Overcarbing does not cause bog. Not tuning the pump shot causes that. A 3k is not too big for the street. I drove around with one for 6 years. You couldn't tell it was that high until a foot break hit the convertor. The cam was only a 224@.050, single pattern. Nothing fancy, 80 technology. Now the 3500 stall, you could tell it had a big stall in it. I'm not talking about a moderate build with 194/160 valves in a stock heads. I mean the guys rolling around here with 302/331 with AFR 185's. When I threw on the 750, I picked up 2 mph in the 1/8, and I'd say that's a little more than 5hp.
If a street motor spends most of it time under 3k, your not having enough fun with it. Just get a 2 barrel. That way from idle to 3k will feel great.

 

Currently Avenger 770 on my 390 but toying with changing to a 750 double pumper.
Custom Grind Cam, Solid lifters, RMP heads, Hooker headers,Ported StreetMaster manifold, forged pistons, Crane Roller Rockers,
Pertronix points replacement & coil. 2.5" Magnaflow exhaust & K&N filter feeding into a C6 and thence nodular 3.5 TT.

 

Basically a work of ART!!!

 

Ultimately carb size also has to be dictated not just by the requirements of the engine, but the metering signal that the carb responds to. If you had a carb that could meter with 0.00000000000001" of vacuum, then you could run a 1,000cfm carburetor and be fine. Well built carbs with good boosters and good shear can get away with being larger cfm, but most people aren't running really good carbs with really good boosters. Ultimately the job of a carburetor is NOT to get air into the engine, though it will restrict flow to the point of decreasing power if too small. The job of a carburetor is to take the velocity in the venturi and turn it into singal to draw, shear and atomise fuel appropriatly so that combustion is as efficient as possible.

Air flow into the engine is going to be basically whatever the engine wants(unless you have a very restrictive carburetor). The cfm rating on a carburetor(4 bbl anyway) is rated at 1.5" Hg, that means if at max power you have less than 1.5" Hg, then you're drawing LESS cfm than what the carb is rated at, if you have more than 1.5" Hg then you're drawing MORE cfm than what the carb is rated at. Having a slightly restrictive carburetor doesn't prevent more than the rated cfm from getting into the engine, but may slightly reduce the cfm the engine would draw with less restriction. Having a carb that is slightly restrictive in terms of flow, may have a negligible impact on power, but having one with poor atomisation can have a huge impact on power. If the fuel isn't atomised/vaporised properly, then it can't combust, which is ultimately what making power is about, combustion. You ideally want a carb with as little restriction as possible(or the lowest intake vacuum at max rpm and WOT) that doesn't compromise shear. The problem with switching from one carb to the other of differing sizes, is they may have different booster/venturi designs, different air bleed setups, and different emulsion packages, all of which can have a HUGE impact on the ability of the carb to help produce power. A modern 850cfm carb with a badass emulsion and booster package will make WAY more power and run better on a stock 289 than a 1965 crap booster crap emulsion package carburetor will. It's not a direct apples to apples comparison in many cases, and I'd be willing to bet that the power people gain from going to a larger or smaller carb 9/10 times isn't so much related to cfm ratings and flow as it is to shear and atomisation.

Also like cpr said, intake design plays a role as well. You could have the world's best 850cfm atomiser sitting on top of an intake, but if it's a stock 60's log style intake, it does no good. The fuel/air charge will just get to the end of the main runner and when it tries to make the 90* turn, the fuel will just fall out of suspension and splat against the runner wall. Crappy intakes like that need a smaller carb to keep the intake vacuum and velocity up so the fuel is less likely to fall out of suspension.

There's no need to get a bigger carb than the engine requires though, if you have a carb that's good at atomising, then going small rather than large just means the atomisation is better than if you went large and the atomisation was "good enough." There comes a point with a carb's size where going bigger has no benefit, if the engine simply will NOT pull more than 600cfm(actual engine draw), then going to a carb that flows more than that at a rated intake vacuum the engine will realistically see is pointless, as the engine can only draw what it can draw. Again, the entire goal is to use a carb that is unrestrictive enough at max power yet still atomises well. No mostly stock 289 or 302 is EVER going to draw 750cfm, period. An engine like that would be lucky if it actually moved 450 honest cfm at full power, so you might as well pick a carb that's closer to the realistic flow of the engine since it will cost less and be easier to tune than trying to buy a high dollar race carb for a stockish engine and making it work. You take the same engine that moves 450cfm and with a 480cfm carb may have 1.3" Hg at full power moving 450cfm, slap the 750 on it and it'll show 0.55" Hg vacuum, but is STILL only moving 450cfm(that's all the engine can do). Now, if the 750 can atomise as well at 0.55" as the 480 can at 1.3", then it's the same either way, but if the 750 can atomise better at that vacuum than the 480 can then the 750 will run better, and vice versa.

