How many CFM for my 340?

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67DART340

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My 67 Dart 340 4 speed is going to get carb replacement. The car has a 6 pack style air scoop on the hood that's got a 16" round cut out for my 14" K&N air filter. Right now the air cleaner is pushed up into air comming in by 2 spacers of about 3" to 4" above the carb. I have a 650 CFM Edelbrock carb on an Edelbrock Performer RPM intake. My 340 has a cam, but I don't know what cam, and it has headers. Can I go with a Edelbrock 750 CFM carb with my 4 speed stick Dart? I'm also going to put a 4 hole 1" thermal spacer under the carb with the change. I read 750 is doable and in 71 the thermoquads were pushing as much as 800 CFM on a factory motor. 750 sounds a bit much for a 340. What's the opinion out there?

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The T-Q's came in two sizes, commonly rated @ 800 & 850.
A 750 would be very doable, however, I'd rather size the carb for the task at hand. Since you do not now the cam specs, what was the car built for?

Tell me your gear ratio and stall converter stall speed (if known)

FYI, (Not being a jerk about it, but...)

The Edelbrock carbs AFB's are 600 and 750
The AVS series are 650 and 800.

The smaller carb would be very happy on the street with gears numericaly at 3.55 and lower.
A 750 could go up top, but will work best with 373's and numericaly higher.
 
Thanks for the reply......

The 650 CFM was a mental typo. Thanks for correcting me. The 1405 Edelbrock is a 600 CFM. I'm planning to put a 1407 (750 CFM) on the Edelbrock Performer RPM, which I believe is a dual plane intake, but a thermoquad (spread bore) won't fit that manifold without some kind of adapter.......

The car is a street muscle car. No racing. It has an over drive 4 speed. The 8-3/4 rear is a 3.58:1, but will be swapped with a 3.91:1, clutch type Sure-Grip, shortly...

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Nice car and with an overdrive trans, cool.

Your car is light weight and 340's love to rev if built right...any carb would work on it..even an 850 holley but I am finding out that the Edelbrock-Carter carbs like stop and go traffic better and think my Holley days are over with (but I'm running two Carter 500's)

The 650 AVS would be a safe pick and get a bit better mpg than the 750 AVS both I feel work work just fine but were you live New Jersey I'm guessing has loads of traffic and police around...you want a nice carb to run around town, not to do WOT high speed runs down the expressways.
 
I have just added a rebuilt 71 Carter 650 AVS to my 340, and I find that it runs great on the street with the 3.55s and 323s before that. The hardest part was setting up the choke, a repo that needed quite a bit of tweaking. If you are going 3.91 you will probably want more cfm.
 
The 750 will do nicely on top of that dual plane RPM for sure. You'll have superior top end with that carb and the 3.91's.
 
Hi, very nice car, it sits with a very beefy look. Color is a plus. I have ran allot of carb's on small blocks. The big spread bore AVS Carter is the nicest for a 340. But as you stated you have a square bore intake so that's okay, the Competition AFB is a great square bore carb. and you can purchase it with an electric choke. Mine was a polished one with 625 CFM and it had a very nice snappy throttle response. It also featured the option of stepping the idle circuit up or down depending on cam to stall ratio or in your case 4 speed ratio. And if you want more CFM a polished Edelbrock 750 CFM is available in the same design as the 625 Carter with the same idle circuit set up and electric choke. They are very nice carb's, very good for the street. Like one of the other poster's said the higher the CFM the higher RPM range. But I wouldn't go with less than 625 CFM on a 340.
 
Yea, the OE AVS carbs were rated from Carter at 630 cfm. Those stock carbs were fine. A noticeable improvement allways came from the larger version (750) from the 440 engines. This is an old trick from back in the day.
 
67DartGT340 said:
I have just added a rebuilt 71 Carter 650 AVS to my 340, and I find that it runs great on the street with the 3.55s and 323s before that. The hardest part was setting up the choke, a repo that needed quite a bit of tweaking. If you are going 3.91 you will probably want more cfm.
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So Im guessing Mark the gears are In Place? Everything works well? But I would say 750 is just fine, I run a 750 Holley On my 340 works little better than the 650 it replaced. Little more mid and top and really lost nothing that I can tell bottom end. The track will tell if all is well or better... my 2 cents D
 
Yes the 750 would let the motor breathe better on top but why not try this / wait until you swap the 3.91 rear end and put an open 1/2 inch spacer and see how it is...have you messed around jetting the carb at all...they are set up close but I would try 1 or 2 steps richer and see if its any better..is it an AVS carb now, if so lighten up the secondary spring...stock is 2 1/2 turns, try 2 turns and if no bog in high gear down low when floored then try 1 3/4 turns.

