1. AJ/FormS

    AJ/FormS 68 B'cuda fb, Form S clone ... 367/A833/3.55s FABO Gold Member

    Messages:
    22,054
    Likes Received:
    9224
    Joined:
    Jan 19, 2014
    Location:
    South-Central Manitoba,Canada, 900ftelevation
    Local Time:
    8:41 AM
    Well you asked for it, lol.
    The TKX-ratios are;
    2.87-1.89-1.28-1.00-.72 ; and splits of; .66-.68-.78-.81
    3.27-1.98-1.34-1.00-.72 ; and splits of; .61-.68-.75-.72
    For reference, my ratios, (IMO, nearly perfect), are;
    3.09-1.92-1.40-1.09-.78 ; and splits of; .62-.73-.78-.72

    First notice that the 2-3 splits are the same, and so become the hinge-point for all the rest, up or down.
    Next notice the short 3-4 of the .78, set up for small engines, so the rpm does not drop so far. at 5500 the difference is 165rpm. at 6500 it is 360 rpm.
    Overdrive is 65= 2243 versus 1994, so which one you chose depends very much on your cam; as both are pretty low..

    Now the thing you are concerned about;
    the effective first gears are ; (corrected to a 24" tire)
    3.55 x2.87 x24/28= 8.73, and
    3.55 x3.27 x24/28= 9.95 . I run
    3.55 x3.09 x24/26.6=9.90 And I can tell you that with a hi-Torque, 430 hp (by the 93mph trapspeed in the Eighth) 367SBM, that I would NOT run any less. I have run a lil less and a lot more, and for my combo, 9.90 is just about right, and that is why it has been this way since about 2005ish.
    Now; trap rpm at 60mph, in second gear; the thing I was concerned about;
    With 3.55s and 28s your rpms are; 4830 with the 1.89, and 5060 with the 1.98. My cam peaks at closer to 5100, so, of the two choices, I would chose the 1.98.
    And second to last, and of real importance to me is;
    what is my slowest possible speed. At 700rpm with 28s and 3.55s this comes to 5.72 with the 2.87, and 5.03 with the 3.27s ; at 550 idle-rpm, it would be 4.5 and 3.9, now you are getting close to walking speed.

    And finally; My Opinion
    Since, your lowest in-gear speed is not low enough to walk/parade at, and
    since your rpm at 60MPH in Second gear is only 4830/5060, and
    your rpm in overdrive at 65mph is only 1994/2243
    If it was mine;
    I would swap the 3.55s out for 3.91/4.10s, and run the 3.27 trans because;
    With the 3.91s;
    If I can idle that engine down to 550 in gear, that will be 3.58mph. and
    60mph in Second gear is 5574. and
    65mph in od is 2200, and
    I don't have to put up with that short-fourth.
    Your starter gear would be;
    3.27 x 3.91 x24/28=10.96 corrected to a 24" tire. I know what yur thinking and by the math, you are sorta right. But in practice, you will love it. And besides, with a 414stroker at WOT, you will be spinning the street tires all the way to 60 mph, and beyond, anyway. But even better is, when you go to take off, you can almost just put it in gear and drive.
    In mine, I just give the heavy stock flywheel a lil blip, ease the pedal out, and go. If you were in a car in the next lane at a stoplight, you might not even catch that my car has a manual trans. There is none of this Mustang slip it halfway to next Sunday bs.
    Just blip,slip,and Zip

    I think you got stuck on something; you say you don't care about the splits but check this out.
    2.87-1.89-1.28-1.00-.72 ; and splits of; .66-.68-.78-.81
    3.27-1.98-1.34-1.00-.72 ; and splits of; .61-.68-.75-.72

    Taking off normally, revving to say 3000 for the 1-2 shift, the drop into Second will be to;
    3000 x.66=1980 with the 2.87 trans and
    3000 x.61=1830 with the 3.27
    You got no TC to multiply your torque, so at these rpms your 414 will need some grunt, and that is where the 3.91s come in, multiplying whatever is coming out of the trans by 10% more than the 3.55s would. Cuz let's face it, it's a Mopar; you don't really want to rev it to 4000 to put it into Second, while just tooling around,.... like one of those other cough-cough, small-block, classic, also-rans; now do you? lol
    IMO, the splits are more important than the actual numbers.
     
    Last edited: Dec 23, 2020
    • Agree Agree x 1
    • srduster340

      srduster340 FABO Gold Member FABO Gold Member

      Messages:
      909
      Likes Received:
      102
      Joined:
      Jan 17, 2007
      Location:
      Decatur,Ga.
      Local Time:
      8:41 AM
      Damn dude that's some awesome info! I'm gonna keep the 3.55s. I've got the rear dialed in. No noise, whining, howling, etc after about 5000 miles. I'm afraid of touching it.
       
    • srduster340

      srduster340 FABO Gold Member FABO Gold Member

      Messages:
      909
      Likes Received:
      102
      Joined:
      Jan 17, 2007
      Location:
      Decatur,Ga.
      Local Time:
      8:41 AM
      I've decided that the 2.87 TKX 1st gear may be best for my combination. With 3.55's and 27560R15 tires, I cruise 60mph@2500-2600. Motor makes 465ft;lbs@3000, peaked 510ft;lbs@3900. Car weighs 3400 with me in it. Now, which OD do I get? .81 or .68? Pros & cons?
       
