Carb for 340

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70Dart340V8

70Dart340V8

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Let me start off by saying I have looked through a few different articles on this topic and have read though them quite a bit. This is to more so get a second opinion on my specific set up and what you guys would recommend. I'm not just being lazy and not searching the forms. I love coming here because I always get really quick solid answers. Thanks guys :D

Now on to my question. I have a 1970 340 that seems fairly built. Everything was done by a buddy of the previous owner so I don't really have any specifics on the cam that was put in it. It does seem like a very chunky can. It has a very rough idle to it. (the ignition timing is set correctly as well) it has long tube headers with a full 3" exhaust. It has a Weiand intake manifold and 360 J heads. I looked around at a lot of the carbs people were recommending and I really like the Edelbrock Performer 750. However 750 CFM seems a bit much for a 340 SB. I don't know if the level of mods on this engine would justify a 750 CFM. All the CFM calculators I've seen have suggested 600-650 CFM, however there's no way to take any of the modifications into play in those either. Any help would be greatly appreciated. Thank you
 
Calculators are low for best performance. While the 600-650 would be correct in it's abilty to make the power, I'd skip over it and move up the notch. You will like the bigger carb for sure.
 
Mostly street driven. I've been spending the last year fixing everything on the car. I have it running pretty well though but it feels like it should have more power than it has. The carb on there is either a 600-650 CFM old school Holley. I'm going to put the new carb on there and have it dyno tuned to get it running better than it is now.
 
70Dart340V8 said:
I have it running pretty well though but it feels like it should have more power than it has.

a 600 - 650 carb will give better throttle response.The bigger carb might not help but may
make a difference at the big end
..what gears and converter?A highway gear, lo stall converter and long duration cam ( read lumpy)
can make a good car lazy.
Click to expand...
 
Edelbrocks can be a little bigger than needed as the air valve secondary only opens as far as the motor can use. Holleys are a little different but can be manipulated the same way with vacuum secondary springs in the pot. DP carbs with no vacuum pots are generally better for 4sp apps. Check out the Thunder series AVS style edelbrocks or even the Demon TQ clones. They are a bit more adjustable than the AFB. Even a prepped TQ would work great, Demonsizzler can hook you up if he's still around.
 
70Dart340V8 said:
Mostly street driven. I've been spending the last year fixing everything on the car. I have it running pretty well though but it feels like it should have more power than it has. The carb on there is either a 600-650 CFM old school Holley. I'm going to put the new carb on there and have it dyno tuned to get it running better than it is now.
Click to expand...
You should find out what you have now. Model # is on choke tower. A few common examples: 1850 600 vac, 3310 750 vac, 4776 through 4781 are dp numbers 600 to 850. If you have a 4777 650 it can be made into a 750 with one of these: http://www.summitracing.com/parts/pro-67100c
 
70Dart340V8 said:
I'm going to put the new carb on there and have it dyno tuned to get it running better than it is now.
Click to expand...

If your going to have it dyno tuned then run what you have and let the tuner tell you if you need more or less carb or if it has issues. Why by something you may not need or be incorrect?
 
750 is not too big for a 340. Especially one with the right cam. It will need the CFM at the upper RPM range. You can run a 340 on a 650 area carb. But it will lose some top end. A race 340 can take 850+. One guy on here, I forget who, has run a dominator 1050 on his 340 at the 1/4 and the engine loved it.

But you mention dyno tuning. Why not run what you got, see what the dyno guy says, see if he's got some in the shop you can play around with and try, see what results you get. There is no "magic" number for CFM.
 
750 would be my vote if you are buying a new carb, I run a 750 Holley DP with pro form main body on my 340 with a .480 lift 238@50 solid cam.
 
The top end is really where it feels like it lacks power. It takes off fairly well it just seems to have some issues getting up to speed. I understand that it can also be partly due to a sloppy torque converter or highway gears in the rear. I'm just tackling one area at a time for now.
 
I had a 750 holley with vacuum secondaries on my 71 dart it worked great. It was a full bodied car with 3.91 gears 2500 stall stock except for small cam and performer intake.
 
I love my edelbrock thunder series 650 on my 400hp 340, of course all depends on build, originally had a 800 on it and when my engine builder went to dyno it and told me that the engine didnt want the 800 and it was too much so i got a 650 and night and day difference
 
The calculators are not usually about performance. You're going to have to tune anything at all really, so it comes down to what suits it best. Sure, a 600 is fine on a 318 that doesn't see over 4500 RPM or have much power but a 340 can see gains with a 750 or 850 stock. Holleys are easily tunable and serviceable. Do agree on the run what you have to dyno-tune, if you can get your hands on a 750 to borrow like a shop carb and get to play with the tune that could lead you in the direction of what you'll want to buy.

I've heard possibly 3 SBMs not sound mean. Sounding good doesn't mean it has much cam to it- there's a definitive difference in note but many can't even tell a very small cam's note from a mild one. Poor tuning can make a rough idle too.
 
I think the Edelbrock 650 Thunder AVS would be perfect here. Yes, a 750 is too big for a 340 if it is to be street driven. You will be spending most of your time on the street under 3500 RPM. This is where a smaller carburetor will shine through much better responsiveness. The 650 AVS has small primaries and larger secondaries. This enables you to cruise on a small two barrel, enjoying both the economy and very crisp acceleration the primaries have to offer. Believe me, you will thank yourself for not choosing the larger carburetor. Regardless of what others say.
 
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