Carb Guru Opinions Requested

Discussion in 'Fuel and Air Systems' started by ocdart, Oct 17, 2018.

  1. ocdart

    ocdart Inland Mopars Car Club FABO Gold Member

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    * Car: '69 Dart
    * Planned engine combo: 408cid, 10:1 compression ratio, EQ Magnum heads with 62cc chamber and 2.02"-dia. intake valves, cam specs 230 in./234 ex. @ .050 w/108 centerline .501 in. lift/.513 ex. lift, Edelbrock RPM Airgap intake, TTI 1.625"x1.75"x3.0" step headers, 3.0" exhaust to the rear bumper
    * Trans: 200-4R, manual-shift reverse pattern w/2800 stall lock-up converter, 2.74/1.57/1.00/0.67 spread
    * Rear: 8.75 Sure Grip w/4.30:1 gears (2.88:1 gearing equivalent when in OD)

    I'm strongly leaning toward a 750 cfm double-pumper carb w/annular boosters in order to maximize fuel vaporization for best possible mileage (yeah, I want the best of both worlds), but I realize the annular boosters are larger and cause a slight reduction in air flow volume - resulting in increased air flow speed.
    Is this air flow volume reduction enough for me to consider going to an 850 cfm carb instead in order to have enough volume for the engine? Or is the volume reduction too small to be concerned about.
    I ran a strong 340 build on the street back in the mid-70s and had to go to an 850DP in order to keep it from shooting ducks on the top end.
    I want to buy the correct carb the first time and not have to build an inventory experimenting.

    Thanks for your input!
     
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    • AJ/FormS

      AJ/FormS 367 FormS clone 3.09-1.92-1.40-1.09-.78od 3.55s

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      I don't know anything about strokers, but just wanted to say, nice combo!
       
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      • Cuprum-74

        Cuprum-74 Moparus Fanaticus

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        I'm currently using a Street Demon 750 tuned to 4500 ft above see level.
        So far I am extremely pleased with this carb. It's like a mashup between an AVS and a Thermoquad.
        I get almost 18 mpg highway, running 70 mph at 2900 RPM (8-13 around town depending if I drive Mad Max or not)
        And that's with a hopped up 1990's Roller cammed LA360 with 219/227 at .050 dur, .515/.530 lift Hyd cam (Lunati Voodoo) , SpeedPro pistons at 9.7 compression.
        Behind (currently for now) is an A999 non lockup transmission.
        Typically Vac. Secondary carbs are a little more street friendly especially behind an automatic transmission.
         
        Last edited: Oct 17, 2018
      • dartgarth

        dartgarth Well-Known Member

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        Nice combo BTW.

        If you want maximum performance I'd get the 850... but as with cams and carbs, its usually better to err on the smaller side. It would probably run better on the street with the 750. With that being said, if you are willing to strap the engine (or car) to a dyno to tune it, just go with the 850.

        I run a Sniper injection which is 800 CFM on my 383 with a cam that has less duration but more lift than yours with A518 transmission and 3.91 gears.

        Garth
         
      • AJ/FormS

        AJ/FormS 367 FormS clone 3.09-1.92-1.40-1.09-.78od 3.55s

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        Since we are waiting for the stroker guys to chime in, here's some food for thought
        I know it may not count for much, on account of it pertains to a smaller engine , but........
        but I run a similar combo in a 367; a lil more compression(10.9) and a lil more lift(549/571), but on a 110. And similar gear ratios with 3.55s,allbeit with a manual trans (3.09-1.92-1.40-1.00-.78od), and with a FinalDrive of 2.77
        >What I can tell you is in my combo, fuel economy does not exist. I have run it with 3 different intakes and 4 different carbs,all with similar results. So you might as well strap on the 850 annular discharge carb. I also ran it with a 292/108 Mopar cam, and a 223/230/110 Hughes, and currently with a Hughes 230/237/110.
        >So I think I know why this 230/237/110 cam of mine sucks gas, and why yours will be worse. The answer is IMO, in the power extraction duration. For this cam the duration events are;
        intake 276, exhaust 286, compression 116,extraction is 103, with overlap of 61, and compression plus extraction =219.
        By my cruise rpm of 65=2236, the vacuum has just peaked so overlap is not hurting me yet. But check out the meager extraction degrees of 103. When that exhaust comes out the dual 3" turndowns, there is still a lot of energy left in it. Energy that could have been used to propel the vehicle. Since it wasn't used, my theory is that I have to drive a wee bit deeper into the throttle to maintain cruising speed.
        >And I'll tell you how I came to that theory; the previous cam from the same manufacturer was a 223/230/110 cam with advertised of 270/276/110. The duration events were;
        intake 270,exhaust 276,compression 116,extraction 111, with overlap of 53, and compression plus extraction of 227*.
        >Ok notice the difference in extraction is 8 measly degrees.
        With the rest of the engine being exactly the same including the 750DP carb (with slightly leaner MJs of course), this combo made amazing fuel economy; even better with a purpose built 600VS
        >If you have a Magnum roller of 230/234/108, then your mileage will be worse for several reasons; starting with the 4inch stroke,the 10/1Scr, and ending with even more meager extraction.
        >If you have an LA FTH, it won't be quite as bad because they have faster ramps out of the gate, leaving a larger compression plus extraction total degrees.
        You know what the extraction is for an old 318LA? answer 120*
        >Some guys might blame the poor fuel economy on the overlap. And typically I would agree........ if you didn't have the overdrive. Cruising with 4.30 might get you 65@~3500 and at that rpm the headers could be yanking a lot intake charge right across the piston and out the still open exhaust; that is what headers do. But at 65=2330, reversion in the intake has barely stopped and the header is not yet yanking, IMO. So, I am reluctant to blame the poor economy on the overlap.
        >So workout your compression and extraction degrees and see where your specs compare. If you need help just ask.
         
      • MOPAROFFICIAL

        MOPAROFFICIAL Well-Known Member

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        750 cfm