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.JMOI 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?
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 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 hugewhat formula does GM use when using Q-jets on 307s/305s?
same for Dodge with thermoquads?
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 thingWhat 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!!!!!!!
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.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?