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I have a 350 Chevy that came with a 750cfm 4barrel Holley. This carb is just too big, and I'm done trying to adjust it to compensate, I got it close but it needs to be better. I was looking on Holley's website and using there interactive carburetor I came up with models ranging from 600-670cfm. If somebody could explain how much of a difference the 70cfm will make that would be great. In case you need to know- mildly modified Chevrolet 350, in a 79 camaro, with auto trans.
 

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Remember that rating are just guesses. A bigger engine will pull harder on the same size carb and change those ratings.
 

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Murdoc101 said:
I have a 350 Chevy that came with a 750cfm 4barrel Holley. This carb is just too big, and I'm done trying to adjust it to compensate, I got it close but it needs to be better. I was looking on Holley's website and using there interactive carburetor I came up with models ranging from 600-670cfm. If somebody could explain how much of a difference the 70cfm will make that would be great. In case you need to know- mildly modified Chevrolet 350, in a 79 camaro, with auto trans.

750 really isn't too big for a 350....... depending what 750 carb it is....

It needs to be a vac secondary or an air valve secondary... whatever size you get.

If buying I'd go with an 600 Edelbrock AFB or AVS. No leaks, easy to tune, air-valve secondary.

70 cfm is insignificant usually. There is a bigger difference in booster types, pumps, etc. that make up tuneability. Over the years I just got to where I dislike Holleys because of the constant messing around necessary. That's why BG. Demons are better.

I believe that the majority prefer Holley for bragging rights..... "I have a 750 double pumper, a 240* cam, 410 gears and shift at 6500!!!!!"

So what, line em up. :thumbup:
 

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I dislike Holleys because of the constant messing around necessary
Let's not blame the carb for being a precision metering device. Every carb changes with the weather and BP, humidity, and other outside influences. Some might be far enough off already that its' not noticable though. If you need to mess with it constantly to keep it close enough, you have other problem unrelated to the brand name.
 

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Ive got a Holley on my 454 that I havent touched since I put it on & made its adjustments to it almost 4 years ago.
I do agree that 750 is to much carb for 350 CI... unless its being reved to 7000 RPMs. There is a reason they make carbs of differant cfm ratings...even vacuum secondary carbs. a smaller carb is going to have much better response at lower & midrange speeds.
A q-jet, or other spread bore style carbs are more forgiving due to there small primary design, allowing 800 cfm to work well on a 305 Chevy to a 500 Caddy.
If this 350 is your typical stock 350 with a lil baby cam, headers, dual plane intake, itll probaly never see the other side of 5500 RPM anyway, 600 CFM is more than enough. John
 

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johnsongrass1 said:
Let's not blame the carb for being a precision metering device. Every carb changes with the weather and BP, humidity, and other outside influences. Some might be far enough off already that its' not noticable though. If you need to mess with it constantly to keep it close enough, you have other problem unrelated to the brand name.
Agree/disagree

Some Holleys seem to change more than others because of their designs in boosters, transitions, etc... their adapability to varying conditions is not as good as other designs. BG improved a lot of the basic problems. JMO.
Fuel injection cures most all the problems... :D
 

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A holley 570 Avenger is quite adequate for a mild 79 350. How do I know? Lots of dyno work with small, mildly modified 350 low compression Chevies.
 

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Rick, I wont disagree there.I think its a tad small....a very tad for a low rpm 350, however, Id rather be a touch small than way big. That is unless driveability isnt an issue.
Also, if the 570 does seem a bit small, a 1" open spacer would probably make up the differance by increasing the plenum volume a bit, making the engine react like it has a slightly larger carb. Id still propbaly use a 600. BTW.. its all personal opinion, I prefer Holley myself, they just work best for me., never used a BG, Im sure there great as well.
 

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454, your gut is correct, better a tad too small. The closer the match of the carb and engine the better the signal through the circuits and the better it will deliver good air fuel ratios over the rpm and load conditions.
 

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Discussion Starter #12
Here's some more info might help-

Not stock motor, sorry should have said-

Mild Crane cam
Vortec heads
around 10:1 compression
I don't wind it up past 5500rpm
 

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i would think a 750 holley would work just fine on your motor. especially with that much compression and the vortech heads. sounds like you may have a tuning problem either in the carb or ignition.
 

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you can make a 750 work , & run well on that motor. & I agree there is probably a tuning issue with the carb or ignition, but a smaller carb well offer better overall performance. a good example is my 454. I run a 770 Holley on it...I take it to just shy of 5500.
 

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Discussion Starter #15
I agree, and I have tuned it to work, but I'm looking for that little extra performance, specifically in the low end. Am I correct in thinking the smaller carb will give me a little more low end?
 

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Murdoc101 said:
I agree, and I have tuned it to work, but I'm looking for that little extra performance, specifically in the low end. Am I correct in thinking the smaller carb will give me a little more low end?
If it is a well tuned vacuum secondary on a dual plane intake.... you will never notice the difference.
 

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xntrik said:
If it is a well tuned vacuum secondary on a dual plane intake.... you will never notice the difference.
My view is differant, yes, it well be more responsive. again, why do they make differant cfm carburaters ?
 

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stepside454 said:
My view is differant, yes, it well be more responsive. again, why do they make differant cfm carburaters ?
Marketing.

Someone's friend has headers and a cam while this guy has headers, cam, and a dist. so he thinks he needs a slightly bigger carb. He looks online and finds something bigger than his freinds 650. Bammo, sold.

OR, a guy is looking online thinking he want's a bigger carb becasue the performance isn't what he want's but the little voice is telling him "every says smaller is better" so he picks the middle of the road.

In reality, carb CFM fluctuate beween 550 and 650 CFM on the same 600 cfm model.
 

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Carb CFM fluctuates based on the CFM requirements of the engine. The more HP an engine makes the more air it needs to consume. All carbs are rated from the manufacturer based on airflow versus pressure drop. The rating on a carb can not change only the amount of airflow drawn through it.

Pull too much air through the carb and power will suffer. Oversize the carb and driveability, fuel mixture will suffer, with no corresponding increase in power

A typical small block making around 440 HP will show around 530-550 or so CFM on the airflow meter with no manifold vacuum restriction.

Our limited late model motors will show 6 to 7 inch of manifold vacuum sucking through a 4412 500 CFM carb. We are pulling a lot more CFM through that carb than 500 CFM but doing it as a restriction and at a great power loss as compared to a larger 4 bbl. Hence the rules for a restricted motor.

When dynoing larger small blocks such as 434's with 4150 CFM 750 HP's we see 5" of manifold @ the 600 HP level pulling 810+ CFM on the airflow meter. Again, more CFM than rated but at a huge cost in power.

These are the reasons carbs are made in different sizes, all having to do with the various demands imposed on them of the engine airflow requirements.
 

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your question was will smaller help for more bottem end

350/268 cam/10-1CR (as an example).....plugged it into my antique desktop dyno, changed only the carb cfm

at 2,000rpms

600cfm=105hp, 276tq
670cfm=106hp, 278tq
750cfm=106hp 278tq

you want more bottem end, keep the 750 and swap to a different cam and/or swap to lower rear gears for more torque at lower speeds
 
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