holley cfm formula?

Discussion in 'Classic Mustang Specific Tech' started by porkchop, Jul 2, 2004.


  1. porkchop

    porkchop Founding Member

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    I was looking at my holley manual and i cam across the cfm formula

    ( cubic inches divided by 2) x (maximum rpm divided by 1728) x volumetric efficiency

    well i figured out what i said i would need for my motor a 351C 4v that is bored .30 over and i figure that my max rpms will be around 6100 rpms. and i put 80% as my volumetric efficiency because they said 80% was a mild engine. anyway that formula says i need a caruretor that is about 500cfm so i would guess i would go up to a 525 cfm carb. But what do yall think about this it seems that this is kinda a small carb compared to what every one else talks about putting on their motors. so is this fomula right?
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  2. porkchop

    porkchop Founding Member

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  3. Ronstang

    Ronstang New Member

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    Get a 600 cfm carb. It is kind of like the universal small block carb. I have had great success with a 600 cfm carb on my first 66 Coupe and on the Shelby when it still had a singe 4 barrel. The 312 n my T-Bird even has a reworked 600 Holley on it and it works quite nicely also.
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  4. mak6363

    mak6363 New Member

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    That formula gives you carburetor size for best throttle response, fuel mileage and driveability. On my previous 70 Mach 1, I used a Carter 625 and loved the way it made my car perform. It had a Holley 600 on it but changing
    to the Carter made a big difference in the way it ran. I am planning on getting one for the Mach 1 I have now when I recam the engine in a few weeks. :)
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  5. ratio411

    ratio411 Founding Member

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    That chart shows the minimum carb size you need.
    Also, it doesn't take a wild engine to actually run well over 100% volumetric efficiency.
    The comment about economy and off idle response was dead on. That is why it is the minimum.
    Do the 3310 Holley (750 vacuum). It will only run as a 375 cfm carb unless the engine needs more. That way you will always be on the safe side and economical too.
    Throttle response is good to because the throttle bores are smaller than a stock 2v carb.
    Dave
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  6. D.Hearne

    D.Hearne Banned

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    The thing you have to rememeber about that formula, is the fact that it uses a fixed figure of "pull" for the vacuum level ( there's probably a better way to explain this, but it's saturday morning :D ) The problem here is, not every engine pulls air into itself at that level.Therefore the formula shouldn't be depended on for actual air flow needs of a certain engine. As ratio411 said, it's a good way to figure the MINIMUM size carb you should use, but not everybody's happy with the minimum. ( me for one) I would also recommend at least a 750 cfm carb for a 351C 4 bbl motor.A 600-650 will give you good response and mileage, but won't let the motor open up to it's maximum potential. A 500 cfm is WAY to small for it, hell I'm running a Holley 600 on the roller 5.0 in my 89 Ranger and it's perfect for it. :D
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  7. 12sec67

    12sec67 Active Member

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    351c 4v--- run the edelbrock 750 since the edelbrock carbs are over rated or go with 750 vacuum sec. 750 edelbrock is approx 700 holley if holley made a 700 :rlaugh:

    650 is going to be way to small for a big block, but i guess it all depends on what your going to do with it. ;)
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  8. D.Hearne

    D.Hearne Banned

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    Uh , Last I checked Holley's been making a 700 cfm carb for many years now. List # 80778 :rlaugh: :rlaugh:
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  9. 10secgoal

    10secgoal Active Member

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    Yeah,but when did edelbrock start making 650 is what I want to know. Everyone in here seems to have one. :D
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