How much power is lost through a 518

Transmission and Drivetrain Tech

  1. peterOZ

    peterOZ Active Member

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    I am looking at doing a conversion from 727 to 518. Benefits are the o/d + lock up. They are a heavier trans and I hear they can rob h/p. Has anyone compared before and after their conversion to confirm how much is lost through the 518?
     
  2. Syleng1

    Syleng1 Karma is real and Life is short... FABO Gold Member

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    The benefits of OD outweigh the loss of extra weight when you factor in momentum, fuel economy and the engine spinning a lot slower.
    Plenty of people here have installed the OD trans with great results.
    Loss of HP? Not sure as it is not really going anywhere unless you have slippage.

    Try it out and i believe dependent on the floor a few modifications need to be had to fit the darn size of the trans in place.
    Good luck!
     
  3. toolmanmike

    toolmanmike FABO Staff Staff Member FABO Gold Member

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    What's the rest of your combination? Do you even need the 518?
     
  4. AJ/FormS

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

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    if your combo can take advantage of the overdrive trans, then you can upcam one size and better than breakeven powerwise. Or just upsize the rear gear 5 to 10% and you will again net more performance. In fact, going from a street-friendly 3.55 you could go to 3.55/.69=5.13s for the same cruise rpm. But, the 5.13s would have 44.5% more TM (Torque Multiplication), making your engine feel that much more powerful.
    If a streeter, therefor, you can rear-gear it to redline at 60mph in second gear, and it will be a killer combo. For example;you could run 4.56s to hit 60=5100 (5% slip) on a cam that peaks at 4900 (say a 218@.050), and annihilate the zero-to 60 ET you mighta had with a 223@.050 cam and 3.55s; which would only hit 4000@60mph. And with the 4.56s plus od the cruise rpm would be; 65~2550, compared to 2870 with the 3.55s
    Plus you can use an A500 behind most any SBM, saving you a few pounds, and the A500 has a slightly lower First and Second gears ..... allowing you to run 4.30s instead of 4.56s, bonus!
    Knowing what I now know; I wouldn't put a Hi-performance dual-purpose combo together without an overdrive; cuz running 4.30s is killer quick to 60, even with a modest engine. Perfect for a streeter.
    The only thing you have to be careful of is to not undergear for your cam.
    What I mean is, if you keep the 3.23s in the back, then your 65=1800rpm. So then if you are going for fuel-mileage, then your cam needs to be out of reversion, with the manifold vacuum being on the plateau, by 1800. Else the fuel curve will be hard to get right, and the intake will get very dirty inside. So to prevent that, I highly recommend not to try and run the DC 292/292/108 cam,lol.
    That 1800rpm cruise points to a 220ish@.050 or less, IMO
    Or just drive faster,lol.
     
    Last edited: Aug 15, 2019
  5. BigBlockMopar

    BigBlockMopar BigBlockMember

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    My Dart went from a 904 to a 518 (LU+OD) with no benefit in mileage. MPG even got worse for a bit.
    This was with a low power 8.6:1cr 318ci and 3.23 gears in the lightweight 7-1/4" axle.
    First 518 went bad, changed for A518 w/OD only.
    Changed rear axle to 8-3/4" w/3.23 gears, no change.
    Changed to 3.55 gears, no change.
    Changed engine to 11.3:1cr 360ci, no change.

    I did notice a difference in revving the engine with the smaller lockup stall convertor. The other 518 got a large late '60s stall convertor.

    What DID change was the car got way more fun to drive. The added 4th, OD gear really brings an old car into the present day, by being able to drive along modernday traffic without turning a silly high rpm.
    I'm currently preparing an A500 to replace the A518.
     
    Last edited: Aug 15, 2019
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    • AJ/FormS

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

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      If you think about that, it sorta makes perfect sense. With 3.23x .69=2.29; you're asking a smogger-teen to pull 65 at about 1840 rpm. That would require a completely different carb and timing tune, over the 904... which you don't mention.
      As to the 11.3/ 360, well that takes a complete different tune as well.
      With 3.55x.71x.78= 1.966 final drive (A833od/GVod), I got my 10.95 Scr 367 cuber up to 32mpgs, without hardly trying.
      Now, I know you are a smart man, but I think you left out a few details,lol.
      I was running a 223@.050 cam and nearly 190psi cylinder pressure. The carb was an ancient Holley1850 set up strictly for this run. The gearing gave me 1850 @~75mph,right on the money. I put a lotta timing into her, a lot.
       
