Speed Demon 650 or 750 for this setup?

Discussion in 'Classic Mustang Specific Tech' started by allcarfan, Dec 10, 2003.


  1. allcarfan

    allcarfan The Answer Man Founding Member

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    lightweight 331 stroker
    Mildly ported twisted wedge heads (290/210cfm)
    xe274hr cam:
    * Advertised duration: 274 intake/282 exhaust
    * Duration at .050 in.: 224 intake/232 exhaust
    * Gross valve lift: .555 in. intake/.565 in. exhaust
    * Lobe separation: 112 degrees
    Ported and notched Performer RPM intake
    1 3/4" headers

    Engine will probably see 6700 rpms or so once in a while.

    So...Speed demon 650 or 750?

    I read an article where an engine made 400 at the flywheel and the 750 speed demon was too big.

    Shane
  2. gp001

    gp001 Founding Member

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    650. My 347 makes about 400 fwhp and the 750 was too big for it as well.

    Using 331 ci, 6700 max rpm, and 100% ve the formula says 709 cfm. Using 90% ve it comes out to 567 cfm. I don't think you'll ever see 100% ve, and you would still be over carbed.
  3. allcarfan

    allcarfan The Answer Man Founding Member

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    im probably going to be making closer to 475/500 hp at the flywheel. To my understanding the 650 flowed wet at 753cfm.

    Thanks for your explanation GP. It doesnt get much better than that.
  4. foghorn67

    foghorn67 I'll save you the time and choke myself Founding Member

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    650. especially with them demons, they flow alot more then holleys.
  5. brianj5600

    brianj5600 Active Member

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    A similar motor in MM/FF went over 100% ve. I would call tech at BG, they know there products better than any of us.
  6. Clark

    Clark Founding Member

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    Defintely 650, I have a 625 Road Demon on my 393W and it runs good.

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