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    #1

    old style AOD?

    Hi!

    Visited an old friend yesterday morning, who had gotten into car repair.
    He was working on an old british car, austin healey? It was very bare-bones; the doors had no vapor barrier and hung trim, no radio, no power anything.

    Attached to the throttle body there was a switch to activate the overdrive. It was a separate mini transmission just behind the main transmission, so my friend explained.

    Now, I know some bicycles have 2 transmissions, if you will. Say, 7 speeds on one side and 3 speeds on the other, for a combined total of 21 speeds. Bike transmissions needless to say, are not under much stress.

    So, is this what's happening, with the old style overdrive?
    Is the overdrive a 2-gear transmission, such that the original 3-gear car effectively has 6 gears, and say once you reach 40mph, you'd flip the overdrive switch, shift down to first gear, and start all over...?

    In modern cars, is it JUST a 4th gear?


    thanks for the clarifications!
    -Bernard
    Tags: None
    #2
    no. Overdrive is only meant to be used with the final gear in cars. you can put undue stress on a transmission by using OD on the lower gears. Technically you're right and you can do that with the gears... but it wouldn't be that shift pattern. it would probably be more like 1, 1OD, 2, 2OD, 3, 3OD. The OD gear is not as big a change as a high/low transfer case on a truck. The proper way to shift the tail add-on OD is 1, 2, 3, 3OD.

    In modern cars the OD setup is sometimes just a 4th gear that is higher than a 1:1 ratio, but sometimes it's just a band that engages a panetary bypass gear to get that high ratio IIRC.

    Only two things are infinite, the universe and human stupidity, and I'm not sure about the former. -- Albert Einstein
    rides: 93 Crown Vic LX (The Red Velvet Cake), 2000 Crown Vic base model (Sandy), 2003 Expedition (the vacation beast)

    Originally posted by gadget73
    ... and it should all work like magic and unicorns and stuff.

    Originally posted by dmccaig
    Overhead, some poor bastards are flying in airplanes.

    Comment

      #3
      Originally posted by slymer View Post
      no. Overdrive is only meant to be used with the final gear in cars. you can put undue stress on a transmission by using OD on the lower gears. Technically you're right and you can do that with the gears... but it wouldn't be that shift pattern. it would probably be more like 1, 1OD, 2, 2OD, 3, 3OD. The OD gear is not as big a change as a high/low transfer case on a truck. The proper way to shift the tail add-on OD is 1, 2, 3, 3OD.

      In modern cars the OD setup is sometimes just a 4th gear that is higher than a 1:1 ratio, but sometimes it's just a band that engages a panetary bypass gear to get that high ratio IIRC.
      OD is always an additional "gear" in modern automotive transmissions which lack a low and high range selector. It has nothing to do with "stress". Most transmissions have multiple planetaries in them with multiple bands. One thing which makes the C6 stronger than the C4 and FMX(what the AOD is based on) is that it only has a single band. There are 3 parts to a planetary system. You can hold the shell, the sun gear or the planet gears or a combination in order to get reverse or an underdrive or overdrive gear ratio. Most transmissions use a clutch in the converter to lock it up. The Ford AOD uses a clutch pack within the transmission and a solid shaft going to the converter hub in order to lock it up. There is 3rd gear lockup in an AOD with which only some of the power is being transferred directly from the converter shell.

      Comment

        #4
        Originally posted by BerniniCaCO3 View Post
        In modern cars, is it JUST a 4th gear?
        In a modern automatic like the AOD and 4R70W, Yes it is.... In a T-5 it is the 5th gear. In a T-56 both 5th and 6th are OD ratios.
        Last edited by Mercracer; 09-05-2010, 12:33 AM.

