Loading ...
Sorry, an error occurred while loading the content.

Formula for figuring carb size?

Expand Messages
  • Juan Ayon
    Posted by Dan(The Old One) on 12/1/2002, 11:12 pm , in reply to Re: What size carb should I use? on the MTS Messageboard.. The problem with the standard
    Message 1 of 3 , Dec 3, 2002
    • 0 Attachment
      Posted by Dan(The Old One) on 12/1/2002, 11:12 pm , in reply to "Re:
      What size carb should I use?" on the MTS Messageboard..

      The problem with the standard formula is that it doesn't figure in
      the fact that a carb is tested with 1.5" vacuum. You DO NOT want any
      manifold vacuum at wide open throttle. If you get a manifold vacuum
      reading at WOT, your carb is to small.
      I posted a link to Edelbrocks sight below. Click on "Technical
      Information" (lower feft). Now 2nd one down, "Technical Discussion".
      In here you will find many formulas, including the CFM= CID X RPM X
      VE / 3456.

      Using this formula, I get this: 500 X 5600 X .8 / 3456 = 648CFM.
      This is the amount of air flowing into the motor. To find what size
      carb, you would need to convert these numbers to "carb test
      numbers". 1.5" for a 4bbl and 3" for a 2bbl. A 650CFM carb would be
      to small. BY a bunch.

      Near the bottom of the page, is a section titled "CFM Rules". In
      this section they state that if running a single plane intake, a
      carb that is 110% to 130% bigger is needed. With your dual plane,
      120% to 150% bigger.

      Now if we take the 648 CFM and figure the 120-150% in, we get: 778-
      972 for a carb size. The 800 being right in there on the low side of
      the scale. For street driving, better throttle response and fuel
      mileage could be expected over, say a 950. The 950 would probably
      edge the 800 out in total hp numbers though.

      If the VE numbers where raised to 90%, a 5600 rpm 500 could in fact
      use a 1050 Dominator on a dual plane intake. This would explain why
      at least 2 are using a Dominator, and a bunch more an 850 DP.

      My informed, or misinformed opinion is that a 800 Edelbrock carb
      will work fine on a street strip car. If you are racing only, a HP
      series 950 Holley would be nice. Dan

      --Previous Message--
      : Try this standard formula for choosing
      : carb size: Multiply your cid X
      : your maxrpm. Divide that by 3456.
      : Then apply a chosen factor for
      : "volumetric efficiency",
      : which in most street applications,
      : should be around an optimistic
      : 0.8. Any good carb book (Holley,
      : Carter, Rochester) will have this
      : formula and elaborate on it in
      : depth. If you go by that, it is
      : hard to imagine that you could go
      : wrong. All you have to deal with
      : then, is informed or misinformed
      : opinions. I would recommend
      : getting one of those books, which
      : are available at most any
      : speedshop. Then you'll be armed
      : with facts, for when you start
      : getting bombarded with all the
      : "personal facts".
      :
      : --Previous Message--
      : I'm building a 500 cad with 10:1
      : comp.,
      : MT15 cam, 76cc heads w/ oversize
      : valves, Edelbrock intake manifold,
      : HEI dist. and headers. I was
      : going to use a 800 Edelbrock but
      : I'm being told this is too small.
      : Is anyone running this combo w/ an
      : 800 cfm carb? Does it work? Any
      : help would be great.
    • C. Gregory Talbott
      If you eliminate the ve from the formula you come up with 810 CFM. I have used the CID x RPM / 3456 for years and it has always come pretty close if the max
      Message 2 of 3 , Dec 4, 2002
      • 0 Attachment
        If you eliminate the ve from the formula you come up with 810 CFM.
        I have used the CID x RPM / 3456 for years and it has always come
        pretty close if the max rpm's are correct.
        780 CFM w/ vacuum secondaries works good on 500 street engines.
      • Juan Ayon
        Yup, leaving out the ve works too. I won t argue with you on that one. That formula should work for your average street rodder, like myself. lol...
        Message 3 of 3 , Dec 4, 2002
        • 0 Attachment
          Yup, leaving out the ve works too. I won't argue with you on that
          one. That formula should work for your average street rodder, like
          myself. lol...
          --- In Cadillac_Performance_Association@y..., "C. Gregory Talbott"
          <cadillacsgalore@y...> wrote:
          > If you eliminate the ve from the formula you come up with 810 CFM.
          > I have used the CID x RPM / 3456 for years and it has always come
          > pretty close if the max rpm's are correct.
          > 780 CFM w/ vacuum secondaries works good on 500 street engines.
        Your message has been successfully submitted and would be delivered to recipients shortly.