Question about gearing for Top Speed

General Discussion

  1. harrisonm

    harrisonm Well-Known Member

    Messages:
    3,504
    Likes Received:
    3778
    Joined:
    Jan 16, 2005
    Location:
    Topeka, KS
    Local Time:
    11:29 AM
    I have a question about gearing your car for top speed. I mean like Bonneville Salt Flats top speed. Let’s say you have a 69 340 barracuda and at 6000 RPM you are at 120 miles an hour with 3.55 gears. And let’s say that same 6000 RPM would give you 140 miles an hour with 3.23 years, and 160 miles an hour with 2.76 gears (all theoretical-i know there are charts available online that give speed for given RPR/tire size and ratios). I know there is a point at which continuing to increase the overdrive gear ratio will actually start to slow you down. I assume the ratio will get to the point where the engine’s torque and HP will start lo lag. At some point, to go faster, you are going to need more horsepower not just more overdrive gear ratio. I just wonder if anybody knows where that is. I am guessing it would probably be somewhere around a 1:1 ratio rear axle
     
  2. George Jets

    George Jets 1967 Dart 2 Door FABO Gold Member

    Messages:
    5,378
    Likes Received:
    7385
    Joined:
    Jan 18, 2019
    Location:
    Farmington MN
    Local Time:
    5:29 AM
    Let's see:
    2.94s at 70 mph was running 2500 rpm, that equals 5000 rpm at 140 mph

    Then 7000 rpm would be 170 mph.

    Wind resistance and how much power under the hood are going to be the limiting factors.

    Probably going to need at least 350 horsepower to get to 140 mph. From there on is your guess.

    Screenshot_20210422-063232_Gallery.jpg

    This guy might know, 500 plus cubic inch.

    ☆☆☆
     
  3. diymirage

    diymirage HP@idle > hondaHP@redline

    Messages:
    25,679
    Likes Received:
    23453
    Joined:
    Oct 12, 2012
    Location:
    michigan
    Local Time:
    6:29 AM

    But zero hp

    :poke:
     
  4. AJ/FormS

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

    Messages:
    21,478
    Likes Received:
    8707
    Joined:
    Jan 19, 2014
    Location:
    South-Central Manitoba,Canada, 900ftelevation
    Local Time:
    5:29 AM
    It's unclear to me what you are actually asking. Are you asking; "how fast can I go with a 340 that makes peak power at 6000rpm?" If so then you would need to know how much real power it makes up there. And you would need to know a few things about the chassis it is going into.
    Power required increases with the square of the speed. So if your car takes 30hp to go 50 mph, then the formula predicts 640hp to go 200mph. To hit 200 at 6000 rpm with 28s, would take a final drive ratio of 2.50

    here is the formula;
    mph = (rpm x TC)/1056 x FD
    where TC is tire circumference and FD is your Final Drive
    Example;
    200= (6000 x 88)/1056 x 2.5= 200
    Below is a power curve for a hotrod 360 sorta like the one I built; with a 230/237/110 Hughes cam. You can see that at 6000 it is just beginning to slide down the backside of the curve. The point is that the 340, to peak at 6000, is gonna need more cam than a 230/237/110, lol.


    upload_2021-4-22_15-32-49.png
     
    Last edited: Apr 22, 2021
  5. TT5.9mag

    TT5.9mag Two atmospheres are better than one

    Messages:
    1,777
    Likes Received:
    1765
    Joined:
    May 5, 2016
    Location:
    So cal
    Local Time:
    3:29 AM
    My experience with bonneville, when you think you have enough horsepower to achieve a desired mph at your selected gear ratio...........double it.
    :lol:

    7965E9DE-3638-4CA7-A1AC-3A668E1C5324.png
     
    • Like Like x 1
    • 33IMP

      33IMP Well-Known Member

      Messages:
      1,651
      Likes Received:
      1735
      Joined:
      Jul 3, 2020
      Location:
      Taxifornia, soon 2b arizona
      Local Time:
      3:29 AM
      In my opinion, gear ratio is not a deciding factor, near as much as horsepower and drag, unless of course, the gears are too low.
      Theoretically, a slant six with 5000 rpm capability, with 2.76 gears is capable of 150+ mph. Do you really think it will?
      Theoretically, at advertised peak horsepower, (390 at 4900 rpm) at 4.10 geared A12 car wont break a hundred.
      We all know thats not true.
      My friend had a 305 camaro, with 2.29 gear in the back. That gear certainly limited top speed of that car, cause there was NO WAY that engine could pull that gear. Top speed would have been better with 3.55s.
      Im guessing there is a rough guideline that land speed record racers are familiar with, but i dont know it. Even with the right gear, power and drag will determine how fast a car is.
       