        Comment

          #5
          The "stress" I was referring to is the "add-on" OD tailshaft... if it's left in OD all the time, it will put more load on the engine/trans than they were designed for because it essentially will turn the rear diff gears (say 2.73 for grins) into higher gears (something like 2.20 or some such depending on the OD ratio).

          you can still get this setup today (mostly aftermarket though, haven't seen any stock)... so not ALL modern transmissions have OD as just the final gear or 2. Think performance C6 with that tailshaft OD add-on that costs as much as the C6.

          Only two things are infinite, the universe and human stupidity, and I'm not sure about the former. -- Albert Einstein
          rides: 93 Crown Vic LX (The Red Velvet Cake), 2000 Crown Vic base model (Sandy), 2003 Expedition (the vacation beast)

          Originally posted by gadget73
          ... and it should all work like magic and unicorns and stuff.

          Originally posted by dmccaig
          Overhead, some poor bastards are flying in airplanes.

          Comment

            #6
            Originally posted by slymer View Post
            you can still get this setup today (mostly aftermarket though, haven't seen any stock)... so not ALL modern transmissions have OD as just the final gear or 2. Think performance C6 with that tailshaft OD add-on that costs as much as the C6.
            I can add a jet engine aftermarket, that does not make the statement that... "No modern automobiles have jet engines" any less true...

            Comment

              #7
              Originally posted by Mercracer View Post
              I can add a jet engine aftermarket, that does not make the statement that... "No modern automobiles have jet engines" any less true...
              that is true... but you said modern transmissions...
              OD is always an additional "gear" in modern automotive transmissions...
              and I was pointing out that you can get a modern C6 and then tack on the OD tail and it's still all new parts.

              Only two things are infinite, the universe and human stupidity, and I'm not sure about the former. -- Albert Einstein
              rides: 93 Crown Vic LX (The Red Velvet Cake), 2000 Crown Vic base model (Sandy), 2003 Expedition (the vacation beast)

              Originally posted by gadget73
              ... and it should all work like magic and unicorns and stuff.

              Originally posted by dmccaig
              Overhead, some poor bastards are flying in airplanes.

              Comment

                #8
                Originally posted by slymer View Post
                that is true... but you said modern transmissions...


                and I was pointing out that you can get a modern C6 and then tack on the OD tail and it's still all new parts.
                Aftermarket only..... The last factory C6 in an automobile was 1980... Back to the jet engine example....

                Originally posted by BerniniCaCO3 View Post
                In modern cars, is it JUST a 4th gear?
                To put an end to this silliness, yes, from the factory, it is JUST a 4th gear. As it was pointed out by slymer, you can get an aftermarket Gear Vendors or other unit to add an overdrive independent of the transmission gears to some factory transmissions.
                Last edited by Mercracer; 09-06-2010, 10:51 PM.

                Comment

                  #9
                  Originally posted by Mercracer View Post
                  Aftermarket only..... The last factory C6 in an automobile was 1980... Back to the jet engine example....
                  was that just cars, or trucks as well? I could have sworn the 87 F250 we had at my old job had a C6. It was some big nasty 3 speed slushbox whatever it was.
                  Last edited by gadget73; 09-07-2010, 07:35 PM.
                  86 Lincoln Town Car (Galactica).
                  5.0 HO, CompCams XE258,Scorpion 1.72 roller rockers, 3.55 K code rear, tow package, BHPerformance ported E7 heads, Tmoss Explorer intake, 65mm throttle body, Hedman 1 5/8" headers, 2.5" dual exhaust, ASP underdrive pulley

                  91 Lincoln Mark VII LSC grandpa spec white and cranberry

                  1984 Lincoln Continental TurboDiesel - rolls coal

                  Originally posted by phayzer5
                  I drive a Lincoln. I can't be bothered to shift like the peasants and rabble rousers

                  Comment

                    #10
                    Originally posted by gadget73 View Post
                    was that just cars, or trucks as well? I could have sworn the 87 F250 we had at my old job had a C6. It was some big nasty 3 speed slushbox whatever it was.
                    They ran the C6 in HD trucks well into the 90s...

                    Comment

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