It's all about the right carb for the right job. All carburetors are NOT created equal, not by a long shot.

 

Like it or not, most street motors DO spend most of their time under 3k. That's what driving on the street is. Winding it up to 6k on the street every singletime you shift isn't fun, it's stupid. There's no point in doing that. The entire upper half of the rpm range is for performance, not for getting around town. If I drove around like that, the only things that would come of it would be pissed off cops(noise) and an empty gas tank.

Also, most people don't have strokers with AFR's that are built to wind up to 7k and are more weekend warrior cars than anything else. There are probably only a small handful of people here that have that kind of motor. Most people have mostly stock 289's or 302's. This is what we're talking about. You can't drop a 750 on an even mildly built 289 and expect it to work right. There just isn't enough vacuum signal to properly pull fuel out of the carb, not to mention, as has been said already, the engine will NEVER pull that amount of air through the carb. There just isn't any point.

 

I had a question about the differences in carb size. I'm debating on buying a 500 or a 600 Edelbrock. Pricewise the 500 seems to be more expensive [:-]and plus in the future (far future) if I ever stroke the 289 would I be better off going with the 600?

 

One persons driving habits different than yours is not stupid. I didn't say up to 6k everytime. I take mine to AT LEAST 3k every time. Why own a muscle car to putt around ? That's neither here, nor there. That's opinion also. But if are interested in MPG so bad, I would gladly trade you my focus for that fastback. But then it could be argued that driving too low in RPM with leave deposits on the intake valve from low air velocity.You have missed my point ENTIRELY. I said that a 750 is not ALWAYS too big for a 302. Nor did I say everyone would benefit from one.I even said by looking at his sig a 750 would be too big. I think you are just stuck too much on the number.


67mustang302, you get where I was going with it. But if I remember right, aren't they rated to 3.0hg for a single 4bbl? I can't remember.Anyways, you understand how and why, and that the flow rating does not dictate everything. It's a number for reference. A 750 with downlegs will not be a restrictive as a 750 with holley econo boosters. One will still much higher velocity in the venturi than the other, but they are both 750's ? The number 750 nowadays more or less refers to the venturi size. Which Demon runs the same size, but flows way more than 750, but they still call it a 750 because of the venturi size. You had somewhat of the idea of my thinking with the intake, and while yours makes sense....I'm thinking more along the lines of a completely divided dual plane vs a open big plenum single plane.A dual plane divides amotor into two parts on a carb. 4 cyls per side of the carb. At any givien moment, that side of the motor is only pulling thru that side of the carb. A single plane....those same four cyl, will now have to create a vacuum in a larger plenum area, and then pull thru 4 bbls now, instead of just two. So the exact same motor can have the same velocity going thru the carb, on either size carb, but with a different intake. Or I could be totally off my rocker.

 

Holley 700dp on a 351C with cam, intake, and exhaust upgrades.

 

i will have a autolite 4100 with a weiand intake in near future. and yes, its the 1.08 venturi or 480 cfm one. going on a basic 302 with some gt 40 heads. the way i look at it with my limited knowlege is this; all the carb ratings at at fixed vaccuum amounts and RPMs and such, so that doesnt nessesarily mean a smaller carb cant go higher then its rated cfm. and a smaller venturi means more velocity, and that makes things a little smoother. just my line of thought.

that and aoutolites are really underrated and not so common, and i like that

 

We've got an Edelbrock 1408 on ours now. I just heard back from Edelbrock today, apparently it's an old carb designed for EGR purposes and discontinued 15 years ago.

Now come spring we're looking to go down a few notches in the carb and intake department (we've also got a Torker II intake to match the carb). Sad part is the carb is basically unknown so we won't be able to recoup much cost of a new carb when we make the switch.