Help me pick my next Carburetor please!

Discussion in 'Engine Topic' started by Vortec80, Mar 4, 2019.

What Carb should I use out of these?

  1. Holley 670 street avenger

    6.3%
  2. Holley 750

    68.8%
  3. My current Holley 600

    0 vote(s)
    0.0%
  4. Other...Please drop other suggestions in the thread

    31.3%
Multiple votes are allowed.
  1. Scott Sievers

    Scott Sievers Veteran Member

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    CFM = Cubic Inches x RPM x Volumetric Efficiency ÷ 3456.
    Any ordinary stock engine will have a volumetric efficiency of about 80%. Most rebuilt street engines with average bolt-ons have a volumetric efficiency of about 85%, while race engines can range from 95% up to 110%.

    Example: Using a 355 CID engine x 5,500 max rpm = 1,952,500

    Take 1,952,500 x .85 = 1,659,625

    Then 1,659,625 ÷ 3456 = 480 CFM

    Even with about a 10% cushion, a 500 CFM carburetor will handle this engine great.
    https://www.speedwaymotors.com/the-toolbox/what-size-carburetor-do-i-need/28634

    I built my first SBC in 1980, and back when I was sixteen, the bigger the carb the better... (We thought) Over the years, the struggle to get those big carbs to run right, eventually taught me that it was easier to get a smaller carb to run "right" than it was bigger carb. Jetting the bigger carbs down is harder than jetting the smaller carbs up. It is usually not the airflow that is the issue, it is the fuel to air ration, along with the accelerator pump (squirter) volume that needs tweaked.

    The last 434 I built had peak power at just over 7000 rpm, so works out to need 747.1938 cfm carb. Using this formula and then tweaking a 750cfm Black Diamond very quickly brought the AFR into line where we wanted it.

    https://www.holley.com/products/fue...l/black_diamond_series/q-series/parts/BDQ-750

    I have half a dozen old carbs on the shelf from those days when bigger was better. I have literally not touched, or found a good use for ANY of them in close to 20 years now.
    Don't be afraid of your 670. I am thinking even it might be too much carb, but it should be possible to make it work splendidly with a little tinkering. Use the closest thing you have, find someone with an AFR gauge, or buy one and install it. I have one on my break in stand, and have found it to arguably be the most important thing I ever bolted on that stand.

    https://www.amazon.com/AEM-30-4110-UEGO-Ratio-Gauge/dp/B00N3VGPYS

    You will be WAY happier with the results, and so will your engine! No more guessing.
     
  2. 1980RS

    1980RS Veteran Member

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    I have run just about every carb size on the street short of a Dominator. My current street ride has a bone stock Vortec 350 with headers and shiny intake, that's it. I have had at least 4 Eddy 500 to 750 carbs on it and all have some sort of drivability issues, know what I did? got out the Holley 750HP street VS that did not go too well on my race and stuck on that 350, guess what it runs perfect. Do I have a bunch of bad Eddy carbs, probably not but I do not want to spend hours on end fiddling with them. One was a new out of the box thunder 650 that run so fat at idle it make you thing there is a top fuel car running, don't have the patience to mess with'em anymore . It is no different with my race car, I have a 1000hp that runs to fat on it so it's a mis-applied carb. You just got to find the right carb that runs good, for me that more important than the beat mileage or being the fastest.
     
  3. COPO

    COPO Veteran Member Lifetime Gold Member

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    I would not use that cfm calculator. In '85 after a rebuild on my LT1 with 9:1 cr, a 600 Holley was running out of fuel/ air at 5,000 rpm. I sold that carb and bought a 750 Holley and it was fine all the way to 6,500 rpm.
     
    cadillac_al and 1980RS like this.
  4. Scott Sievers

    Scott Sievers Veteran Member

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    Wifes 355/LT1heads etc in her 82 camaro ran 10.9 at 134mph with a 650... as everything in life should be, you have to take it with a grain of salt, then modify and adjust. that calculator is for a baseline... having said that, I have been using that baseline since 79 and it has saved me a lot of time and headaches with customers.... typically the guy that brings in the magazine and points to a motor in a drag car and says... I WANT THAT... in my K5 blazer isn't going to be happy with the results. The same goes for choosing a carburetor.
     
  5. QUAKE_WARS

    QUAKE_WARS Veteran Member

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    You will have good luck with demon
     
  6. Vortec80

    Vortec80 Camaro project is a go

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    Thanks for all the help everyone. After trying different carbs, the trusty Holley 3310 was the best for my setup. Did not need much adjustment out of the box. Did not dyno it, but went by seat of the pants. Pulls noticeably harder than the others especially compared to my original Holley 600. Street avenger seemed a little problematic and needed a ton of adjustment. Ended up not trying the quick fuel. Couldnt get one from my buddy.

    If I can be of any help for info on the 750 I'll be glad to assist
     
  7. COPO

    COPO Veteran Member Lifetime Gold Member

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    Glad it worked out.
     
  8. G72Zed

    G72Zed Veteran Member

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    Glad to hear, but not at all surprised that the "old trusty" 3310 worked out the best. There's a reason why GM put them on so many SBC/BBC HP combo's.
     
  9. MoarThrottle

    MoarThrottle Veteran Member

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    Holley Sniper, leave the carb behind!
     
  10. 1980RS

    1980RS Veteran Member

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    The 3310 is a winner for most people. I just got a QF Hot Rod 750 DP for a song and it came with a real nice fuel log plus these neat little Thompson Power Plates that go under the Squirters. They are supposed to help the low end and my thinking is that it will help on a big open plenum intake so you won't need a giant squirter. Now I just have to see how the carb runs.
     

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