I CAN’T DRIVE 145mph

Discussion in 'Mopar Performance Issues' started by loganscuda, Jan 10, 2019.

  1. loganscuda

    loganscuda DON’T reMember

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    Thanks just got me a new hood have not painted yet.
     
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    • AJ/FormS

      AJ/FormS 367 FormS clone 3.09-1.92-1.40-1.09-.78od 3.55s

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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.
       
    • MOPEkidD-3

      MOPEkidD-3 Torsional 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
         
      • MOPEkidD-3

        MOPEkidD-3 Torsional 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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        • MOPEkidD-3

          MOPEkidD-3 Torsional Member

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