Stock 340 is a holley 570 big enough?

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dogpro

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I have a stock 71 340 duster that I bought with an incorrect carb (some avs carb) and I want to get a new one. I went on the holley site and it suggested a 570 avenger I thought most stock 340's came with 700 to 750 cfm. Is a 570 big enough or should I step up to a 670? This 340 has alot of miles and will not be raced just driven to car shows and some putting around. Thanks,Lou
 
what intake do you have on it, if its a stock one #'s ending in 2100 its a spread bore for a t-quad and you would need an adapter plate to run a square bore, i would look for something in the 650 700 cfm range if its stock. even the 750 vacuum sec.
 
It does have the stock intake and it had the adapter on it so the avs would fit so I can go with the square flange. I also thought about a 750 vac. secondary. Thanks, Lou
 
A 670 Avenger would work great, and will leave you some room for other engine upgrades. the 570 would be ok, but marginal.
 
The correct carburetor was a Carter Thermoquad that flowed 850 CFM. So no, a 570 ain't gonna make it. I'd try to find a 850 thermoquad for it if it was mine.
 
StrokerScamp said:
The correct carburetor was a Carter Thermoquad that flowed 850 CFM. So no, a 570 ain't gonna make it. I'd try to find a 850 thermoquad for it if it was mine.
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You can't make a comparison between a spread bore thermoquad and a square bore carb when it comes to cfm ratings. The Thermoquad likely has smaller primaries than even the 570 Holley and a stock 340 would never have opened the secondaries fully to realize that 850 cfm rating. If you have one it's certainly a good choice but just don't use it's cfm rating as a guide to buying a square bore carb.

I have run a 600, 670, 725 and 750 on my 360 which is far from stock (~370 HP). The 600 would give up a little on the top end (above 4500 rpm) to the 670, 725 or 750 as measured by the butt dyno but it had it all over the 725 and 750 in low/mid rpm throttle response. The 670 is ideal, same low/mid response of the 600 and no difference as measured by the butt dyno on the top end. Also, the 600 and 670 get about 5 mpg more than the 725 or 750.

If you are buying a new carb the 670 would be a great choice. If you have the 570 it will work but you may give up a little on the top end but for a street car you will likely never notice.
 
I've had a couple of "stock" 340's with holley 750's v.c. secodary carbs. on them ran great,as for a thermoquad or "thermo-bog" as we called them back in the day the best place for those "plastic" carbs. is in the trash...
 
I wouldn't go so far as to say they were trash. A lot more is known about them now, compared to their earlier years. I still think the 670 would be ideal for THIS motor.
 
thermoquads have been run on super stock cars for years and dominated. they are also very good carbs for the street. you can take a 440 thermoquad off, put it on a 318 and never give it another thought. they are hardly trash. people who think like that have never run one or never learned how to tune them. they are great carbs.
 
dgc333 said:
You can't make a comparison between a spread bore thermoquad and a square bore carb when it comes to cfm ratings. The Thermoquad likely has smaller primaries than even the 570 Holley and a stock 340 would never have opened the secondaries fully to realize that 850 cfm rating. If you have one it's certainly a good choice but just don't use it's cfm rating as a guide to buying a square bore carb.

I have run a 600, 670, 725 and 750 on my 360 which is far from stock (~370 HP). The 600 would give up a little on the top end (above 4500 rpm) to the 670, 725 or 750 as measured by the butt dyno but it had it all over the 725 and 750 in low/mid rpm throttle response. The 670 is ideal, same low/mid response of the 600 and no difference as measured by the butt dyno on the top end. Also, the 600 and 670 get about 5 mpg more than the 725 or 750.

If you are buying a new carb the 670 would be a great choice. If you have the 570 it will work but you may give up a little on the top end but for a street car you will likely never notice.
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I can compare whatever the heck I want to. don't school me on carbs. i built my first one in 1975 and have done it ever since. I think I know the differences.
 
StrokerScamp said:
thermoquads have been run on super stock cars for years and dominated. they are also very good carbs for the street. you can take a 440 thermoquad off, put it on a 318 and never give it another thought. they are hardly trash. people who think like that have never run one or never learned how to tune them. they are great carbs.
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Stock and superstock cars run thermo-bogs because they have too,i'm sure if given the choice they'd be using a much better performance carb,and we did run them back in the day/ or tried too they always were nothing but a big pain, but as for performance they can't/couldn't touch a properly tuned Holley
 
put a holley on it like everyone else, anyone can tune it. If you have the time and like to get the most performance with mileage supertune a T-quad or even a Q-jet.
 
Thanks for all the replies! Like I said I don't really have a carb right now so I am wide open on choice but I do know the correct carb for a 71 340 is mucho $$$ and right now I am just looking at a driver. Thanks,Lou
 
If you can find a "GOOD" one, my 1st. choice would indead be the TQ, you'll notice that everyone is a carb/cam & even intake "expert", carbs are like cams when it comes down too it, bigger is not always better, theres no need "at all" to have anything bigger then a 670 (standard bore) on that stock/mild 340 period, so my next choice would be the 670, just my 2 cents.
 
I guess the simple answer to your question of "Is a 570 big enough or should I step up to a 670?" given the parameters of "This 340 has alot of miles and will not be raced just driven to car shows and some putting around. " would be yes its big enough for what you plan on using it for and if you can afford to step up to the 670 it might be a good idea.
 
StrokerScamp said:
I can compare whatever the heck I want to. don't school me on carbs. i built my first one in 1975 and have done it ever since. I think I know the differences.
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The way your original post was written implied that because a 340 came with an 850 thermoquad that a 570 or 670 was not big enough. My post was for the benifit of the original poster. Putting an 850 square bore carb on a stock 340 is grossly over carbing. A real skillful tuner can get it to work but something smaller will work better and be easier to tune.
 
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