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I have spent this past winter rebuilding my 350 small block Chevy engine. I was wondering if there is a way to figure out what the best size carburetor is I had a 750 cfm and it was surging too much at slow speeds. Any suggestions?
 

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Don't blame the carb for your surging!!
It's timing that's the culprit!
More info needed though!
What other mods?
Intake?
Cam specs?
Compression?
Heads?
Headers?
Etc!!
 

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I have spent this past winter rebuilding my 350 small block Chevy engine. I was wondering if there is a way to figure out what the best size carburetor is I had a 750 cfm and it was surging too much at slow speeds. Any suggestions?
I believe a 600 or a 650 cfm would be a better choice, 85% of your driving will be in the lower rpm range, the smaller carb will give you more low end torque and better response, the 750 will give more on the upper end but not alot.If you just think bigger is better then why not 850.It depengs on what you mainly going to using the vehicle for.But then again we don't how you built up the 350, if it is stock or a mild build, then i stick with the smaller carb, now if you have built that 350 up in the 400hp or higher i would say go with the 750.JMO
 

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Figure out how many cubic feet of air your engine pumps per revolution. Then figure out what your primary revolution per minute operating range is, then you can use those two figures to come up with a cubic foot per minute figure which will relate to the cfm of the carb to run.
But as a general rule, for a 350, 600-700 cfm should do OK depending on how you run it. I would think 650, would serve it well, and anything over 700, would be a waste.
 

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. Hello... welcome... where are you located? ( ours is off to right --> )


. Yeah, we need more info... is this a 125 HP 350" for a California spec. 1975 Monza? Or a 500 HP strip car 350"? Or somewhere's in between?
.
 

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There is actually a formula (THE CARBURETOR SHOP / Carburetor size selection criteria) to put you in the ball park for carb size. You do have to make some assumptions based on what you are using the car for on a regular basisi. If it's a daily driver with an occasional floor boarding just because you can, that's one. If it's used every weekend, 1320 feet at a time, that's another. Then you have to make a guess at volumetric efficiency - and 80-85 percent is about all a street driven car will have.

Carb CFM = CID x RPM/3456 x VE
350 x 5000/3456 x .85 = 430 (HUH!!)

Read the reference above and see why a 750 is too big for most small block engines that aren't being raced
 

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cubes x RPM divided by 3456 = CFM for a 1960's base level engine... subtract 10-15% for an early 1970's slug engine... but add 10-15% instead for a performance street engine...
.
 

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Don't blame the carb for your surging!!
It's timing that's the culprit! More info needed though!

. Could be ignition timing and/or cam timing... less likely a problem in the carb... but NOT the size of the carb...
 

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carb size

There is actually a formula (THE CARBURETOR SHOP / Carburetor size selection criteria) to put you in the ball park for carb size. You do have to make some assumptions based on what you are using the car for on a regular basisi. If it's a daily driver with an occasional floor boarding just because you can, that's one. If it's used every weekend, 1320 feet at a time, that's another. Then you have to make a guess at volumetric efficiency - and 80-85 percent is about all a street driven car will have.

Carb CFM = CID x RPM/3456 x VE
350 x 5000/3456 x .85 = 430 (HUH!!)

Read the reference above and see why a 750 is too big for most small block engines that aren't being raced

what formula does GM use when using Q-jets on 307s/305s?

same for Dodge with thermoquads?
 

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what formula does GM use when using Q-jets on 307s/305s?

same for Dodge with thermoquads?
What does this have to do with the discussion? When was the last one of those carbs made? But ........ \/\/\/\/ see below. Those carbs really aren't that huge

Thermoquads have small primary venturis and large secondaries. They went from about 500cfm to about 1000 in the competition series - and with the plastic body, IMO, tender junk. The way that they metered fuel/engine needs(linkage and an air door or valve - think Edelbrock or Carter AFB), works out just fine, but there sure as heck are better choices for the secondary market.

Rochester Quadrajets are generally about 750 cfm with some more, others less. As with the Thermoquads - small primaries, large secondaries but the total cfm, again, controlled via the linkage and an air door. Find a good one of these, set it up well and it will run every bit as efficiently as any race Holley or clone. Either one, those back two barrels would seldom be 100% open, effectively reducing CFM.

I'll stand by the formua - it works!!!!!!!
 

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not trying to start an arguement,I respect your postd

What does this have to do with the discussion? When was the last one of those carbs made? But ........ \/\/\/\/ see below. Those carbs really aren't that huge

Thermoquads have small primary venturis and large secondaries. They went from about 500cfm to about 1000 in the competition series - and with the plastic body, IMO, tender junk. The way that they metered fuel/engine needs(linkage and an air door or valve - think Edelbrock or Carter AFB), works out just fine, but there sure as heck are better choices for the secondary market.

Rochester Quadrajets are generally about 750 cfm with some more, others less. As with the Thermoquads - small primaries, large secondaries but the total cfm, again, controlled via the linkage and an air door. Find a good one of these, set it up well and it will run every bit as efficiently as any race Holley or clone. Either one, those back two barrels would seldom be 100% open, effectively reducing CFM.

I'll stand by the formua - it works!!!!!!!
why pick a carb that has to max out the flow to make the power you want? Thats like taking a set 882 heads that flow 175 cfm and building a 380hp engine. The engine would have to be perfect but 175 cfm is enough to make 385 hp mathematically it is possible. Better flowing heads make the power easily and they are not maxed out,,,,carbs,,,,same thing
 

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I have spent this past winter rebuilding my 350 small block Chevy engine. I was wondering if there is a way to figure out what the best size carburetor is I had a 750 cfm and it was surging too much at slow speeds. Any suggestions?
It shouldn't surge at low speeds, this is a function of cam against gearing, the enigne is telling you its time to downshift or get rid of the 2.73 rear axle or 40 inch tires. Add more advance in that RPM range, or get rid of the unheated intake manifold. Go to a milder cam, etc. There are lots of contributors to engine surge, the carb can or not be one of them. The old saying that "most carburetor problems are electrical" meaning distributor still holds a lot of truth.

Most formulas will tell you 600 to about 610 CFM for a 6000 RPM 350 engine. So a 650 which is a common size is in range. However, on a dyno the 750 always pulls about 20 more horsepower on the top end with good manners everywhere else.


Bogie
 

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If you are just wanting an engine for transportation, an A to B vehicle, some sort of reasonable gas mileage, don't hot rod it every other day, etc, etc... and very rarely put your foot in it...a 600 is perfectly fine. Jeez, a 350 2 bbl would be right up that alley.
Now, if you are performance minded, drive in a spirited manner, don't really care too much about gas mileage, take it to the track and "rat whip" it occasionally, etc, etc....a 750-780 is just what the Dr. ordered!
 
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