Lots of different reasons.

First, there are a couple different 289s.  A regular A code 289 (4 barrel) had a 390cfm carb stock.  Peak power is 225 at 4,800RPM.  A K code (4 barrel solid lifter) had a larger carb and made peak power of 271HP at 5,700 PRM.  Very few guys are running stock anymore, so once you change the heads, cam, exhaust, intake, etc. you move the power into the upper RPM, where it needs more fuel. 

Second, different manufacturers rate their carburetors differently.  My supposition is that Edelbrock for example rates their carbs at a different depression (vacuum measured in in. of Hg) than Holley.  When you change the depression through the carb you change the air that is drawn through it.  Its somewhat dishonest, but think of it like how advertised duration on cams can vary widely between manufacturers.

Third, you never know what people really mean when they say an engine runs good.  Some guys think that the poor idle that comes from a carb that's too big is how its supposed to run.

I can tell you I do in fact have a stock A code 289 that is a Ford reman engine.  Nothing on it is modified except for the timing.  The Edelbrock 500 I run works very well.  In truth the Autolite it had ran pretty well too.  I swapped to the Edelbrock because parts are scarce for the Autolite and I wanted to be able to tune the carb. 

I certainly can't teach any lessons on cfm, but I can say that a 500cfm Edelbrock has worked well on my 289 both when it was mostly stock and even still after swapping the cam, doing some mild head porting, and adding Tri-Y headers.  I'll also add that the metering rods in the Edelbrock make it easy to make small adjustments without disassembling the carb.

If you do decide to go that route be aware that the extra metering rods, jets, springs do not come with the carburetor, so you may want to order the tuning kit when you order the carburetor.
https://www.summitracing.com/parts/edl-1486

I was advise to use a 750 Edelbrock on my engine and was told it would be to big and a 600 would be to small. So I would need to de tune it. I've been messing with it for ever leaning it out till I finally figured out that the distributor was no good. When I put in the Duraspark it changed the way it ran and I was able to tune the carb a lot easier. Turns out I had it too lean. It runs a lot smoother now.

In this video by Summit, the presenter shows some carb size math at time index 3:40.  His calculations for a street engine 302 came out at 445.65 cfm.  Even for a race engine 302, his calculation was 576.73 cfm.

https://www.youtube.com/watch?v=6JdXrblkbd4

I get where he's going with the math.  Engine size (the volume of air the engine can theoretically ingest) x RPM (this gives you the number of times you have to fill the engine each minute) x volumetic efficiency (engine's can vary on this based on how efficient they are, a street engine is usually in the 85% range, race engines get closer to or even exceed 100%, though that's tough naturally aspirated because you have to actually create a ram effect to slightly overload the cylinders to get beyond 100%) / 3456 (took me a little while to figure this out, but its actually pretty easy)  You have to convert cubic inches to cubic feet, there are 1728 cubic inches in a cubic foot.  Then you multiply that by 2, because what you are really doing is halving the volume you just figured out in the first part.  Why are you halving it?  Simple, RPM is a measure of crankshaft revolutions, but you don't fuel each cylinder on each revolution; you only fuel every other revolution in a 4 stroke engine, like so:

Revolution 1 starts at TDC having just expelled the exhaust from the last combustion event, it travels to BTC as the intake vale opens to fill it with air and fuel, then returns to TDC to compress the air and fuel.

Revolution 2 starts the the spark plug fires as the piston approaches TDC on the compression stroke sending it doward to BDC on the power stroke, and finally it returns to TDC as the exhaust valve opens to expel the exhaust.

Now that's a great formula, I know I've seen it before, and it should get you close.  Of course there's more to it than that, and I suspect there's some tweaking being done with the number used for VE to compensate for that.  In general a street engine tends to run better with a smaller carb because they idle better, have better throttle response, and bolster low speed torque.  A race engine only cares about power at higher RPM and that means a bigger carb tends to serve better, even if the engines are nearly identical (e.g. class rules mandate stock parts). 
 

jkordzi wrote:

Please see my recent previous post on using a Holley 600 on a small block.  I have on one my car and have successfully run them on many small block engines with great success.  If you have 19 inches of vacuum, then the power valve you should start with is a "10."  That's assuming you read the vacuum from a point below the throttle blades and the engine was fully warmed up.  Which one is on the carb?  What size jets are in it?  Did you set the fuel level correctly?  Did you adjust the idle mixture screws on the side of the metering block (not the idle speed screw on the throttle)?

Sorry for late reply ...
I prefer follow the suggestion about selling it , instead of making a revolution just to make it work.
More , this one was refurbished , and may something went wrong ...
Yesterday i sold it and now i am looking for something else littlier.

I got just one question: i have read the vacuum from the carb. spacer inlet - is it correct ?  

Now i am looking at edelbrock 500 because is easyer to dial ( as people says )

Thanks for info ! 

 

You can read vacuum anywhere below the throttle blades.  If you see a port that is above the blades its called a "timed" port, meaning it only gets vacuum when the throttle blades are open (anything but idle).  Don't read vacuum there.  You can experiment with whether or not the engine runs better with the vacuum advance for the distributor connected to timed vs. manifold vacuum if you want, but that's about it. 

As for the different series, I would run a 1403.  That's what I run.  Its an electric choke with mechanical secondaries.  The AVS uses Air Valve Secondaries, translation: vacuum secondaries.  I've never been a fan of vacuum secondaries on a car with a manual transmission.  The 1403 is also a little cheaper than the AVS carbs. 

I have the 600 Summit on my 347 at 7000ft elevation. Runs nice and crisp. Have a few people I know have also gone that route and all happy with results.
Read the reviews on their website, not much negive feedback.

Didn't realise it till yesterday but Summit also has a 500 cfm.
https://www.summitracing.com/parts/sum-m08500vs

The transmission will get it's vacuum from the vacuum manifold tree as usual, the carb secondary gets it's vacuum signal from within the carburetor it self.

Not sure about the blow-by? I was running a 4100 on my 67 with a mild 302 and an automatic 279? rear gears. Ran fine, no real performance to be expected especially at 7000 ft.
I built a 347, put in a t-5 manual and 4:11 rear with the Summit 600cfm vac secondaries. I have run the light spring on the secondaries and have been real happy with all aspects of the carb. Starting cold or hot has been flawless now for 2 years.
I do have an AFR gauge and dropped a couple jet sizes front and back for the elevation. I can hit the pedal one time and start it and walk away even with a snotty cam. Never slobbers. Light car and light secondary has worked pretty good  even with the manual. I have 10-11 inches idle vac and it has good street manners.
For $299 I would put that 8500vs on a 289 without hesitation. Read the reviews on it and the 600.