    • BigBlockMopar

      BigBlockMopar BigBlockMember

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      I know details are scarce, because I didn't feel like typing out my life story :D
      Pretty much from day 1 I got this car, I've been playing with timing and ways to optimize the engine tune.
      I also didn't mentioned the fuel used is LPG (Propane/Butane). That offsets the mileage back usually.

      At one point I also went digital with the ignition and installed a MegaSquirt3 ECU, which made tuning a lot easier. This in turn made the engine run better and give back more fun. But.... mileage hardly changed for the better.
       
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      • ir3333

        ir3333 Well-Known Member

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        i have a friend who installed a 518 od in his 3.91 geared 318 dodge 4 x 4 truck...mileage did not improve.
        as for power loss to 518 from a 727 i don't know but 904 to 727 i have read may be 20 hp or more and 35 lbs heavier.Lots of guys going to 904's.
         
      • BigBlockMopar

        BigBlockMopar BigBlockMember

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        Somewhat of a topic-update;

        I made a real attempt to get a good mileage number again recently.
        I had to go on a 2hr one-way trip so decided to drive the same speed as I drove years back when I got my car and managed 17mpg with the low cr 318/904/7-1/4" in it.

        Well, after driving like a senile old fart for some 4 hours at a max 60mph and become the most hated guy on the road, it turned out the car managed 16.5 mpg.
        With some more air in the tires I'm sure the 17mpg could have been reached again.
        But, during this trip the drivetrain now consisted of a 11:1cr 360/A518/8-3/4...
         
      • 89on35s

        89on35s Inferno Red Duster FABO Gold Member

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        My Duster went from a 360/904/3.23 8.25 rear with a 25.5" tall tire to the same 360/A500/4.10 8.75 rear with a 28" tall tire and the mileage stayed the same but it changed the personality of the car immensely. Dropping 500 RPM at cruise is so nice, and the 4.10s wake the bottom end up too.

        Cley
         
      • YY1

        YY1 Well-Known Member

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        MPG HAS to improve with the addition of an overdrive gear.

        Unless your daily drive doesn't have any opportunity to use the OD gear at moderate, steady throttle.

        ...or you just can't keep your foot out of it.
         
      • ir3333

        ir3333 Well-Known Member

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        not so fast...a friend added an overdrive to his dodge pickup and changed nothing else.His mileage went down.
        Your gearing must allow you're cruise rpm to be at maximum lo speed torque, or the most efficient lo speed torque.
         
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        • YY1

          YY1 Well-Known Member

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          I'll spare you the red x but I disagree.

          Peak torque on my 5.9 is something like 4000 RPM, but my best MPG at about 2000 RPM in OD.
           
        • YY1

          YY1 Well-Known Member

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          peak tq is peak efficiency, but not necessarily peak MPG
           
        • TrailBeast

          TrailBeast Slightly Twisted Member

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          An interesting conversation for sure.
          I went from 17mpg average at 75-80mph with a stock 318 four barrel with headers and a 904 to a mildish 5.9 Magnum and the A500 with lockup (barely ever use lockup and have it turned off most of the time.)
          2:73 rear gears and a 25 inch tire.

          At that same 75-80mph with the Magnum and A500 I am running right at 2,100 in the rpm's and get 25mpg on the hiway.
          My cam is 214/224@50 with a .512 lift and carb is an Edelbrock 1406 (600cfm)
          I also run it on the lean side (around 15.5-16:1) and about 18 degrees initial with a total all included timing of 52 degrees or so.
          So basically at 75-80 which takes about 1 inch of throttle at the pedal I am running a slightly lean condition and lots of timing.

          Now ya got me trying to figure out why some peoples mileage stays the same or even goes down.
          I'm about ready to swap in a 3:55 geared A body 8.25 sure grip, so I'm expecting I might be into the throttle a little more out of pure enjoyment, but it will be interesting to see how much mileage I loose.
          Don't expect it to be much, but I do expect some loss with an 800 rpm increase at freeway speeds.
           
        • ir3333

          ir3333 Well-Known Member

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          no no...not peak torque...the maximum or most efficient lo speed torque.Beyond that you are diminishing your returns.Everything mechanical has a sweet spot...where you get the most for the least.
          Even paddling a canoe. At some point you paddle easily with minimal effort.You can double the amount of power in your paddling but you won't double your speed.
           
        • 89on35s

          89on35s Inferno Red Duster FABO Gold Member

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          Not saying my tune is close to optimal (likely why no improvement for me) but it is true the drop of 500 rpm at cruise should definitely improve mileage.

          Cley
           
        • ir3333

          ir3333 Well-Known Member

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          the most efficient rpm is largely affected by the cam.
           
