfm. 500 cfm center carb and 350 cfm each for the outer carbs. and how do you measure as a 4 barrel ??? and why ???

02-14-2007, 02:20 AM
You have it backwards. The 6 pak is a 350 center and 500 outers, for a 2bbl rating of 1350CFM. The reason that it flows less when measured as a 4bbl is because, for some reason I don't understand, the accepted pressure drop for measuring 2bbl carbs is 3 inches, and for 4bbl carbs it is 1.5 inches. Here's what I do know. The change in flow is proportional to the square root of the change in the pressure drop. In other words, if the pressure drop doubles from 1.5 to 3, the flow increases by the square root of 2 or 1.414 times more flow rating for the exact same carb, using the 2bbl rating instead of the 4bbl rating. So, going from 2bbl to 4bbl ratings we divide by 1.414, and get a flow of ~955CFM from the 6 pak. Clear?

Okay, so why 600s on a tunnel ram? Same reason. If you're running a tunnel ram, presumably you are after all the performance you can get. You don't want 1.5 inches of vacuum at full throttle, that's too much. You won't get the maximum charge into each cylinder with that much vacuum, you want just enought to be able to correctly meter the fuel. Any more than that and you are reducing your volumetric efficiency, and your power. So, let's keep the math simple and say that you want no more than .75 inches of vacuum at max RPM. That has again cut vacuum in half, so we divide by 1.414 and get not 1200CFM, but only 849CFM. Hey, just about the 850 that is recommended for the strip, how about that?

Jack_440 Savoy
02-15-2007, 01:54 PM
Thanks for the reply. My teachers always said I was a little backwards !!! I'll have to let that explanation sink in for awhile. Jack

02-16-2007, 02:57 PM
Hi 68Roadrunner440, Good discussions above on carb flow ratings and pressure drop !

I run two (original) 750 cfm Carter AVS carbs on my CH-28 and it doesn't seem like too much carb. The manifold has excellent air flow and mixture distribution. Run it cold with the exhaust cross over blocked. You'll have to let the engine heat warm up the manifold a bit for smooth running.

Currently, I'm also running the Mopar 0.509" cam, but the same carb/manifold combo worked well with both the stock 440 magnum cam (0.458" lift) and a Lunati Bracket master II cam (0.480" lift). The CH-28 is in my 4-spd Roadrunner (3.91 gears, 275-60-15's on the rear).

To make it work I had to drill the carbs for "narrow AFB" style mounting. Edelbrock doesn't warn you about this. The reason a pair of 750cfm carbs is overall better than say 500 or 600 cfm ones - even with a progressive linkage - is that the second carb (the front one) will crack open with about 15 degrees or so of throttle opening on the primary carb (the rear one). This doesn't give much air flow on the primary carb for normal highway cruising, or rapidly moving from a stop light. You DON'T want to crack the front carb open for normal driving, unless you're racing ! All hell breaks loose !

Remember, the 440-six pack used vacuum operated secondary carbs - a mechanical linkage will have a totally different operating feel. Now I have to admit, I don't have a pair of decent 625 cfm AVS's or 600 cfm AFB's to try, but I can tell you that 750's will work very well.

If you have the bucks, a pair of Edelbrock's new 650 cfm AVS's might be the hot ticket-air valves help "size" the carbs for the manifold,

Mark H.

02-18-2007, 01:21 AM
Vizard has a formula that says that a carb needs X amount of pressure drop across the venturi to properly meter its F/A ratio. You need a certain amount of velocity across the venturi to pull the fuel into the airstream. When you put a 6 bbl carb setup on a 340, you are only going to get about 770 CFM out of that setup because that is all the motor can pull through 6 bbl's. There is just not enough vacuum across all that venturi area to pull serious CFM's. Now put a 4bbl on it and you have enough to pull more CFM's out of it. Sounds ass-backwards, but it is a fact: the more the motor is starved for air, the harder it will pull on the carb(s). Look at the NASCAR 390's; 9000 RPM's pulling through a 390 CFM carb equals a whole lot more than 390 CFM's because the insane amount of vacuum, or for a better term; velocity across the venturi. Put a 600 CFM 1850 on a 700 ci motor at 6000 RPM and itll pull about 1200 CFM, pulling 6inches across all barrels. 2 600's on a tunnel ram will allow only about 700 CFM because the motor just cant create the pressure drop across 8 barrels to allow 1200 CFM. So most of the time a large 4bbl is better than 6 or 8. conversly, a 2 is much better down low than a 4, hence the progressive 6 bbl setup (vacuum is always better for progressive linkage type carbs)
Reference: Vizard, How to build horsepower, Vol. 2, pg. 62

02-22-2007, 08:10 PM
thanks guys!

lots of good info!!

02-23-2007, 07:18 AM
Would you consider a six pack ?

I have been reading up on Barry Grants new six pack setup

from the BG site:

This new induction system not only produces better torque and horsepower than the original Six-Pack arrangement, but also out-performs today's best single four-barrel street combinations. Attractive and simple to install and adjust, the SixShooter has only three adjustment screws-no synchronization needed. Economical to run, the SixShooter comprises three two-barrel 250 cfm Demon carburetors (electric choke on center unit) with mechanically operated progressive linkage; an optional custom air cleaner; and all the necessary fuel inlets and throttle linkages. Installs on existing factory or aftermarket Six-Pack intake manifolds and works with factory-style air cleaner.


SixShooter™: New Six-Pack induction system for Mopar

Dahlonega, GA - Barry Grant's company, Triple-D Induction, has produced a SixShooter carburetor package to suit Mopar A, B, and RB engines from 318 to 440 cid

02-23-2007, 08:07 AM
E could you explain further, I always thought the 6 pak was 1200 cfm. 500 cfm center carb and 350 cfm each for the outer carbs. and how do you measure as a 4 barrel ??? and why ???

If memory serves me right 2 barrels are measured at a different vacuum pull than 4 barrels thus the different cfm ratings, it is a good idea to look at the throttle bores and venturi(sp) when making a comparison.