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Thread: Need help identifying a carburetor.

  1. #11

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    Not to hijack this thread, but I've used the "carb calculator" that's on the Holley site, and it says I should have no more than a 490 cfm. This is for a 340 c.i. Plymouth. Is this not correct?

  2. #12
    Join Date
    Jan 2010
    Location
    Brisbane, Australia
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    1,541

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    My personal opinion, and many others will agree.
    Depends on few factors, including state of tune.
    A 340 in motorhome is vastly different to a 340 in a strip car.
    A 340 in mild street car can run a 670 vacuum secondary, about 2 cfm per cubic inch.
    You could run a 570 if you are very mild.
    What are your vehicle details, camshaft, compression, weight, differential ratio, etc.? Gary
    Last edited by Gaz64; 07-10-2019 at 08:58 PM.
    Regards, Gary

  3. #13

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    Well, it's in a 1970 Duster, 8.75 rear differential with 3.91s. Compression ratio is stock, Edelbrock heads, 4-speed, XE268 cam. Can't say what the weight is.

  4. #14
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    Jan 2010
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    The thread has deviated so it might be better to start a new thread.
    I would be running at least a 670 vacuum secondary.
    There are guys out there running 950DPs on 355s. Gary
    Regards, Gary

  5. #15
    Join Date
    Mar 2013
    Location
    Baton Rouge, La
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    235

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    Quote Originally Posted by Gaz64 View Post
    Yes, if any engine flows enough air to flow the Holley rating system of 1.5 inches Hg, then the engine is under carbureted and power is left on the table. Example would be 355 with a 465, the engine will get to 4500 RPM, then the manifold vacuum will start to climb, as if you're lifting the throttle, this shows the carb is then a restriction. I run a 750 annular on one of mine, baby 308, and an 850 annular on a 434. Gary
    Hmm, OK am I understanding this correctly? If at WOT, at any RPM you have more than 1.5" Hg the carb is a restriction causing vacuum?

    Never noticed this before, most of my tuning started after I bought a Stealth Ram with an HP EFI system. (Before that I was shade tree by the ear. LOL) It flows 1000 CFM, so I never even contemplated the engine being anything other than atmospheric pressure at WOT.
    Richard
    1969 C-10 SWB Step
    355 Small Block
    700R4 Trans, 2500 stall & 3.73 gears

  6. #16
    Join Date
    Jan 2010
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    Any form of throttle valve control, carb or EFI, that allows the engine to develop vacuum in the manifold towards the high end, is limiting the engine power. A throttle body that flows 1000 cfm can only do that on an engine capable of flowing that amount of air. The throttle body will not flow 1000 cfm if fitted to an engine that can't. Gary
    Regards, Gary

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