        • AJ/FormS

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

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          I can tell you the yardstick I use, which has served me well;
          In neutral,with a vacuum gauge plumbed to the intake manifold, slowly rev the engine up until the vacuum plateaus and does not rise any further, or at least drastically slows down in the rising. The lowest rpm on the plateau is the rpm at which the pistons are no longer pushing air charge back up into the intake; so all that has been inducted, is now being compressed. That will be your lowest EFFICIENT cruising rpm. You can cruise at a lower rpm but the intake is gonna get messy. You can cruise a lil higher and not lose too much.
          To find your cruise timing at that rpm; (edit; minimum cruising rpm), just keep tugging on the timing, while simultaneously reducing the rpm back down to the minimum cruising rpm, until the engine no longer builds rpm. Edit; I use the fast idle cam to set the minimum cruising rpm.So for example; you add 3 degrees timing and the rpm goes up say 200rpm. You back it down the 200 and add 3 more degrees. This time it might only rise 100rpm. So you back it down, and add 3 more and this time it goes up only barely, so you take the last 3 degrees out and call it done. End Edit.
          Badaboom! Now put the timing light on it, record the number,return the engine to idle and retard it back to the "Before-test" numbers.
          Don't be surprised to see mid-fifties for timing; or more.
          and do be surprised to see rpms less than 1800 with typical street cams that have Ica's between say 58 and 66 degrees.I'm gonna guess plus something like 80rpm per degree of Ica; so a 64* Ica would need ABOUT 2280rpm minimum.
          Now you have the test-drive numbers; all you gotta do is gear the car to achieve whatever speed you wanna cruise at, and blast up the timing, and start leaning it out, until it won't maintain the speed, or starts acting funny, or burning stuff up; then fatten it back up a lil. And of course a lil fine tuning.That's the best she's going to do at that speed; if you want higher numbers, yur gonna have to either; slow down and regear, or lose weight, or get a slipperier body-style..
          There are just two hiccups;
          #1) how do you give your small-cammed engine, 56* of cruise timing at 1800 rpm? Good luck with that................. I think that it cannot be done with any factory distributor ever produced, which if you really twist up your timing, might get 50*, but more typically with that small cam, hi-compression combo, you might get low 30s* which Ima thinking is 18*@1800mechanical, plus , 12 to 16 in the Vcan.
          And hiccup#2) is; you automatic transmission guys who maybe can't even cruise at 1800, without a lotta TC slip. But if you have a loc-up, this applies equally to you, except with a hi-stall, for some reason you guys like to run 30 degrees at 2400 or something equally wild,and you can,so you might have an easier time getting the 50, but you still might not get to what the engine truly wants.
          This is where cylinder pressure really shines; and why modern EFI cars are again being built with high compression ratios, and computer controlled timing..

          I solved the problem with a dash-mounted, dial-back, timing box. Mine has a range of 15 degrees. So I can optimize my power timing, and optimize my PartThrottle timing, and my Vcan assist, and once at cruising rpm, dial in whatever more she wants.
          The hard part is remembering to dial it back out, when I slow down for a town,lol..
          But that's another reason that 367 is wearing aluminum heads; they put up with a lottachit.
           
          Last edited: Nov 14, 2019
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          • BigBlockMopar

            BigBlockMopar BigBlockMember

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            Good info AJ.
            My ignition timing is fully digital.
            I can tell exactly how much advance my engine is running at a certain rpm and at what vacuum level (engine load).
             
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            • mbaird

              mbaird mbaird FABO Gold Member

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              @AJ/FormS

              So you are saying to kepp bumping timing at idle and backing off idle screw to maintain highest vacuum @ reasonable idle speed ?

              What happens when your throttle plates won’t shut any further ? And doesnt that mess up your off idle transition from the idle ports ?
               
            • rumblefish360

              rumblefish360 So close, yet so far away

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              That of course is to far. You wouldn’t go there.
               
            • AJ/FormS

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

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              No you keep it at the previously determined minimum cruising rpm, using the fast idle system or whatever it takes. this minimum cruising rpm is likely gonna fall between 1800 and 2400 rpm, depending on the cam's Ica mostly, and on the ignition timing secondly.
              I edited the post for clarity.

              On a side note; check out TrailBeast's post. He's working out real sweet street-combo.
               
              Last edited: Nov 14, 2019
            • mbaird

              mbaird mbaird FABO Gold Member

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              You will need to clarify what you are talking about here .
               
            • MOPAROFFICIAL

              MOPAROFFICIAL Well-Known Member

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              Middle to low end of cam range IS where a motors mpg efficiency occurs, peak is peak of power/cam range.
              If you drive below the range it only gets worse.
               
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