Question about Edelbrock carbs

[Fury440]

Interesting, Ma Mopar used 525 cfm on a standard 440 350hp, maybe a bit bigger on the 375hp version and then jumped to around a 1,000 cfm on a 6-pac. The 6-pac wasn't as far as I know ported and polished to increase the engine flow rate. So it appears 6-pac engines were tuned for drags and were probably dogs in stop and go traffic.

 

[WorkerBee]

Ive read that the 340/383 AVS was 480 CFM and the 440 AVS was 580CFM. I'm pretty sure that was done by Chrysler.
Isn't the 6bbl carbs 350 CFM X 3 .....1050CFM???

No the center one is 350 cfm and the two outers are 500 cfm = 1,350

caveat diff carb manf flow and rate their carbs differently and I've read that the 2 BBL holley's are flowed and rated differently than the 4 BBL holley's and just from a weak memory that the true comparable flow of a 440 six BBL set up was really around something like 980 cfm .. but memory is not always accurate

I remember that 980 CFM number. It seems more realistic to me.
Just as an example (not scientific), take a 440 AVS rated at 580 CFM with (4) X 1-11/16" butterflies compared to only (2) X 1- 3/4" butterflies for the 500 CFM Holley 2BBL.
500 CFM seams a bit optimistic.

 

[WorkerBee]

Interesting, Ma Mopar used 525 cfm on a standard 440 350hp, maybe a bit bigger on the 375hp version and then jumped to around a 1,000 cfm on a 6-pac. The 6-pac wasn't as far as I know ported and polished to increase the engine flow rate. So it appears 6-pac engines were tuned for drags and were probably dogs in stop and go traffic.

Those outboard carbs are basically vacuum secondaries, so the CFM is an on demand thing.......whatever the motor needs in CFM.

 

[tallhair]

Interesting, Ma Mopar used 525 cfm on a standard 440 350hp, maybe a bit bigger on the 375hp version and then jumped to around a 1,000 cfm on a 6-pac. The 6-pac wasn't as far as I know ported and polished to increase the engine flow rate. So it appears 6-pac engines were tuned for drags and were probably dogs in stop and go traffic.

As far as performance differences the 70 6 BBL had a faster taper cam, smaller diameter lifters (slightly domed IIR) and a .5 or so higher compression ratio and I think a dual point ignition. The center 350 carb was the one in operation in stop and go duty so the outer two only opened when called upon like workerbee said. I'd ask Big John and BlueFury61/Will about them being a dog (unless they carbs were messed with)

 

[70bigblockdodge]

Ive read that the 340/383 AVS was 480 CFM and the 440 AVS was 580CFM. I'm pretty sure that was done by Chrysler.
Isn't the 6bbl carbs 350 CFM X 3 .....1050CFM???

There you go I knew they were in the 500 area. On a 6 pack I believe the outboards are rated at 500 cfm and the center at 320 for 1320, but 2 bbls are rated at a different vacuum level I can't remember why.

 

[67 chrysler300]

Thanks all for the replies!! I DO have to admit that my car ran without any drivability issues and with decent fuel economy with the 600 cfm carb. BUT, it ran like a "raped ape" with the 800 Thunder carb!! Fuel economy was terrible though and there was always a bog when leaving a traffic light. It also REACKED of fuel when shut off and had serious drivability issues when driving slowly on a hot day. What to do??!!!

 

[commando1]

I couldn't sleep nights knowing my car has a bog in it.

 

[70bigblockdodge]

Any of the bigger carbs will need some tweaking, especially on a stock or near stock engine. With exhaust manifolds and stock air cleaner/even a dual snorkel you are probably not giving up anything with the 600. The 800 should work on any size engine just needs tweaking and air door adjustment.

 

[bluefury361]

Surging at cruiseing speed, backfire....... I would look for a restricted fuel filter first.

 

[70bigblockdodge]

Thanks all for the replies!! I DO have to admit that my car ran without any drivability issues and with decent fuel economy with the 600 cfm carb. BUT, it ran like a "raped ape" with the 800 Thunder carb!! Fuel economy was terrible though and there was always a bog when leaving a traffic light. It also REACKED of fuel when shut off and had serious drivability issues when driving slowly on a hot day. What to do??!!!

Sounds like floats are not adjusted right to start with, some fiddling with the accelerator pump adjustment might help.

 

[67 chrysler300]

Thanks; to give you the complete picture, my mechanic rebuilt my Eddy 750 cfm carb because he said the idle screws were unresponsive. After his overhaul, the backfires started....I told him about that and he took the car back and returned everything pretty much to stock....NO more backfires and fuel economy has returned to about 10 mpg instead of SEVEN!!
BUT, there are still issues; it does surge at cruising speed and the last time he changed the plugs, they were BLACK as all hell. NEVER seen them that way before. I've been driving the car for awhile now since he returned most things to original....Have not checked the plugs yet; I'll pull a couple of them tomorrow and see how they look.

All I can really say is that I've NEVER been happy with this carb. I've had it about 4 years now. I've been tempted to just go ahead and buy another Carb....But this is the 5th carburetor I've had on this car!!! I hate the idea of throwing another 300 to 400 dollars at it not knowing if I'll be any better off!

 

[67 chrysler300]

I pulled some plugs today. After 2-300 miles of mixed driving they were clean as a whistle... NOT black like before.

 

[QMopar]

This is why the factory put 525 cfm on a 440.

Resurrecting an old thread for a history lesson - isn't the Thermoquad on my 75 Imperial rated at 850CFM? I'm fighting with a Street Demon 750 and it's displaying many of the same things (bogging, hesitation, poor economy) as the original poster was/is. Thought I was doing the right thing in getting a 750 with vacuum secondaries; but some say a 600 works better than factory 850? I'm confused (which is VERY easy) - help me understand. Thanks...

