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Easiest carb CFM formula ever...

Discussion in 'The Hokey Ass Message Board' started by Fat Hack, Dec 24, 2012.

  1. Fat Hack
    Joined: Nov 30, 2002
    Posts: 7,709

    Fat Hack
    Member
    from Detroit

    I get asked fairly often "What size carb should I run on my ________ engine?", and it's a question that pops up on this board from time... so I figured I'd offer up my time-tested, rule-of-thumb 'formula' for all to use...

    Double the engine's cubic inch displacement.

    That's it, really. Honestly, it works out perfect for almost ALL street/strip applications.

    A 350 engine? 700cfm carb.

    Got a 302? 604cfm.

    454? 908cfm.

    Obviously, a little rounding up or down will be in order...usually to the nearest 50cfm incriment to be able to buy a commercially available carburetor in the required size, but I've used this method since high school and it's always served me well.

    Hope it makes your next carb purchase a little simpler for you, too.

    Merry Christmas! :)
     
  2. the other me
    Joined: Jun 21, 2007
    Posts: 387

    the other me
    Member

    So I'm to believe the 500ci caddy motor waiting to go into an essex coupe will need a 1000cfm carb on it? Not too sure how tried and true, but to say it works perfect most all the time, doubtful. Yes, the caddy mill is not bone stock, it's been mildly persuaded to run a we bit better, let's just say it will easily be a street/strip ride. There are just way too many variables to have such a basic rule. Thanks for the input tho, and Merry Christmas to you and yours.
     
  3. Fenders
    Joined: Sep 8, 2007
    Posts: 3,922

    Fenders
    Member

    Easy formula, but is seems a bit high... Maybe double then subtract 100 ?
     
  4. 3wLarry
    Joined: Mar 11, 2005
    Posts: 12,804

    3wLarry
    Member Emeritus
    from Owasso, Ok

    triple cubic inches, then divide by Pi, multiply by two, then subtract 28.

    this only works on thursday though...
     
    kidcampbell71 likes this.

  5. George
    Joined: Jan 1, 2005
    Posts: 7,383

    George
    Member

    One is...CID X 2 X .8=CFM Reality is that a much wider envelope will work w/o much problems.
     
  6. Geee according to all you naysayers the 1200 cfm on my 306 Ford might be to much??
     
  7. tfeverfred
    Joined: Nov 11, 2006
    Posts: 15,792

    tfeverfred
    Member

    Check the manufacturers application chart? Works for me.
     
  8. el Scotto
    Joined: Mar 3, 2004
    Posts: 4,357

    el Scotto
    Member
    from Tracy, CA

    I've never understood people's inability to figure out carburation... :rolleyes:
     
  9. This is the same one I use!

    As long as you can manage your calendar, it never fails! :D
     
  10. Here's mine: Put a 750 Quadrajet on it.
    Used on 230 cammer 6 cylinders to 500 c.i. Caddys
     
  11. 3wLarry
    Joined: Mar 11, 2005
    Posts: 12,804

    3wLarry
    Member Emeritus
    from Owasso, Ok

    or spell it...(carburetion) :p...sorry, couldn't help myself...
     
    kidcampbell71 likes this.
  12. Shane Spencer
    Joined: Oct 3, 2009
    Posts: 2,159

    Shane Spencer
    Member

    Agreed, it really isnt hard to figure out carb cfm CORRECTLY

    Sent from my DROID device using the TJJ mobile app
     
  13. 333 Half Evil
    Joined: Oct 16, 2006
    Posts: 1,440

    333 Half Evil
    Member


    306 x 2 = 604 so your theory must be cid x 4!!! :eek:
     
  14. 333 Half Evil
    Joined: Oct 16, 2006
    Posts: 1,440

    333 Half Evil
    Member

    that what test-n-tune is for!!!

    sent from my hemroid desktop usin fat fingers and a keyboard no app
     
  15. Fenders
    Joined: Sep 8, 2007
    Posts: 3,922

    Fenders
    Member

    Especially with vacuum secondaries.
     
  16. carbking
    Joined: Dec 20, 2008
    Posts: 2,978

    carbking
    Member

    A guy tries to make life easy, and he gets beat down.

    So, a little quiz.

    What size carbs would you guys put on the following:

    1947 Ahrens-Fox truck 6 cylinder (935 CID)

    Seagrave Fire Truck 6 cylinder from about 1930 (1013 CID)

    or somewhat off-topic as it is industrial

    Caterpillar model 70 4 cylinder (1308 CID)

    and the largest car engine or which I have documentation

    Pierce-Arrow model 66 6 cylinder (825 CID)

    Merry Christmas to all!

    Jon.
     
  17. slammed
    Joined: Jun 10, 2004
    Posts: 8,151

    slammed
    Member

    One of the reasons it is hard to figure for some (or most) is the rodder mentality of 'more is better'. Next common mistake: Camshafts! They want that rad, bitchin', radical, wicked sound and still cruise without a hitch.
     
