Question about gearing for Top Speed

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  1. harrisonm

    harrisonm Well-Known Member

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    Thanks everybody. It was a question that just popped up in my mind (or what's left of it). You all gave me a lot of good information. I think I understand now.
     
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    • 273

      273 Well-Known Member

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      Torque and hp aren't separate powers that do different things in an engine. Yes the op needs torque and rpm to reach his objective but we normally call combined torque and rpm "Horsepower", hp is torque with rpm and torque and rpm are hp, torque with rpm move accelerate maintain highway or even top speed or whatever work you want but use and call it hp so we can calculate the work and engine can do. The main difference as to hp between 2 engine that make the same hp but different torque is one will need more revolutions in the same amount of time "rpm" to do the same task.
       
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      • cojohnso

        cojohnso Well-Known Member

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        Aero resistances is the enemy. But it's not the only aero enemy. Your first enemy will be lift. Because the top of the car has more surface area the underneath? This causes natural lift. (And the car wanting to fly is typically bad. Lol) Usually just starting at 140 MPH. This is typically controlled with front spoilers to limit/disturb air flow underneath the car. But like everything? That only gets you so far. When pushing towards 200 MPH? You most certainly will be adding rear wing to generate down force. But even if you found the correct formula for a given package? Over all wind drag will eventually stop even the most powerful engines. No matter how gradual you gear to get to final high gear drive. So the question becomes. What is the drag coefficient of a particular vehicle? And as we can all imagine? A whole lot of variables. It's why wind tunnel data becomes far more prevalent with the high speed guys.
         
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        • StreetSleeper

          StreetSleeper Member

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          horsepower is torque over time. that's it. a measure of acceleration of torque. end of story.
           
        • d5667

          d5667 Member this?

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          And that is why 53 champions are used quite often
           
        • SixPaksRule

          SixPaksRule Member

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          Show me that car that goes 160 mph...I doubt it ran 160 mph without crashing. I owned a mopar car that ran an easy- no vibration 150 mph and would do so for a good while with ease. I spent a whole lot of money making it able to do that. Aerodynamics & proper suspension play a huge role above 130 mph. A tremendous amount of air starts pushing under the car above 130 making it unstable.
          Above 150 it gets more interesting - I spent seat time in other faster cars so i have been 160 mph on a closed course. Ford GT, Corvette, Mustang track cars, Older former SCCCA Mustang race cars, Viper.
          Show me that 160 mph rocket ship!
           
        • cojohnso

          cojohnso Well-Known Member

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          My '69 GTX. This is my 2nd GTX. (Replacing my loss from years ago.) This car is same exterior color. Same a little over stock 400+ HP 440 performance package. Same 4-speed and same Dana 3.54:1 rear. What is different? This car has 15" rear rim with 28" tires. Compared to 26.5" 14" rims tires of first car. As expected? This GTX is not as quick thru the gears. But my first GTX was done at 130 MPH. With a bit of a spooky vibration. This car? Is not happy until 90+ MPH. Then it's 440 starts pulling. At 130 MPH? It's still pulling. With a very comfortable float on the road. On a very lonely interstate. I saw its 150 speedo bury. And tack at around 6100 RPM. (I'm guessing speedo was doing its best on accuracy.) But tach confirms with tire height and rear gear to be 140+ MPH. Car was still remarkably stable. Only valve float ended its pull. (I shutdown more on fear of tire qualifications to sustain that speed and temps.)

          It's remarkable that it's that stable. But I believe (Besides just lucky on the suspension setup.) That the car lifted about 2" higher in the rear balances the car better thru the air.

          This car is non numbers matching. (But period correct '68 440) So is a decent candidate for a performance upgrade build. I had an opportunity for a Gen III swap. But passed. Mainly due to I just can't pull a very good running 440 to gain 90 HP with a GEN III hemi 392. It would certainly change the car balance. And I am very capable of screwing up a setup that doesn't need fixing?

          Moral of this story? Every car is different. Quick walk around vid of this car below.

           
          Last edited: Apr 25, 2021
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          • RustyRatRod

            RustyRatRod I was born on a Monday. Not last Monday. FABO Gold Member

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            My 73 STOCK 340 Rally Charger wrapped the 150 speedo all the way straight down goin out I-16 just east of Macon with six of our dumbasses in the car about 80 or 81. All because one of um asked "Rob, how fast will it go?". I held it on the floor till it quit speedin up and said "bout right there."

            Now, here's the rest of the story. That was on a Saturday mornin on the way to Pat Patterson's junkyard to see some cool stuff. That followin Monday, me and my best friend Kirk, jumped in the car on our way to auto shop class downtown. Soon as I turned in the driveway to the shop, my right front tire blew out. I was goin probably 3MPH. Both front tires has the polyester cords showin through. Yeah, I've done some stupid stuff.
             
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            • cojohnso

              cojohnso Well-Known Member

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              They had 150 MPH speedo? And a 340 having the torque to push that heavy a car to 150? That's even more amazing. That guy was not stock for '73? Someone did some parts and wrench time.
               
            • RustyRatRod

              RustyRatRod I was born on a Monday. Not last Monday. FABO Gold Member

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              Yes sir. Stock with 79K miles. I got it form the original owner. Rally cars had the 150 speedo. Had 2.76 geared 8 3/4. The car was unusually fast for a 73. It already had a reputation in Macon when I got it. We always "guessed" maybe it was one that got some extra blueprint work or "something", but it was stock, right down to the original blue engine paint and manifolds. There were only two or three other cars in high school that could out run it. One was an original LS6 454 4 speed Chevelle, one was a really hot small block 400 70 Camaro and there was something else....but I cannot remember what it was. There was even a 70 SS396 Chevelle I outran on a regular basis....he never did get me. Really pissed that guy off, too. Now we did have it super tuned with the timing jacked up and the Thermoquad tuned all up.....and of course back then you could still get 108 out of the pump.

              Here I am in the auto shop with the car about that time period.

              ME AND CHARGER.jpg
               
              Last edited: Apr 26, 2021
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              • Krooser

                Krooser Building Chinese Free Engines since 1959...

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                Fastest I've been is 146 MPH in my brother's '55 Jag XK140MC. Two lane WI Hwy 57 in 1959 coming back from the races at Elkhart Lake. Fun.
                 
              • 66fs

                66fs FABO Gold Member FABO Gold Member

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                God was watching out for you! I was like Cojohnso, always worrying about the tires. Also that was a very aerodynamic car. Aerodynamics is huge when going very fast.
                 
              • Dartswinger70

                Dartswinger70 I been North I been East to the California beach

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                I seem to think at that speed 130, the air gets under the car, lifts it to where the steering gets loose, at that point I could see the car jumping around from lane to lane pretty easily.
                 
              • 66fs

                66fs FABO Gold Member FABO Gold Member

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                I'd take my 64 Barracuda up to 140 mph once a month when I lived out west. No funny business straight and smooth. Every car /body style is different. I've had B Body friends tell me about being able to turn the steering wheel at a little over 100mph with little effect...
                 
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