The 750 AVS will bring the best ets over a 650 carb but you may find those 3.91 gears wake the motor up nice...and those loose gears will allow the secondaries to open quicker..even now with your 3.58 gears the secondaries are likley not opening as fast as they could.
 
got any side view pics of your car?,,,,very nice,,,,i just bought a '67 post,but it is still in stock stance.
 
I've had an Edelbrock 750, Holley Street Avenger 670, and now have a 750 Holley double pumper on my 340 in my Duster. The 750 DP is by far the best driving and quicker carb out of those 3. I have no trouble driving it on the streets even having an auto trans. I do have 4.10 gears though.
 
Thanks for all the interesting replies and for the nice comments on the car. The car still needs more work, I just need the time. Anyway, the Edelbrock 1407, 750 CFM carb goes on tomorrow. And it goes to a car show Saturday. Right now we're having great muscle car weather

And for 67yelodart, here's a side view. This was before the Cragar SS mags were swapped to my 63 Newport and the Weld Pro Star wheels and street slicks were installed, and before the front end was rebuilt with new upper and lower control arms, bushings, torsion bars, and the front end raised (for more clearance of the oilpan and headers). Also, before the rear air shocks (at 60 PSI) and half inch spacer were added to give the tires clearance, and before the dual snorkel non-functional hood scoop was swapped for a 6 pack style (functional ram air) scoop on a flat black hood stripe. Good luck with your ride and enjoy.

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Careful with those air shocks and that much lift. I had a dart as a kid and
had it lifted like that and over time it tore out the shock mounts in the trunk.
Keep and eye on that. Looks awesome though. FWIW
 
True, just enuff air to make it level and a touch more at best.
 
I'm running only 60 PSI right now (25 PSI is minimum) and most of the lift you see is from the 6 leaf springs and slightly bigger rear shakels. Although, when I removed the old shocks, I noticed the right side upper shock bolt was loose to the body. After assembly it was still loose. Later I ended up putting a thick washer (spacer) in the trunk side to tighten it up. Back in the day - I had a 66 Dodge Coronet 500 (440, auto, 4:10.1 Dana), I put too much air (over 150 PSI) in the air shocks and hit a bump and boom, down it went. Tire clearance was a problem then and air shocks were cheaper than new springs at 21.
 
I have a 360 that is producing approximately 370 HP. I have had a 600, 670, 725 and 750 cfm carb on it. The 600 gave up some noticabel seat of the pants power above 4500 rpm to the 725 and 750 but had it all over those two in driveability below 4500 rpm. For a street driven vehicle the 600 is the best choice of those three.

The 670 Street Avenger is the current carb I am using and is the one that has caused me to stop playing with carbs. It has every bit as much power above 4500 rpm as the 725 or 750 plus has all the driveability of the 600 rpm below 4500 rpm.

IMHO, unless you plan to race you are better off going with a smaller carb. Even the 400 HP 5.7 Hemi crate motors come with 600 cfm carbs.
 
You should be just fine running a holley 750 v.c. on a 340 street car,i ran one on a few of my 340 street cars..lose the air shocks,i've seen them rip the top mounts right out of a few mopars,and the traction bars are for chevys,mopars don't need them...
 
the formula for determining carb size is displacement in inches times maximum r.p.m. divided by 3456 times 0.85
 
O.K., I know about the math calculations for CFM, that's why when I read 750 would work on a 340 had me questioning if it was too much CFM, but the concenus has been it works. That all depends on it's use.

And after rebuilding an Edelbrock 1407 and replacing the 1405 (which I just started having problems with) and running it today, I'd have to agree, the 750 CFM works. Not the same as before, certainly differently. I can run in 3rd or 4th at a slow speeds and not have to down shift too get the car moving quickly again. But, I also noticed if I'm in 1st or reverse and I goose it several times, while riding the clutch while turning it loads it up to stall, whereas it did'nt before. But, then again that could be accelerator pump settings, jetting, or metering rod variables. The car does accelerate more quickly. It sounds like the 800 CFM thermoquad (71 factory) would be the best for low speed driveability (spread bore) and upper RPM go power. Back in the day a small block Mopar A body (340) was a force to be feared.

If I get rid of the air shocks, I have to cut the wheel wells, or get rid of the street slicks and Weld Pro Stars and I'm not ready to do that. Besides I like the way the car looks and it's intimidating. If I break it my son can weld it better than it ever was.

The traction bars were installed in an attempt fix a rapid acceleration condition feeling like wheel hop or spring wind up at higher speed. It didn't fix it. This is my first HP small block with a 4 speed stick. But, interestingly for the lack of fixing the problem, they match the car body color and look mean. I'm also going to install subframe conectors next to see if that helps the problem, or look elsewhere.

Thanks again for all the responses..........
 
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