  18. gimpyshotrods
    Joined: May 20, 2009
    Posts: 17,908

    gimpyshotrods
    ALLIANCE MEMBER

    Yup, it is just not terribly common that it actually happens.:eek:
     
  19. gimpyshotrods
    Joined: May 20, 2009
    Posts: 17,908

    gimpyshotrods
    ALLIANCE MEMBER

    750, 800, 950, 600. All vacuum secondary.
     
  20. Let me guess, they all use a 275 cfm carb, right?
     
  21. oj
    Joined: Jul 27, 2008
    Posts: 6,257

    oj
    Member

    What is so slick about Jons' quiz is that they are inline engines - the Cat might be a V style - but there won't be any overlap on the intake stroke. In short, a carb large enough to feed one cylinder is whats' needed - unless the intake is split.
    With a V8 engine and overlap of intake into a common plenum the carb sizing is entirely different.
     
  22. tjm73
    Joined: Feb 17, 2006
    Posts: 3,386

    tjm73
    Member


    That is not entirely inaccurate. Until you get into wild, over the top builds it's fairly accurate. A 302 Ford needs between 650-750cfm, My brother has a pretty stout 418, upon which he is running a heavily worked 750, that while not flow tested we think is closer to 850cfm.
     
  23. el Scotto
    Joined: Mar 3, 2004
    Posts: 4,357

    el Scotto
    Member
    from Tracy, CA

    [​IMG]

    :D
     
  24. gimpyshotrods
    Joined: May 20, 2009
    Posts: 17,908

    gimpyshotrods
    ALLIANCE MEMBER

    Better take the 500 off of my 305, and put on a 610, then. Wonder what that will do to my 28.5 MPG.
     
  25. saltflats
    Joined: Aug 14, 2007
    Posts: 10,599

    saltflats
    Member
    from Missouri

    How about a Crower stack injection.
     
  26. GregCon
    Joined: Jun 18, 2012
    Posts: 689

    GregCon
    Member
    from Houston

    The problem with CFM formulas is they ignore mixture quality. They also ignore the fact that carb must flow fuel as well as air. They also seem to predicate their results on the wrong idea that if an engine needs 700CFM at full throttle then a 700CFM carb must be the best when in fact it's not such a great idea to have your carb operating at its max capability, ever.
     
  27. Deuces
    Joined: Nov 3, 2009
    Posts: 19,692

    Deuces
    Member
    from Michigan

    I'd rather have a Hilborn setup...:)
     
  28. dad-bud
    Joined: Aug 22, 2009
    Posts: 3,884

    dad-bud
    Member

    CFMs depend on motor size, max revs and volumetric efficiency (without trying to get all technical on Christmas Eve).
    All the go-fast magazines that do motor dyno test seem to find that an over-size carb will eke out an extra couple of horses at the very top end, but off-idle response suffers.
    But a calculation based on CID/1728 x max RPM x 60 x Volumetric Efficiency (depends on how 'good' your motor flows) will give you CFM.
    600 CFM is about as big as you want to go for a 350 that won't see more than 6000 rpm on the street. For a 500 CID Cad that won't see more than 5000 rpm or so, a 750 cfm carb will be enough.
    Spreadbore is good for light throttle openings - you only run on the small primaries until you step on it.
    That's pretty much the arrangement GM used with their Rochesters and it kinda worked on millions of cars for them.
    Anyway, I hope this helps.
    In the meantime, Merry Christmas to all.
     
  29. You all have missed the biggest point here and that is V.E.




    Yes, Volumetric Efficiency of a engine in the calculations and theory of chosing carb CFM. For most of us a 80% V.E. factor should be considered into thier formula whether you use the O/Ps formula, or some long drawn out one from one of the available carb manufacturers. Simply meaning, take you fiqure and multiply by .8. A 1000 CFM carb now becomes 800 CFM, a 700 CFM carb now becomes 560.

    And now you are saying TR doesn't know anything, a 560 carb on a 350 is rediculous. Not if you are looking for economy as well as efficiency, ease of starting, emissions and average performance.

    You start adding or changing componants to increase V.E., and the requirements for CFM go up accordingly. Even changing to roller rockers, and or correcting valve train geometry such as pushrod legnth, you have increased V.E., and the CFM requirements have now increased from the 80% baseline. I do this for a living folks, and have taught the theory as well in a Tech school.

    On the other end of the spectrum could be a Forced Induction engine such as a Roots style Supercharger, which could easily be over 115% V.E., and Turbochargers even more again. If you want to split hairs, a Draw-Thru carb although on a very high V.E. turbocharged or supercharged engine requires a smaller CFM carb than the normal calculations would recommend. That is a lesson for another day, Merry Christmas All, TR
     
  30. I was running a 650 eddy on a 300 cid Buick engine. That large a carb on that small an engine would not perform up to its full potential. At rpm below 2500 it was not flowing enough volume of air to properly mix fuel at the ventury's. And it just was pretty "flat" (and thirsty) around town. I switched to a 550 cfm eddy and now it has much crisper throttle response and pulls 20+ mpg freeway(vs. 16 mpg before). I used the formula fat hack used, but subtracted 50cfm for less than perfect v.e.
     

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