How much gain with lower gears?

Mopar Performance Issues

  1. Garry in AZ

    Garry in AZ Building a 9 second Valiant FABO Gold Member

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    The original question will be very difficult to predict without knowing the torque curve of the engine. In very general terms, the reason a gear change makes some cars go quicker and others run the same ET is because of the torque curve of the engine. Better ET's come from maximizing the amount of time the car spends in the RPM range with the highest torque. If your car makes lots of torque down low, and falls off as RPM gets over 5 grand, the car will likely perform better with a higher street type gear. If the engine makes good torque higher up, a lower gear will improve ET. You said it was a "mild" 408. That might mean it's got a good street cam that works better at lower RPM. If that's the case, a lower gear might do more harm than good.
    I'd spend a little money on a couple of dyno pulls before making a decision on a gear ratio.

    Garry
     
    Last edited: Aug 1, 2017
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    • moper

      moper FABO Gold Member FABO Gold Member

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      A chassis dyno will only give sort of usable numbers. Get the car on the track. Get it consistent. Then change the shift points by 500 rpm. If it goers faster, it could probably use deeper gears provided it pulls cleanly in high gear past the stripe. If it doesn;t change much, don't plan on seeing any major night and day change. Because more gear will change the consistency. The convertor will change, and hit the suspension and tires a bit differently.
       
    • AJ/FormS

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

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      Well here's my 2cents
      With 3.23s and 25.5s you're gonna hit 100@ 6500 or so in second gear. The shift into third will drag the Rs down to 4500, and 112 will get you up to about 5000 so the engine is just getting back onto the power. This might not be the best situation.
      If you expect 112 mph, then you want to trap at an rpm where all the power is just used up. If your power peaks at 4700 or 4800, then 3.23s might be the right gear.
      but more typically, if the power peaks at 5400,and the roll-over might be several hundred rpm. So maybe you want to trap at 5800. Still at 112, this would require 3.73s. So now you stand the best chance of getting a low-ET with an engine that makes best power at say 5400 and is able to hit 112mph.
      Your engine may vary.
      if you have enough power to hit 118, that would be 3.55s.
      Or 106 would be 3.91s.
      But if your engine power peaks at 5100 and a 5500 rpm shift is appointed, then everything moves down one gear. 118 will now require 3.38s,112s will now require 3.55s, and 106 is 3.73s
      All math was done with a 5% slip factor in the TC, so again your results will vary.

      But if you had more gears, well, that would require more calcs,lol
       
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      • roccodart440

        roccodart440 Well-Known Member

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        As much as a half second. My car goes 12.20's in 4th gear. 11.70's in 3rd gear.

        What you want is a gear that will put you through the traps at about peak hp RPM. If you don't know what RPM that is you are pissing in the wind.

        Also keep in mind where you want to be at cruise RPM. Balancing street and strip without an OD gear takes some conservative thinking.

        3.55's will put you at about 2500@ 60mph with a 28" typical tire. 3.73's will put you at about 2800.

        I'd be running 3.55's if I were you. Or a GVOD and some big ole gears......
         
      • 68dodge

        68dodge Well-Known Member

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        This is correct. You also need to look at your cam specs, most cams are made to use a certain stall convertor, motor compression and gear ratio to work together. Once you have this set up and some track time, you can change one thing at a time to see what works better. Like gear ratio. I run 4.10s, I go from street use, right to the track. With just by changing tire pressures. But I also lost some street ability because of RPMs and ride comfort with my suspension set up.
         
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        • cosgig

          cosgig MoBro Inc. FABO Gold Member

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          In my bone stock GTX, I went from a 3:23 to a 3:91.....no difference! Since then I put in a converter and really lit the car up, but the only time I've had to try it at the track was at the Nats last year and I had a fuel delivery problem that kept me from full power thru the traps!!
           
        • Hot Metal

          Hot Metal '69 440 Cuda

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          Here are 2 pure on track weight/mph examples>

          My stock '71 340 Cuda, 3.55's x 26" slick, 3500 verter, .484 purple cam, went 13.3's@101 in street trim, trapped@5700 = 301hp...added>
          .590/312 Solid ft cam, more carb, HSD intake. bigger hdrs, 4.30's x 28" slick, 4200 verter, went 12.39@108.7, trapped@6300 = 367hp, so +60hp is approx 4/10ths, the rest was gear/verter, 60ft.

          Opposite example>
          '69 440 Cuda, best n/a with 32" slick, 4.88's =10.7@125, trapped@6500,
          +N02 went 9.96@133.9, trapped@7350, not good with 6pk rods/pistons, back off the gearing to 4.56/4.30 or tighter verter with less slip, or both.
           
          Last edited: Oct 8, 2017
        • Moparbear

          Moparbear New Member

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        • 318willrun

          318willrun Roadkill '80 D150

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          I went from 2.76's to 3.55's on a very mild 340 in a Demon and I dropped 6 tenths. However, it wasn't the same day, and it was a little cooler. At least 4.5-5 tenths were the gears.

          However, I went from 4.30's down to 3.91's in a mild 360 in a Dart Sport and the E.T. was exactly the same.
           
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          • inertia

            inertia Well-Known Member

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            Get it to hook first.
            Can't get any accurate info if you're spinning the wheels 60'.
            Cheers.
             
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            • MAPS

              MAPS Well-Known Member

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              Oh God, I hear you 100 % most people will not believe you but doI , I swapped 3.23 for 4.10 and they actually felt the exact same, I did not feel any difference let alone that "Smash" off the line with the lower gears (4.10) Even on the highway wasI doing about 2500 (give or take 1-200 rpm difference) at 60 mph.
              Finally someone who see what felt.
               
            • AJ/FormS

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

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              At Zero-TC slip, your 2500rpm and 4.10s translates to a 33" tire. With 30" tires this computes to 10%slip.
              At Zero-slip your 2500 and 3.23s translates to a 26" tire. With 5%slip, this would be 27.3s.
              But going down the hiway, the TCs often/usually slip in the opposite direction; with the actual running rpm being less than the Zero slip.
              If we toss 100rpm into the mix, then ;
              at an indicated 2500, and zero-slip being 2600, then;
              4.10 needs 31.8s, and
              3.23s need 25s

              If you used the same tire for both, then the TC .........mustabin IDK, maybe toast? Or real loose
               
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              • 66Valiant528

                66Valiant528 Member

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                I recently blew up the 3.58s (early 60s case) after driving to Lebanon Valley for Musclepalooza 130 miles....and changed to 3.91 gears. Car ran 13.1 @ 104 and change put in 3.91s hoping to get a 12. Didn't happen. Torque converter now stalls at a lower rpm. I did change the cam btw but the mph only increased by a little. Bottom line...gear change did not do what I wanted and caused my converter to stall at a lower rpm hurting my et. Put a 3500 Dynamic converter in this weekend past. Lebanon Valley this Sunday for Musclepalooza. 12s here we come!
                 
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