help setting up my 8 3/4!!!

Transmission and Drivetrain Tech

  1. carls72dart

    carls72dart FABO Gold Member FABO Gold Member

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    Hey Fabo members, I'm again in need of your help! My goal is to take my Dad on a trip down Rt.66. I'm building an 8 3/4 w/wilwood disc brakes. My question is what gears can I use in my set up? I'd like to get some mpg's!:) Any and all help is always greatly appreciated!!:) here is my set up:

    I have a rebuilt 340 with a rebuilt 904 transmission. The engine has the following:
    1973 block bored .030” over
    10.5 to 1 TRW forged pistons w/moly rings, rods resized and bushed w/floating wrist pins and new locks
    Pistons and rods balanced
    ARP bolts crank polished
    Clevite 77 rod, crank, and cam bearings
    Windage tray
    Dynagear tru-roller timing chain
    Crane cam 284/480 degreed
    J heads, 3 angle valve job
    Comp Cams locks and push rods
    Mopar perf. valve springs and adj rocker arms
    Edelbrock Torker II intake
    Edelbrock thunder series 500cfm carb
    Mopar perf. black alum. valve covers. Electronic ign. distr.
    Carter fuel pump
    Mellings oil pump

    The 904 transmission had also been rebuilt with performance parts. It has a Dynamic 3000 stall torque converter, reverse manual valve body, and an Cheetah SCS shifter with console.
     
  2. cosgig

    cosgig MoBro Inc. FABO Gold Member

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    Ma Mopar called the 3:23 their "maximum performance" ratio! A pretty good balance between economy and fryin tires!!!
     
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    • replicaracer43

      replicaracer43 Old school member

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      with that converter, 3:55 would be a good match. 3:23 would be ok too, at highway speeds, maybe not as much around town. You really don't want your cruise rpm to be much lower than your stall speed, or you will have slippage, and loss of mpg. Tighter converters are a big help in the mpg department.....
       
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      • carls72dart

        carls72dart FABO Gold Member FABO Gold Member

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        Thanks for the feedback guys!:) I was hoping 3:55's would work. One day I'm putting a passon 5 speed manual w/4.10's!;)
         
      • Woods74

        Woods74 Quick Senior

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        How about this: Stick that sucker in selected second and take it for a romp.

        Where does it make the most seat of the pants power in and around 3k? Stick your cruise rpm right there. That will balance power and economy.

        I see it time and time again, you pick a gear that lugs the engine and when you rev it up a little you get better mileage. Cam and converter selection is a big part of it too.

        Best fuel economy comes around bottom of the torque curve.

        Screw the voodoo. Read this:

        http://www.jalopyjournal.com/forum/showthread.php?t=773608
         
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        • dustoff440

          dustoff440 Well-Known Member

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          • RustyRatRod

            RustyRatRod 30 Degrees Outta Whack FABO Gold Member

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            If you go with too much of a highway gear with that converter you will slap kill mileage because the converter will be slipping badly at highway speeds. I would go with the 3.55.
             
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            • halifaxhops

              halifaxhops It's going to get stupid around here! FABO Gold Member

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              Nice calculator!
               
            • fishy68

              fishy68 Tyr Fryr's Inc. FABO Gold Member

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              One thing not mentioned here is the tire height has a big effect on the "final drive ratio". If you run 3.23 gears with a 25" tall tire it will spin more rpm down the road than a 3.55 gearset with 28" tall tires. Just something to keep in mind when deciding on what gear to run
               
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