What length of transmission do I need for 71 duster?

Transmission and Drivetrain Tech

  1. Wizard

    Wizard Well-Known Member

    Messages:
    518
    Likes Received:
    21
    Joined:
    Jan 23, 2012
    Location:
    Dillsburg, PA
    Local Time:
    3:13 AM
    Have a 71 duster and 318 motor. Want to put in a 3 speed for now and then eventually a 4 speed. So, what year 3 speed can I use and the length of it from mearsuring from tip of input gear to the end tip of trans gear or does it really matter then just find a driveshaft to work? My setup is going to use a 65 B body 8 3/4 rear.

    Thanks,

    Wizard
     
    • Like Like x 1
    • j par

      j par Well-hung Member

      Messages:
      19,367
      Likes Received:
      9474
      Joined:
      Jul 2, 2014
      Location:
      Portland Oregon
      Local Time:
      1:13 AM
      I'm pretty sure as long as it's out of an abody it doesn't matter which transmission...
       
      • Agree Agree x 1
      • valiantwagonguy

        valiantwagonguy FABO Gold Member FABO Gold Member

        Messages:
        777
        Likes Received:
        479
        Joined:
        Oct 20, 2005
        Location:
        Long Island
        Local Time:
        4:13 AM
        Any A body trans will fit but with that rear you are probably going to need a custom length driveshaft no matter what trans you go with. A 3 and 4 speed will be the same overall length so that won't be an issue. 66 and up manuals use a splined drive shaft 65 and down use a ball and trunion so stay away from those.
         
        • Agree Agree x 1
        • j par

          j par Well-hung Member

          Messages:
          19,367
          Likes Received:
          9474
          Joined:
          Jul 2, 2014
          Location:
          Portland Oregon
          Local Time:
          1:13 AM
          The 904 automatic is also the same length as the four Speed just a different size slip yoke...fyi...
           
          • Like Like x 1
          • Agree Agree x 1
          • 66fs

            66fs Well-Known Member

            Messages:
            4,202
            Likes Received:
            1059
            Joined:
            Sep 9, 2009
            Location:
            Greenwood, South Carolina
            Local Time:
            4:13 AM
            An A Body 4 speed uses the same length drive shaft as a 904 Auto Trans. I'm pretty sure the HD 230 3 speed for an A Body is the same length as the 4 speed but not sure. There are a230 versions for B and E bodys that are longer with the shifter locations for those bodies. Darts use a longer driveshaft than Barracuda, Duster, and Valiants.
             
          • 66fs

            66fs Well-Known Member

            Messages:
            4,202
            Likes Received:
            1059
            Joined:
            Sep 9, 2009
            Location:
            Greenwood, South Carolina
            Local Time:
            4:13 AM
            Some 4 speed slip yokes are the same as a 904 as is the a230 3 speed.
             
            • Thanks! Thanks! x 1
            • AJ/FormS

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

              Messages:
              17,092
              Likes Received:
              5626
              Joined:
              Jan 19, 2014
              Location:
              South-Central Manitoba,Canada, 900ftelevation
              Local Time:
              3:13 AM
              use the full-synchro A230 side cover trans, from 70/71 up.It is 23.2" from the front of the case to to the end of the rear housing.
              The ratios are quite far apart but work well with the stock hi-torque 318. But are crappy with a low-compression 340 cammed doggie 318. I kindof remember them being pretty much like the 4spd od box. something like 3.08-1.70-1.00; this was a slanty set-up.
              I think there was a close ratio version; 2.55-1.49-1.00 which would be the smarter buy for a V8, especially; if you have a cam in a smogger-teen. But it will require a matching rear gear of about 3.55s or better.
              Either is identical,dimensionally, so you have to count the output turns in first gear, compared to one input turn.
              xxxxxxxxxxxxx
              You might imagine to like the 3.08 version but that huge 1-2 split of .55 is eventually gonna get old.
              Just don't slip a 340 cam into that 318. And here's why;
              the 340 cam will drop your already very low cylinder pressure of perhaps 135/140, down into the basement to something like 120psi; making your 318 into a total dog below ~3500 rpm.
              So then the 340 cam when driving around town, with the slanty trans, even with 3.55s, is gonna be at 32mph= about 2450rpm in second. When you slam it down into first, the Rs will rise to ~4500, and the 340cam, is gonna want to be shifted about 4mph later,at about 36mph. At the 1-2 shift the Rs come crashing down to 2700; and then,working with that sick, cylinder pressure, your world has just become a dark, dark ,black hole.
              IDK about you but when I'm just taking it easy, I like to upshift 1-2 at about 2800, cuz my engine sounds good there. If you do that with the slanty trans, your Rs will fall to 1550, and with such low pressure you might have to use a lotta lotta throttle to continue accelerating. There is no upshift rpm to overcome this, cuz you will first run outta first-gear rpm; 5000 in first will drop to 2750 in second.
              In my experience this is altogether unacceptable. This not a good combo.
              So in that light, I highly recommend to stay away from the 3.08low trans; when using a big cam, in a low-compression 318. Don't do it. It will be sorta fine with a stock 318 and 3.55s or better.
              BTW, some people say that 340 stick is not big cuz it is only spec'd at 268/276/114. That is very true. But when you install it into an 8/1 318, the engine sees it as the worst nightmare imaginable..... below 3500......... which is where 95% or more, of your driving is gonna be, and you don't even have a TC to help her get thu it. The Mopar 340 cam has extremely long clearance ramps, and worse, if that is possible, is the 114* LSA. These two together conspire to drop your cylinder pressure into the basement, slanty territory, and that is about how powerful your 318 will feel below 3500 rpm. Recall that the 340s were rated at 10.5Scr and nobody ever called a 340 torquey.
              here's what your smogger teen looks like
              Static compression ratio of 8.0:1.
              Ica of 48*, at 500ft elevation
              Effective stroke is 2.89 inches.
              Your dynamic compression ratio is 7.10:1 .
              Your dynamic cranking pressure is 137.16
              PSI. ..................... 137
              V/P (Volume to Pressure Index) is 114 .................................. 114
              Notice the 114VP, that is your low-rpm performance benchmark.

