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Thread: Calculating Carburetor CFM

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  1. #1

    Default Calculating Carburetor CFM

    Hello everyone. So I took a Holley 1850 and milled off the choke horn, then installed thinned primary & secondary throttle shafts along with thin stainless throttle blades. How can I find out approximately how much airflow this this is pulling? Thank you.

  2. #2
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    Have it flow tested.
    May God's grace bless you in the Lord Jesus Christ.
    '92 Ford Mustang GT: 385" SBF, Dart SHP 8.2 block, TFS TW 11R 205 heads, 11.8:1 comp, TFS R-Series intake, Dominator MPFI & DIS, 36-1 crank trigger/1x cam sync, 160A 3G alternator, Optima Red battery, A/C, 100HP progressive dry direct-port NOS, Spal dual 12" fans/3-core Frostbite aluminum radiator, Pypes dual 2.5" exhaust/off-road X-pipe/shorty headers, S&W subframe connectors, LenTech Strip Terminator wide-ratio AOD/2800 RPM converter, M4602G aluminum driveshaft, FRPP 3.31 gears, Cobra Trac-Lok differential, Moser 31 spline axles, '04 Cobra 4-disc brakes, '93 Cobra booster & M/C, 5-lug Bullitt wheels & 245/45R17 tires.

  3. #3
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    Probably not much improvement over a stock 1850 600 Vacuum secondary. What engine is it fitted to?
    And as Danny has already pointed out, have it flow tested. Holley rates their 4 barrels at 1.5 in Hg. Gary
    Regards, Gary

  4. #4

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    Stock 383 Mopar.

  5. #5
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    Although it's good experience to do all of this to a Holley carb, you'll find little gain with a vacuum controlled secondary carburetor.
    On a 383, it would need to spin to 6000 RPM to flow 600 cfm at 1.5 inHg.
    You need a passenger to observe a manifold vacuum gauge reading at wide open throttle towards your RPM peak.
    If the gauge starts climbing, and reaches 1.5 inHg, then the carb is flowing 600 cfm.
    If the gauge gets to 3 inHg, which I doubt, then the carb would be flowing about 700 cfm.
    If you put this carb on a 632, it could flow 800+, but the engine would be pulling maybe 6 inHg, as the power starts falling off.
    So after your mods, and you have the carb flow tested, it may well have more flow, but will your stock engine pull that additional air?
    Only if the engine can spin higher RPM. Gary
    Regards, Gary

  6. #6

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    Thank you Gary, for the information.

  7. #7

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    So my 1850 Holley is flowing 640 CFM. Would it be in my best interest to put 650 vacuum secondary metering blocks on this? Or keep the 600 metering blocks on it?

  8. #8
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    There would be little to no difference between 600 & 650 4150 metering blocks. Have you fitted a secondary metering block in place of the metering plate? Does it have center hung float bowls? Gary
    Regards, Gary

  9. #9

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    Yes, I have a 34-6SA metering block on the secondary, and yes on the center hung bowls.

  10. #10
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    So now you have a 1850 that flows 640 at 1.5 Hg, and converted to metering block secondary, separate fuel inlets etc. It now depends on what secondary spring and main jet as too how much more power it will make over a stock 1850. Jet it two sizes up, and one spring lighter (from plain to green or purple), on the secondary as your first setup. Gary
    Regards, Gary

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