      • Agree Agree x 1
      • harrisonm

        harrisonm Well-Known Member

        Messages:
        3,504
        Likes Received:
        3778
        Joined:
        Jan 16, 2005
        Location:
        Topeka, KS
        Local Time:
        11:29 AM
        I understand the math. Maybe I wasn't clear. I totally understand that higher gear ratios (ie 2.76) will produce a higher speed at a given RPM compared to a lower ratio (ie 3.55). I was just assuming there was a certain point at which the engine would bog down as it tried to overcome an extremely high gear ratio. Kind of like trying to start out in 6th gear. Or maybe it becomes a matter of overcoming friction from wind drag. In which case, you need more HP. Just wondering.
         
      • AJ/FormS

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

        Messages:
        21,478
        Likes Received:
        8707
        Joined:
        Jan 19, 2014
        Location:
        South-Central Manitoba,Canada, 900ftelevation
        Local Time:
        5:29 AM
        Ok , I wasn't sure, but
        I have seen that with a 350 motorcycle, published in an FSM, back in the 60s or 70s. So you're on the right track. I just can't help with the numbers.
        However; I got this;
        A friend and I headed out to BC from Manitoba in July 1975; he on a 750 and me on a 350. The only way I could keep up to him at 75/80 mph was If I lay down on the saddle and hung my ankles over the rear signal lights. A few hours after we left Regina Saskatchewan I vowed to get rid of that lil 350 and a few more hours we came to Tabor Alberta and the first thing I saw was a big ol' Suzuki dealer.
        A couple of hours later, I was sitting tall on a brand new 1975 WaterBuffalo! We were flying! and I was sitting tall.....
        Now If I remember right, that 350 was rated 29hp, and the 750 was 70hp.
        So yeah yur on to something lol.Wish I could help. There was an article in some muscle-rag back in the 90s, about a certain E-body Barracuda that was built to cruise at high speed. I cannot recall the numbers but I think it was brown, and had custom fender flares; does that count for anything,lol. That article was somewhat technical, from what I do remember.
         
        • Thanks! Thanks! x 1
        • 273

          273 Well-Known Member

          Messages:
          3,750
          Likes Received:
          1501
          Joined:
          May 14, 2012
          Location:
          Ontario
          Local Time:
          6:29 AM
          Gearing for top speed generally the car will dead out of the hole lot of speed record cars need a push car to get them going but as you get up in mph you get up into the engines powerband. Say you go a 500 hp engine at 6500 rpm and geared so peak power is at the top speed you want and that's enough power to do so it shouldn't be bogging down your at peak power and have 500 hp going to the ground minus drivetrain loss.

          It depends on the top speed your looking for, a drag is geared to be around peak rpm at that top speed it can do in a quarter mile so like a 12 second car at 110 mph its not really gonna bog anywhere.

          Do you have a goal or is this a general question ?
           
          • Thanks! Thanks! x 1
          • StreetSleeper

            StreetSleeper Member

            Messages:
            22
            Likes Received:
            12
            Joined:
            Jul 21, 2015
            Location:
            32202
            Local Time:
            5:29 AM
            As someone whom has foolishly driven his Mopar in excess of 160, -get a longer, wider, heavier car. I may get beat up on the A body forum, but for YOUR safety I'll risk my reputation. Besides HIGH-END Brakes, a driveshaft safety loop, internally balanced engine, rebuilt differential, new wheel bearings-overpacked with grease,, Z rated tires, new,, an aerodynamic vehicle, with low-body weight and a roll cage...

            ...at any rate high speed steering corrections with a short wheel base are too often followed by lethal loss of control. I feel a B-body would be better for top speed, but you need to spend more on safety than a plan of high horsepower engine, gears, and brakes.

            In my humble opinion, though the physics promotes narrow wheels for speed, I recommend wide wheels for control as well.
             
            • Like Like x 2
            • Thanks! Thanks! x 1
            • StreetSleeper

              StreetSleeper Member

              Messages:
              22
              Likes Received:
              12
              Joined:
              Jul 21, 2015
              Location:
              32202
              Local Time:
              5:29 AM
              And no, the engine won't bog down with high gears
               
              • Agree Agree x 1
              • diymirage

                diymirage HP@idle > hondaHP@redline

                Messages:
                25,679
                Likes Received:
                23453
                Joined:
                Oct 12, 2012
                Location:
                michigan
                Local Time:
                6:29 AM
                I dont know about that
                I grew up on dirtbikes, and I remember several of them where the gearing outran the engine and when you shifted from 4th into 5th, you'd actually start to slow down

                Now I understand we're talking something more powerful then a 2 smoker, but I would imagine the principle carries over
                 
                • Like Like x 1
                • StreetSleeper

                  StreetSleeper Member

                  Messages:
                  22
                  Likes Received:
                  12
                  Joined:
                  Jul 21, 2015
                  Location:
                  32202
                  Local Time:
                  5:29 AM
                  I guarantee it. I have driven 60-90hp/200 ft-lbs torque engines 130.
                  CelloMom on Cars: How much horsepower do you need? (cellomomcars.com)
                  How Fast Can a Car Go? (itstillruns.com)
                  HP required to go 100mph? (pro-touring.com)
                   
                • AJ/FormS

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

                  Messages:
                  21,478
                  Likes Received:
                  8707
                  Joined:
                  Jan 19, 2014
                  Location:
                  South-Central Manitoba,Canada, 900ftelevation
                  Local Time:
                  5:29 AM
                  Like the powertrain combo that GM put into the early 2000s pick-ups that hunted up and down at ~60 mph into and out of lock-up.
                   