 

[saylor]

just dropping some possible knowledge to this thread -

i heard 2bbl and 4bbl CFM ratings are different - they use a different Hg scale measurement.

i was also told that multi-carb setups do not give you the sum total of the carbs - i.e. 2x600cfm = 1200cfm.

i dont know how the hell you check/verify that, or what the 'total' ends up being...

we need a physics guy.

wait didnt someone here on the forum work in the fuel dept at chrysler?

help us 0 wise one!

- saylor

 

[70bigblockdodge]

Thermoquads have small primaries and huge secondaries which is why they are a great compromise carb. I'm not sure the demon works the same. All the square flange carbs have very similar sized throttle valves and the bigger they are makes tip in problems (bog) that have to be masked by accelerator pump( air speed drops off too quickly because of the large throttle valve-no air speed through venturi no pressure differential created to pull fuel through main circuit) this has to be masked or covered for by accelerator pump this is why a 600 will drive better than a 800. It has smaller throttle vavles, air speed gets up to speed faster in smaller bore/carb. A 600 will give up high rpm, in most cases cruiser, stock engine no headers this does not show up as a hindrance like never holding it in low gear to rev it past 5000 rpm. Hope that helps.

 

[QMopar]

Thermoquads have small primaries and huge secondaries which is why they are a great compromise carb. I'm not sure the demon works the same. All the square flange carbs have very similar sized throttle valves and the bigger they are makes tip in problems (bog) that have to be masked by accelerator pump( air speed drops off too quickly because of the large throttle valve-no air speed through venturi no pressure differential created to pull fuel through main circuit) this has to be masked or covered for by accelerator pump this is why a 600 will drive better than a 800. It has smaller throttle vavles, air speed gets up to speed faster in smaller bore/carb. A 600 will give up high rpm, in most cases cruiser, stock engine no headers this does not show up as a hindrance like never holding it in low gear to rev it past 5000 rpm. Hope that helps.

The reason I went with a #1904 Street Demon was because it emulates the TQ in that it is a spread bore - small primaries and a big secondary (they call it a Goggle Valve - it's 1 big secondary) as well as a composite bowl for cooler fuel.

It has gotten pretty frustrating talking to Holley/Demon tech line asking for help and following their advice only to make it worse (not blaming them - I bought the carb). Messing with metering rods, springs and jets trying to get the thing lean enough to cruise and rich enough to accelerate is good experience but I just don't have the time to keep playing with it (using an O2 it won't cruise any leaner than 12-ish and leans out to 14-15 under acceleration). Lost 2 miles per gallon (down to 8MPG) and it just doesn't run like the Thermoquad did (TQ would initially bog and then take off - but it got 10MPG).

Based on the descriptions here and the way the Street Demon acts makes me suspect its too much carb for this fairly stock 440.

I rebuilt a Carter 9000 Competition Series 625CFM and the exhaust sounded more like the TQ and ran better from mid pedal on up than the Street Demon but has other issues. It returned a high of 11MPG though.

Just wondering how a TQ that's 850CFM is fine (fine in that it was a factory piece - not fine from a 40-year old carb perspective) and a new 750CFM is too much.

 

[Snotty]

It generally runs OK but does have some issues. When the engine is cold, it kind of sputters a little at light throttle. Sometimes it backfires! It also kind of surges at steady speeds leading both my mechanic and I too think the cruise setting may be too lean. I tried the metering rod that was one stage rich both cruise and power. Although it didn't surge, mileage was terrible (bout 8 -9 mpg) and performance was no better than with the stock metering rod.

I am not reading all of the responses, so someone might have already said this.

I had the same problem with mine. Backfiring and running terrible when cold. When I replaced the intake valley pan gasket I discovered that my choke arm was not connected to the choke mechanism. The "Jesus clip" that held it was gone. Therefore, the butterflies for the choke simply were not moving. Once I realized that and reattached a clip, these problems vanished and have not been back.

Is your choke properly connected?

 

[Snotty]

Oh, I use the same carb as yours on my Newport as well as the Gremlin. The one on the Grem has never had a problem - runs like a champ!

 

[70bigblockdodge]

just doesn't run like the Thermoquad did (TQ would initially bog and then take off - but it got 10MPG)

If it was only bogging it most likely has a air valve adjustment problem. The big top plate over the rear/secondaries is spring loaded to come on when demand for more air overcomes the spring tension. Yours sounds like it is opening too early before engine speed has increased enough to want more air. Adjustment is on drivers side of carb, calls for a special tool but a large and small screwdriver can be used, large to unlock small to adjust and if I remember right counter clockwise tightens spring but you will be able to tell when you mess with it. The best part is adjust go take for a ride, bog still there tighten adjustment and another ride, repeat till bog is gone.

If street demon is similar it sounds like your spring or springs for metering rods are too strong and your air valve tension is too light, but I have never messed with one of these so don't take that as gospel just putting it out there from Carter/Edelbrock experience.

 

[QMopar]

If it was only bogging it most likely has a air valve adjustment problem. The big top plate over the rear/secondaries is spring loaded to come on when demand for more air overcomes the spring tension. Yours sounds like it is opening too early before engine speed has increased enough to want more air. Adjustment is on drivers side of carb, calls for a special tool but a large and small screwdriver can be used, large to unlock small to adjust and if I remember right counter clockwise tightens spring but you will be able to tell when you mess with it. The best part is adjust go take for a ride, bog still there tighten adjustment and another ride, repeat till bog is gone.

If street demon is similar it sounds like your spring or springs for metering rods are too strong and your air valve tension is too light, but I have never messed with one of these so don't take that as gospel just putting it out there from Carter/Edelbrock experience.

I'll certainly give it a try. Thank you...