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Choosing the right CFM for your engine

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by Paxx, Jan 22, 2004.

  1. Paxx

    Paxx 1/2 ton status

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    How do you determine what the proper cfm carb is for your truck. I used to think that the higher the # the better but I'm starting to think that depending on your application bigger isn't always better.

    How do you know? Is my 750cfm too big for my 355?
    I crave all your free knowledge /forums/images/graemlins/deal.gif /forums/images/graemlins/tongue.gif
     
  2. sled_dog

    sled_dog 1 ton status

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    750cfm, 355 won't have good throttle response. I have a 650 on my 355, if they made a Thunder AVS Off Road 600 I would have gotten that.
     
  3. Jonny-K5

    Jonny-K5 1/2 ton status

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    there is some formula for figuring this out, the higher your engine needs to rev and the more hp it makes the bigger carb you need. i think a 350 that redlines at 5K with 300hp should have a 600cfm. you need to be making like 450+hp and spin to 6000+rpm to accually need a 750cfm.
    if i find the formula ill post.
     
  4. rjfguitar

    rjfguitar 3/4 ton status GMOTM Winner

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    CID x maximum RPM
    -----------------
    3456
     
  5. Jonny-K5

    Jonny-K5 1/2 ton status

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    thats it , dang your quick /forums/images/graemlins/thumb.gif
     
  6. J.R.

    J.R. Guest

    check out this months chevy high performance magazine. there are many factors involved, what works on one 355 may not be the right one on a different 355. cam, compression,intake all are a factor in finding the right carb. for a mild 355 on pump gas with no nitrus a 600-650 cfm vac. carb will do. the easy way to find out is to call a carb or parts company and ask them. give the specs on your motor and they will tell you. good luck, hope this helps..
     
  7. dyeager535

    dyeager535 1 ton status Premium Member

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    yep, saw an article awhile back about carb selection, and the 350 they were testing made the best power with a 1000CFM carb, which by conventional wisdom was way too much.

    But with all the variables, your optimal will vary. 750CFM Q-jets were used on 6 cylinders, 800CFM's on 305's and 307's, so that may give you an idea of how mcuh engine variables and application play.
     
  8. rjfguitar

    rjfguitar 3/4 ton status GMOTM Winner

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    [ QUOTE ]
    thats it , dang your quick /forums/images/graemlins/thumb.gif

    [/ QUOTE ]
     
  9. Z3PR

    Z3PR Banned

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    I ran a 600cfm on a built 355 in a 72 Nova. No problem at all with 600 CFM for a small block in my opinion.
     
  10. bigburban383

    bigburban383 1/2 ton status

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    [ QUOTE ]
    How do you determine what the proper cfm carb is for your truck. I used to think that the higher the # the better but I'm starting to think that depending on your application bigger isn't always better.

    How do you know? Is my 750cfm too big for my 355?
    I crave all your free knowledge /forums/images/graemlins/deal.gif /forums/images/graemlins/tongue.gif

    [/ QUOTE ]

    Here is the formula:

    RPM for max power = Z
    Cubic Inch Displacement = Y
    Volumtric Efficiency = X (stock heads .80, better heads .85, performance .90)

    X((ZY)/3456) = W

    W((1.5/.5)^.5) = CFM needed
     
  11. mudjunkie 82

    mudjunkie 82 1 ton status GMOTM Winner

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    Carb size depends on what you plan on doing with your motor. An oversized carb will probably hurt you on the low end. An undersized carb will hurt on the top end.Call hollys tech support and talk with them and see what they recommend.
     
  12. Paxx

    Paxx 1/2 ton status

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    Sorry but how can I determine what my redline is? I'm not sure since i have an aftermarket tach that you set your own redline for.
     
  13. mudjunkie 82

    mudjunkie 82 1 ton status GMOTM Winner

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    it all depends how your motor is built. What is the upper R.P.M range of your Cam?
     
  14. Paxx

    Paxx 1/2 ton status

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    Thnx for all the replies so far /forums/images/graemlins/thumb.gif
     
  15. SUBFAN

    SUBFAN 1/2 ton status

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    You determine that. Does the motor pull hard to 7000 rpm's?

    Did you put enough money into the rotating assembly?

    Pick the highest your motor will see on a semi regular basis, taking into account the parts used to put it together.

    If you are throttle crazy, lean towards the 'big' side.

    If you like throttle response and don't rev the heck out of it, lean toward the 'smaller' side....
     
  16. Paxx

    Paxx 1/2 ton status

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    lol I've always been too scared to rev it past 6000 cause I thought it might be too high so I don't know if it will go to 7000. I would never rev it that high though. It usually sees no higher than 5-5500 because it starts to lose power after that.
    And I want to do a cam swap in the near future so after that it will likely see no more than around 4500rpm
     
  17. SUBFAN

    SUBFAN 1/2 ton status

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    I believe in rev limiters too....At times the brain does not care how high that motor is getting pushed, and the wallet is saying hold back /forums/images/graemlins/eek.gif
     
  18. 84_Chevy_K10

    84_Chevy_K10 Banned

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    [ QUOTE ]
    I believe in rev limiters too....At times the brain does not care how high that motor is getting pushed, and the wallet is saying hold back /forums/images/graemlins/eek.gif

    [/ QUOTE ]

    I never care, thank God I've got a rev limiter to keep me from being stupid again.
     
  19. bigburban383

    bigburban383 1/2 ton status

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    Make sure your engine gets balanced too or it will surely blow up.
     
  20. k20

    k20 3/4 ton status

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    here is a quote from the holley site [ QUOTE ]
    Engine Size (CID) X Maximum RPM / 3456 =
    CFM @100% Volumetric Efficiency
    EXAMPLE:
    350 CID X 6000 RPM = 2,100,000 / 3456 = 608 CFM


    Approximately 608 CFM would be required for this engine. However, most street engines are capable of achieving only about 80% VE; a modified street engine with ported heads, headers, good intake and carburetor can achieve about 85% VE; a fully modified race engine can achieve 95% or greater VE. The CFM number arrived at with this formula must be factored by this percentage.


    [/ QUOTE ]
     

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