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Help chosing a carb size 650 or 750

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  • #31
    Originally posted by BigAL View Post

    Given the fact that he is running a 1406 Edelbrock with is a 600 cfm carb, I don't care who you are, you don't know what your taking about if you think that is the right size carb for a 400 cubic motor. He's going to have to buy a carb anyways, save the money on the rebuild kit and just get a Holley 3310, it is a 750 vacuum secondary carb that will only meter what the engine needs for airflow do to the vacuum secondaries. It is the most logical way to go and he can get a new one on Ebay for $315.

    That's a nice theory. Unfortunately it doesn't work in the real world. Just like that stupid CFM calculator. You are talking to someone who has had every damn carb from 390's to 1050's on his engine. Over 200 carbs. That Edelbrock 600 can't flow enough air for a 400 ci engine. It will fall on it's face around 4000 rpm.
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    • #32
      Originally posted by Scott Liggett View Post


      That's a nice theory. Unfortunately it doesn't work in the real world. Just like that stupid CFM calculator. You are talking to someone who has had every damn carb from 390's to 1050's on his engine. Over 200 carbs. That Edelbrock 600 can't flow enough air for a 400 ci engine. It will fall on it's face around 4000 rpm.
      Didn't I just say that?

      Also just cause a carb says it a 650 doesn't mean that's all it flows, depending on manifold vacuum at WOT it can flow more or less cfm than it is rated at. Basically Holley's are rated at a 1.5" of manifold vacuum at WOT, that is when it flows 650cfm, if vacuum in the manifold at WOT goes up so does the CFM rating, basically a 650 that sees 6" of manifold vacuum at WOT would be flowing 1200 cfm. The opposite happens when manifold vacuum is less than the 1.5", if manifold vacuum at WOT is .5" then that 650 cfm carb will only flow about 500cfm at WOT. So really the best way to size a carb is to log manifold vacuum at WOT and get a carb that achieves 1.5" or less, as anything over 1.5" is a restriction and will cost you power.
      Last edited by BigAL; November 2, 2014, 11:03 AM.
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      • #33
        By the way on my dyno run, my manifold vacuum was 1.4-1.5 through the whole test range, which means that 830cfm carb was the right carb for the job.
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