I CAN’T DRIVE 145mph

Mopar Performance Issues

  1. loganscuda

    loganscuda DON’T reMember

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    A839B805-921B-4C7E-8351-AB3E9F54F8CB.jpeg
    Thanks just got me a new hood have not painted yet.
     
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    • AJ/FormS

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

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      My "68 Barracuda is very stable at 120/130 ...... in a straight line....lol. no spoilers. If it wasn't illegal to cruise up there, I would.
      My math says 130 with 3.73s and 27s is 6024 (zero-slip) in direct, 4156 in overdrive (zeroslip). Estimating and adding 5% for slip, gets you 6325/4365. Will your engine survive that and for how long?
      20% more rear gear would get you 20% more rpm going into overdrive, so now 5240, and maybe your engine would have enough power at the new rpm to continue accelerating. Or
      10% less rear gear would get you 10% less Rs in direct, so now 5700, and maybe your engine would have enough power at the new rpm to continue accelerating.
      For direct
      10% less gear will get you 10% more speed (if the power is there), so adding 10% to 130 you get 143mph.
      .9x3.73=3.36s and there ain't such a thing in a vintage Mopar ; that is between gears.
      With 3.23s,27s and in direct;145=6110@5%slip
      With 2.94s,27s and in direct; 145=5560@5% slip
      For overdrive
      Adding 20% to use .69od, we get 1.2x3.73=4.56 , and those we have.
      So with 4.56s and 27s,in overdrive;145=5950@5% slip.
      And with 4.30s,27s and in overdrive; 145=5612@5% slip

      The biggest problem for this exercise is the .69 overdrive ratio. It might be hard to get into it and continue accelerating because of the very large rpm drop;eg; .69x6000=4140, and the accompanying power drop.

      Consider this; the A998 non loc-up with a GVOD(.78od) with ratios of 2.74-1.54-1.00-.78od ..
      With the .78 od; We get .78x6000=4680 which is 540 rpm higher.
      145 at 5800@5% slip would take 3.91s and od to get 5770@5% slip. To get into overdrive your engine might like say 6300@124mph in direct,and that would drop to 6300x.78=4900 into overdrive, still a heck ofa job.

      To find YOUR optimum ratio;
      First determine your engine's peak power, where you want to buzz it up to. Second, subtract the TC slip, and third;here is your formula;
      mph= (rpm x Tr)/1056xR1xR2
      and finally; calculate the reciprocal of the result

      where;
      Tr is the Drive-tire roll-out (diameter x 3.1416), R1 is the trans gear,R2 is the od, R3 is the diff.

      >In the following example, I picked peak power out of a hat for purpose of calculations, same as the 85" roll-out, and the 5% slip; You will have to sub in your own numbers.
      Example; for peak power at 6000 less 5%slip =5700, so
      145=(5700x85)/(1056x1.00x.78xR3)
      145=484500/823.68xR3
      rearranging, we get
      R3=145x823.68/484500=.2465 , and the reciprocal of that is 1/.2465=4.057
      So 4.06 is the gear you need to hit 6000 at 145 with 27s

      to verify;
      6000 less 5%=5700, and
      (5700x85)/(1056x1.00x.78x4.057)=MPH=144.99
      >of course there may not be 4.057s available so you may have to choose between 3.91s and 4.10s in this example.
      > and you can work the formula with your current trans, just stick in your own numbers.
       
      Last edited: Jan 10, 2019
    • brian6pac

      brian6pac Well-Known Member

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      Where do you buy the hood from?
       
    • fishmens67

      fishmens67 Well-Known Member

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      I went a buck 50 in my 440 powered 71 Charger in the early 80's on interstate 90 in Washington state, tried it in the 67 fastback and gave up at a buck 15 after a slight wisp of wind from the wrong direction made the car want to steer by itself.lol
       
    • loganscuda

      loganscuda DON’T reMember

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      Thanks professor, this is exactly what I was looking for. I looked for a calculator on line but was confused by a few of them. Thanks for well thought out calculations and information.
       
    • loganscuda

      loganscuda DON’T reMember

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      VFNFiberglass
       
    • brian6pac

      brian6pac Well-Known Member

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      I have to check that out, I had 2 AAR's bought the first one in 76 the other in 83 sold both about 5 years ago, wish I had them back.
       
    • MopaR&D

      MopaR&D Nerd Member FABO Gold Member

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      That sounds a lot like a speed-limiter issue in the engine computer, even with a stock 5.7 Hemi the car would literally start to fly before you'd be overcome by drag.

      I've had my Duster up to 110 a couple times and there was still plenty left with 2.94 gears in the back, I was barely getting into the secondaries IIRC, definitely not flooring it for sure and it was "only" a 360 with about 320 HP. I'd argue the front end on those cars are even less aerodynamic than a Cuda like yours, it's totally flat and the top of the grille is canted forward slightly with nothing below the front bumper like a valance panel (is that what it's called?).

