Then why do all the high performance engine builders recommend a 750 cfm carb for a 350 cid engine?

 

Not all of them do, and just cuz they're a "professional rebuilder" doesn't mean they know what they're doing. It seems like just about EVERYONE that has a carb, overcarburates. If you actually calculate the required CFM taking into account manifold design and VE, and select the best AVERAGE carb size for the top 2,000-2,500 rpm that you generally operate in under full power, you end up with something on the order of 100-200cfm smaller than what most people use/reccomend. 750cfm on a 347 would be great at 6,000rpm on a healthy engine, but what about the other 1,999rpm you move through when you're shifting gears?

 

i can vouch for over carburetion when my 289 had a 2bbl i could lay rubber down and now that i switched to a 4bbl (i knew it was too big at the time ) i cannot do that i am either gunna get a 570cfm avenger or a 450cfm holley cause i only need 426cfm to spin my engine up to 6k rpm and i have a 625 on it now and its a little pig like but it was available to me for free and so was the 800cfm for my 460 that only have a two barrel

 

I like the 570 for 302's, it makes gobs of power everywhere and with good throttle response. Many people clmaim it's actually under rated, and based on how my engine runs with it, I believe it. It'll take my hyd roller 302 to 6k rpm FAST and it doesn't fall flat, just pulls harder with more rpm

 

While this sounds like a great idea , it will still run like total crap. Simply because there isnt enough VELOCITY to the air moving through the 750, like someone else said its just moving too slowly.
600 vacuum secondary is what I would run on a mildly built 302, for a more radical one with aluminum heads, 2.02 valves, and a victor jr. I'd run a 650 double pumper. Even then itll blow a tiny bit of black smoke when you really lay in to it at low rpm's.
-P.

 

Well, maybe you'll listen to the experts...you know...Shelby, Ford, Cheverolet......

1. 1965 289 GT-350...715 Holley
2. 1968 Z/28 Camaro...780 Holley
3. 1969 BOSS-302.......780 Holley

If you have enough motor you need enough carb. Double the cid and add 50 for a hi-po engine. At least that is what Jake King, Jack Roush and Dan Perron told me years ago, but I guess YOU know more than they do. Oh, and Jack told me in 1974 that there was not enough carb available to feed what a BOSS 302 wanted....maybe the BIG inline 4's that were used on BOSS 429's but my 302 had 2 875's and wanted more.
And that formula you speak of is for a motor that is run in exactly the 2-2.5k you speak of...if you are building a 347 stroker I assume you ARE going to use all the rpm capability of all that motor/cam/head/header/gear combo you spent ALL tha time, money, dyno runs etc on. Blow THAT smoke up sombody elses rectum, I'm too old with too much real world experience to allow it.

 

You picked 3 engines that are all known for being way over carbureted and didn't drive that well on the street for 1. Second, carb CFM rating can be misleading and abritrary, carbs flow different CFM's based on the vaccum that's pulling the air through them. It's also common knowledge that engine technology especially in the induction systems/heads from cars in the 60's was very limited, they didn't know how and didn't have the technology to build engines with the efficiency they have today. And I think you misunderstood what I was talking about with the 2-2.5k rpm range. I was talking about the top end of the engine's operation under full power. 3,500-6,000, 6,500-9,000rpm. You need to make the best average power in the rpm range you operate in(unless it's a superspeedway engine, then you're looking at peak hp more than other applications). If you calculate carb CFM size based on max rpm, VE at that rpm, engine discplacement and manifold type, you get the required cfm FOR THAT RPM ONLY. Anything lower in the rpm range requires less carburetion. And as far as carburator sizes and you "being too old to be fooled" or whatever, you're stuck in the past. People like myself an others are running naturally aspirated 302's with 600cfm or SMALLER carbs andmaking the samehp at the WHEELS that the Boss 302's or the Shelby 289's made at the crank, and doing it with less rpm and MUCH better mileage. And as far as "not enough carb to feed a Boss 302," that's a load of garbage. I know guys running turbocharged blow through carb's witha single600cfm 4 barrel and making over 1,200hp to the wheels, and guys naturally aspirated on MUCH larger engines with single 650 or 750 4 barrels that are running 10's all engine. And if you're 302 wanted more than 1,750cfm you had to have been turningat LEAST14,000rpm, cuz that's the only way a 302cid V8 is going to be able to move THAT MUCH AIR with extremely high volumetric efficiency(greater than 130%)

 

67mustang302 thank you for backing me up on this issue.Sorry I was at work and missed this.
I built my 302 from the ground up and did a lot of research before coming up with a formula that works for me.My motor has been running great for 4 yrs now.Gone thru a lot of tires.Sounds great.
I feel I have enough knowledge to share.Old school mechanics are hard to find,we cant let the non computerized car knowhow go to waste.

 

You can adjust the linkage response to help an oversized carb on an engine. The actual capability of the carb to flow is more important than what's it's rated at. You can switch between several carbs of the same rating, and find yourself with an engine that behaves differently with each one. I've seen 302's run great with a 750 carb (older carb), and some that run like you'd expect (crappy).

If you're buying a carb, shoot for something a little on the small side for a street engine. If you already have the carbs, put 'em on and try to tune them in to see what you can do. I run a 650DP on my 302 with a single plane and every part of the RPM range (800 idle to 7k) has good response.

 

Here Valley, use this calculator to determine the CFM you need.
That 570 sounds like it's plenty.
http://www.4secondsflat.com/Carb_CFM_Calculator.html