              Now, lets slip the 340 cam into it;
              Static compression ratio of 8.0:1.
              Ica of 64*, still 500ft
              Effective stroke is 2.57 inches.
              Your dynamic compression ratio is 6.43:1 .
              Your dynamic cranking pressure is 119.89
              PSI. ...................... 120
              V/P (Volume to Pressure Index) is 89 ..................................... 89
              Notice the VP has fallen 22% to a slanty level of 89

              But it gets worse because that 64* Ica does not reflect that the intake valve is not yet closed, but rather is still about .012 open! And compression cannot start until the intake is on the seat and not leaking. IDK exactly when this will occur but adding a conservative 7 degrees to that, puts the Ica at 71*. Plugging that into the Wallace , she spits out;
              Static compression ratio of 8.0:1.
              Ica of 71*, and 500ft
              Effective stroke is 2.40 inches.
              Your dynamic compression ratio is 6.07:1 .
              Your dynamic cranking pressure is 110.75
              PSI. ...................... 111
              V/P (Volume to Pressure Index) is 77....................................... 77

              Now your low-rpm VP yardstick is LESS than what a slanty makes,a lot less, and that is what your 318 will feel like until the Rs get up past 3000/3500 ..... which with 3.55s is gonna be about 23mph, and every time the rpm dips into the 2000s which is after every shift, even if you rev it to 5000 .......................

              Ok but what would it take to run that 340 stick in your 318? Glad you asked
              Static compression ratio of 10.6:1.
              Ica of 71/500ft

              Effective stroke is 2.40 inches.
              Your dynamic compression ratio is 7.95:1 .
              Your dynamic cranking pressure is 159.55 PSI.................... 160
              V/P (Volume to Pressure Index) is 110 ............................... 110
              I jacked the Scr to the max for iron heads, and the VP is still less than your stock 318 was (114). So while it is doable to run that 340 cam, just try and get your 318 up to 10.6 Scr! You would need to get the total chamber volume down to 67.8cc, which can be done, but you'll also want to run the 2.02 valves and big ports...... making it yet more difficult, but more than anything, expensive.
              But I digress; we're a long way from the original question.
              lol
              Read about VP here
              V/P Index Calculation
               
              Last edited: Nov 18, 2019
            • CudaFactHackJob

              CudaFactHackJob FABO Gold Member FABO Gold Member

              Messages:
              802
              Likes Received:
              255
              Joined:
              Mar 17, 2016
              Location:
              Hemet, Ca.
              Local Time:
              3:13 AM
              My priority is always to get the motor to at least 9.5:1 then work around that.
               
            • dano

              dano Evil Handy Man

              Messages:
              2,672
              Likes Received:
              675
              Joined:
              Aug 22, 2005
              Location:
              Gresham, Oregon
              Local Time:
              1:13 AM
              I've seen far more A-body 4 speeds for sale than 3. For the amount of work, cost and effort I'd just go right to a 4 speed.
               
              • Agree Agree x 1
              • Wizard

                Wizard Well-Known Member

                Messages:
                518
                Likes Received:
                21
                Joined:
                Jan 23, 2012
                Location:
                Dillsburg, PA
                Local Time:
                3:13 AM

                A lot of good info here in all the replys I am getting. Keep it coming. Appreciate it all.

                Wizard
                 
              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.