      This Spring/Summer I want to see about getting my Duster into 1/2-mile racing (and road racing) so I'll be trying to improve the aero for sure. And this time around it'll be a hotter 360 with closer to 450 HP.
       
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      • brian6pac

        brian6pac Well-Known Member

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        The factory computers limits the MPH to the tire rating, if the tires are rated to 110 MPH then the computer limits the car to that speed, you have a aftermarket controller so I doubt that is the problem.
         
      • loganscuda

        loganscuda DON’T reMember

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        I have factory ECU but have never seen a tire rating for ECU only tire size
         
      • brian6pac

        brian6pac Well-Known Member

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        ECU's are programed to tire speed rating that the car left the factory with.

        speed rating.jpg
         
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        • MopaR&D

          MopaR&D Nerd Member FABO Gold Member

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          I was gonna say, how does the computer know what type of tires are on the car when you change them? Lol
           
        • 3406pk

          3406pk Well-Known Member

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          • MopaR&D

            MopaR&D Nerd Member FABO Gold Member

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            Definitely getting one of those for my car, that is indeed awesome thanks!
             
          • Jesus Chrysler

            Jesus Chrysler Forgiving Sins Against Mopar Since 1983

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            Where did you get those sweet gauges? That's the only thing I've ever seen that would be an acceptable replacement for the factory cluster
             
          • loganscuda

            loganscuda DON’T reMember

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            Speedhut
             
          • Ottmundr

            Ottmundr 68 Fastback

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            Do you have a part number for those gauges, and pictures of when you installed the assembly?
             
          • loganscuda

            loganscuda DON’T reMember

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            I lost most of my build pictures. If you go to Speedhut Guages you can design what you want. I had to make a bezel ring for the speedo and quad guage as they where about a 1/16 too small for hole in the housing.
             
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            • Jesus Chrysler

              Jesus Chrysler Forgiving Sins Against Mopar Since 1983

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              Sorry, last question on the gauges!

              What size are they and is that an aftermarket rallye dash?
               
            • loganscuda

              loganscuda DON’T reMember

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              I think they are 4 1/2. I will have to check. That is my original I had it restored for $300 at Mr. G’s in Dallas tx. I think he takes your original and sells a restored one to you now. That way you don’t have to wait. Mr. G's Enterprises - Home Page
               
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              • Oldmanmopar

                Oldmanmopar Going left turning right FABO Gold Member

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                Some of these things you may have already done. I do not know your car

                410 gears.

                Ad a piece of aluminum between the lower valance and the front of the K member, Frame rail to frame rail

                Close off the grille so it flows air between the valance and bumper

                Aluminum Radiator and electric fan

                raise the rear of the hood to let the air flow or buy the forbid-in cowl induction hood . And you can even raise that.

                Now back in the day it started with raising the rear of the hood. Then the hood just got taken off. That started when someone couldn't get theirs latched. that advantage was the weight . I would just raise the rear of the hood for now.

                But speaking of weight. Get in the drivers seat and imagine you are in a boat that is sinking. Now look around and start tossing all the extra things out the window. that are not needed to go racing. NOT the Beer! Question: If you drink the beer will you loose the weight.

                Well that wraps it up. You need more motor , Make the body slicker trough the wind, or loose some weight out of the center to tail of the car. keep the nose weight or they take off. I have experience at that.

                Your most expensive change would be the motor. Cam and just a little Boost.
                 
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                • kybatman

                  kybatman Well-Known Member

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                  I have one of these NIB if you are interested.

                  Bruce
                   
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                  • roccodart440

                    roccodart440 Well-Known Member

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                    Have you ever done this? IT makes sense for aero and for cooling.
                     
                  • fishy68

                    fishy68 Tyr Fryr's Inc. FABO Gold Member

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                    I have a 2011 Ram and used the Diablo tuner on it and it has the option to turn the speed governor up to a higher speed. Been 2 yrs. since I did it so I don't know what the max was but I'm sure it was at least 145 as that's what I turned mine up to. I'm surprised you can only run 130 with that combo. I had my Ram up to 132 (gps verified) and it was still climbing at a good rate and I can't imagine my Ram is more aerodynamic than your car. Plus it weighs 4400 with me in it so it is close to 1000 lbs heavier than yours and it has 4.11 gears. Other than the tuner the only mods are a K&N drop in filter and cat back exhaust. In other words probably only a 10 hp gain over stock. Yours should do 145 stock
                     
                  • loganscuda

                    loganscuda DON’T reMember

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                    I bet that is an issue I have a diablo tuner I never looked at that parameter will check it thanks.
                     
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