There is a lot more to it then just the carburetor size. There is also a lot more to it then just using a certain size. I get a lot of heat from several over the years because on all of my builds I use a smaller 600 to 650 size carburetor on every 350 build I have ever done for the most part and have had excellent results. It all depends what the person is looking for out of there build and also how much power does the build make in power and how much does the heads flow and what cam size is being used and how and what is the person going to drive the thing and what do they want out of it.
There are many variables on carburetor sizing and there effect on different style builds which is a lot more to it then formulas. Take a stock 350 making around 250 horsepower and maybe 325 ftlbs of torque and it tops out at 4000 rpm with a very mild cam like a 196/
[email protected] duration and smog heads.That car or truck could use a small holley 500 cfm two barrel and run fine or you could step up to a 600 cfm vacuum secondary and the car would run still just fine with a four barrel vs a two barrel with out the carb being to big for said application. Put a 750 vacuum secondary on it and that makes it a carb that is too big for that motor and will never use all the cfm it can flow as the motor itself won't be able to have enough velocity and air going through to make it open up the secondary side all the way.
Now many will say well GM used a Quadrajet that was 750 cfm rated on many stock engines. But the thing is they primary side was super small to give more better signal for throttle response and the secondary side would only open up to what the engine could use similar to a holley vacuum secondary. You can't compare the two together because even though they both can flow the same amount of cfm, the Qjet will still operate much better on a bone stock motor vs a Holley style carb of any cfm up to a certain point.
On certain engines depending on the build a slightly smaller carb will give better signal on the bottom and mid range, but once you get in the higher mid range and high rpm then the smaller carb will start to fall off and either choke or drop off some and the bigger carb will out do it every time. Just depends on how its going to be used and what the person wants and likes. Bigger does not always hurt it like some say it will like its too big. It depends on what carb they are talking about. An edelbrock AVS2 and a Qjet is different on how its going to do vs a Holley style carburetor of the same size range cfm rating and how it will perform to a certain degree.
Bottom line its all in the build and what the persons preference is and what do they want out of it and what is the best option for what they are wanting from there engine that will make them happy and satisfied for how it performs. For all out racing then you most definitely will want the biggest you can use on your build as that is what you look for the most power you can get and it all counts.
I have currently a Dart SHP 377 small block (4.155 bore x 3.48 stroke) with a set of Dart pro 1 platinum 200cc cylinder heads with close to 10:1 compression and a previous 276/280 228/232 and 110 lsa. Lift was over 535 or more but forget the exact specs. The heads have all been ported out and bowl blended and ran an Edelbrock victor jr intake with that build and a 3000 plus stall convertor and a set of 3.42 rear gears and a turbo 350 in a chevy s10.
That build can use all of a 750 cfm and then some and I ran a Holley 750 HP and also a 650 HP style carburetor and I had a lot of top end with it past 4000 plus rpm. The HP Style is nice but I also needed a choke for the colder months here in Ohio, so I also still had a 650 Holley double pumper and also a Holley 600 vacuum secondary carb from previous builds. On the bottom end and to lower mid range the 600 and 650 carbs gave me more crisper throttle response vs the 750 and would give for a more snappy throttle and better cruising at lower speeds and slightly better mpg but not by much. Only about 2 mpg difference.
I ended up wanting to tune my engine down and make it more for street cruising as I don't race mine and never take it to the track and hardly am ever over 4000 rpm ever unless I get on it but I like to respect the laws and don't do street racing like some younger folks do. I just like the part of having something nice and don't mind if I loose a little off the top to get a little bit more on the bottom. I put in a smaller cam a few notches down and also put in an edelbrock rpm performer air gap and stuck with a 650 ish range carb of whatever one I choose to use since I have several ones.
I like the nicer throttle response and better signal on the bottom end and lower mid range which is where I am mostly at vs the top end. The intake swap and the cam swap added more for what I was looking for and liked better and should have planned it out that way the first time. Over the years I like the bigger and higher reviving engines and higher end rpm rumpy rumpy cams but as time went on for me at least it got old and no longer like the more radical builds.
I am also de tuning my engine for the last time and am done with the bigger rpm builds and rump cams. The 750 carbs were good all the way around don't get me wrong and after 4800 rpm you could tell a bit of a difference on the top end especially once you got up to 6000 rpm as the engine still seemed to want to go as with the 650 carbs it would just be out of air and would just flatten out. Off idle the smaller carbs gave more stronger signal for my build to what I was used to and more to my liking so I kept that over the making more on top end while giving up some signal on the bottom end.
That is how I felt it performed and my opinion so some will vary on how they feel on how there build is doing and how it works for them as all hotrodders are difference on what there opinion on how things run. That is why there is many options out there. Oh and the old myth that a double pumper is crappy on fuel mileage vs a vacuum secondary Holley is just an old wives tale if you don't keep going wide open throttle over and over again but for regular street driving and not getting into the secondary side that much I have gotten identical fuel mileage from both carb styles. It is having the knowledge to do advance tuning such as the idle circuit with the idle feed restrictors in the metering block and also the power valve channel restrictor and some other things that many tuners fail to learn and just do jet changes and adjustments on the idle mixture screws and not much else.
Learning to understand how the different designs works and how to tune each area at at time can bring in much more from the carb itself vs the regular basic tuner that mostly does not know how to go about with the advance tuning many carburetors need out of the box.
Other factors in carburetor application is the booster style that the carb is using itself on the Holley style carbs. Straight leg booster vs down leg boosters will have a different effect on things with also annular boosters being thrown in the equation. The old formula does not always work as like you stated there is a lot more to an engine then just sucking in air and a carburetor does a lot more then what the average tuner realizes as they don't actually read up on how a carburetor functions and how it works in relation to the engine its on etc. It does more then just suck in air and squirt some fuel.
The different styles of carburetors from a Qjet and an Edelbrock AVS 2 and a Holley all have there pros and cons with there designs across the board depending on there use. Same thing with a vacuum secondary vs a double pumper carb. Many things can change how they work and perform vs way more then the cfm rating itself. The transmission being used along with stall rating if using an automatic and the rear gears and cam size and engine rpm cam range along with the weight of the vehicle and how it is setup can effect things across the board.
Bigger is not always better but sometimes bigger is better depending on what you want out of your build and how you plan to use it. Since I am just all street and mostly just cruise and am under 4000 rpm 99 percent of the time I like to be on the slightly smaller conservative side of things, but not to the point I choke it to death.