    • AJ/FormS

      AJ/FormS 68 B'cuda fb, Form S clone ... 367/A833/3.55s FABO Gold Member

      Messages:
      22,054
      Likes Received:
      9224
      Joined:
      Jan 19, 2014
      Location:
      South-Central Manitoba,Canada, 900ftelevation
      Local Time:
      8:41 AM
      Do you have a choice in od gears?

      If the engine is in and running;
      Just put a vacuum gauge on the intake and slowly rev her up, watching the Vacuum. When it plateaus, drop her back a bit watching for the vacuum to drop. As soon as it does, grab the rpm. That rpm is the lowest rpm at which your engine is becoming efficient. The valve timing is finally trapping the mixture in the chamber, instead of allowing some to return to the intake manifold. And that is my minimum cruising rpm for fuel-economy.
      Now, at that rpm, the timing will want to be waaaaaaaaaay more than the mechanical advance mechanism can supply. At say 2600 rpm, the engine may be wanting 50 or more degrees. If you do not supply it, your fuel-economy will suffer.A lot. So that is the job of the Vcan. The slower rpm that you cruise at, the harder it will be to meet the engine's cruise-timing needs.

      To figure out what she wants;
      just rev it up to your chosen cruise rpm and begin advancing the timing, always maintaining the cruise rpm. When the rpm no longer rises with additional timing, at the chosen rpm, read the timing. That is what she wants with no load.
      I build my curve to ~3* less, then fine-tune, with my dash-mounted,dial-back, timing retard box. You will not notice an MPG difference in those 3 degrees so don't spend 3 summers looking for the ideal number. like I did,lol.

      But if the engine is not in and running, then I can offer only this;
      Cruising at too low an rpm for what is going on in the intake,and with not enough timing, is worse for fuel economy than; a little too high an rpm with adequate timing.
       
      • Like Like x 1
      • Agree Agree x 1
      • srduster340

        srduster340 FABO Gold Member FABO Gold Member

        Messages:
        909
        Likes Received:
        102
        Joined:
        Jan 17, 2007
        Location:
        Decatur,Ga.
        Local Time:
        8:41 AM
        I can choose either the .81 OD or .68. With the 3.55 rear axle and .81 OD I can cruise 70@2400, with the .68 I cruise 70@2000. My engine is built for low end torque. It made 465ft;lbs@3000, peaked 510ft;lbs@3900. I can be cruising 60mph@2600, floor it and it pulls really hard. I just want to make sure I don't lug the engine while cruising the highway. I admit I'm a little paranoid about choosing the wrong OD gear. Yes, the engine is in the car and running.
         
        Last edited: Dec 30, 2020
      • AJ/FormS

        AJ/FormS 68 B'cuda fb, Form S clone ... 367/A833/3.55s FABO Gold Member

        Messages:
        22,054
        Likes Received:
        9224
        Joined:
        Jan 19, 2014
        Location:
        South-Central Manitoba,Canada, 900ftelevation
        Local Time:
        8:41 AM
        and that is why I explained how to get your cruise-rpm into the ballpark.
         
      • DionR

        DionR Well-Known Member

        Messages:
        2,764
        Likes Received:
        680
        Joined:
        May 7, 2008
        Location:
        Spokane, WA
        Local Time:
        6:41 AM
        So which one did you go with?
         
      • Muswagon

        Muswagon Well-Known Member

        Messages:
        82
        Likes Received:
        41
        Joined:
        Jan 20, 2020
        Location:
        Winnipeg
        Local Time:
        8:41 AM
        You will not be happy with a 3.27. I had a 2.87 first gear TKO in my 73 Mach 1 with a 408W, 3:91 rear gear and 295/60/15 drag radials. The thing was an absolute beast everywhere, especially off the line, and the close ratio made it stay right in the prime RPM range.

        3.09 and 3.27 type gearing, in my humble opinion are relics from the smog motor era (the king was the 80-81 Z28 with 3:27) where they made a car feel torquey off the line, with nothing after that.

        My suggestion would be to order the 2:87 and enjoy the close ratio. You can always mess with rear end ratio if you feel the low end is not what you want. Lower rear end gearing is kinda the whole point of a 5 or 6 speed anyway...again just my opinion.
         
        • Like Like x 1
        • goldduster318

          goldduster318 Overzealous Car Modifier

          Messages:
          1,764
          Likes Received:
          338
          Joined:
          Aug 25, 2007
          Location:
          Lake Orion, MI
          Local Time:
          9:41 AM
          I would get the 2.87 and the .68 OD. I have a T56 Magnum with the 2.97 1st, 3.23's, and 25.6" tall tires and its just about right for a street car. I'm running a 340, but with EFI and I can pull 1500 rpm and the car is happy, even to go up hills, drive around a lot at 55 or more in 6th.

          My engine made 400 lb-ft @ 2800 rpm on the dyno. I should probably have less torque than you do and I like it. The EFI does help with pulling a low RPM but if you want to drive on the freeway at 70, it will run 1900 rpm which is pretty nice. If you only ever go 60 mph, pick the .81
           
        • DC Dart

          DC Dart Well-Known Member

          Messages:
          51
          Likes Received:
          21
          Joined:
          Dec 18, 2019
          Location:
          Oregon
          Local Time:
          6:41 AM
          SST advised me to get the 2.87 1st and .68 OD with 355 and 225 60 15 tire. Going to run an efi big block 451. Hopefully working on it this weekend.
           
        1. This site uses cookies to help personalize content, tailor your experience and to keep you logged in if you register.
          By continuing to use this site, you are consenting to our use of cookies.