                  • Agree Agree x 1
                  • dICKj

                    dICKj Well-Known Member

                    Messages:
                    84
                    Likes Received:
                    64
                    Joined:
                    Sep 25, 2018
                    Location:
                    Kilgore, TX
                    Local Time:
                    5:29 AM
                    it is strictly a matter of horsepower needed to overcome aero resistance.
                     
                    • Thanks! Thanks! x 1
                    • diymirage

                      diymirage HP@idle > hondaHP@redline

                      Messages:
                      25,679
                      Likes Received:
                      23453
                      Joined:
                      Oct 12, 2012
                      Location:
                      michigan
                      Local Time:
                      6:29 AM

                      i had to look it up, i took a 59 HP, 76 Ft/lbs kadett up to 120...but that was pushing it

                      there have got to be other factors then gearing and HP that come into play though, like the rolling resistance, or air resistance

                      exactly
                      but it wouldnt surprise me if that was an MPG driven ECU micromanaging things
                       
                      • Agree Agree x 1
                      • StreetSleeper

                        StreetSleeper Member

                        Messages:
                        22
                        Likes Received:
                        12
                        Joined:
                        Jul 21, 2015
                        Location:
                        32202
                        Local Time:
                        5:29 AM
                        By horsepower you mean torque and gearing. Acceleration does not have to do with reaching top end power for the sake being at top end power.
                         
                      • StreetSleeper

                        StreetSleeper Member

                        Messages:
                        22
                        Likes Received:
                        12
                        Joined:
                        Jul 21, 2015
                        Location:
                        32202
                        Local Time:
                        5:29 AM
                        there are other factors, wind, traction, friction input vary at varying speeds, (pressure from) altitude, -but basically like you said, "59hp 76ft lbs"
                        gearing is like Archimedes? lifting a ship by himself with pulleys. It makes up for the torque to some extent. As a point of interest, the early steam engines didn't need much in transmission to accelerate-excessive torque.
                         
                      • Dartswinger70

                        Dartswinger70 Catch a wave youll be sittin on top of the world

                        Messages:
                        3,364
                        Likes Received:
                        1798
                        Joined:
                        Sep 20, 2005
                        Location:
                        Southeastern Pa.
                        Local Time:
                        6:29 AM
                        I seem to recall my Dart with 3.91s and a 318 running at 3000 rpm at 65 or 70 MPH
                         
                        • Like Like x 1
                        • dICKj

                          dICKj Well-Known Member

                          Messages:
                          84
                          Likes Received:
                          64
                          Joined:
                          Sep 25, 2018
                          Location:
                          Kilgore, TX
                          Local Time:
                          5:29 AM
                          I agree with TT5.9. The only math that matters at Bonneville is the time slip. All I know is that it took 790 HP to go 199.997 mph, and then $30,000 more to get 840 HP to reach 200.991 mph! Aero is about all that counts when you get up to the big end.
                           
                          • Agree Agree x 3
                          • Like Like x 1
                          • 273

                            273 Well-Known Member

                            Messages:
                            3,750
                            Likes Received:
                            1501
                            Joined:
                            May 14, 2012
                            Location:
                            Ontario
                            Local Time:
                            6:29 AM
                            By horsepower he must mean horsepower, torque is “basically”output of one revolution and hp is all the revolution added up over time “rpm” can’t have an either or situation an engine make power only which we use Hp to represent and hp is the sum of torque and rpm. You won’t find a formula telling how much torque you need to do x amount of speed but you will with hp.
                             
                          • StreetSleeper

                            StreetSleeper Member

                            Messages:
                            22
                            Likes Received:
                            12
                            Joined:
                            Jul 21, 2015
                            Location:
                            32202
                            Local Time:
                            5:29 AM
                            I agree with your definition of Horsepower. But essentially a single piston briggs and straton engine making 75 hp and worn chevy 305 V8 making 75hp NEGATE your opinion;torque is what is going to move the vehicle (outside of gearing) and rpm will dictate the final speed of the OP's objective.
                            Torque AND at max rpm for th eOP's objective is what is needed. Horsepower is also a measure of acceleration and driving constantly at top speed isn't acceleration.
                             
                          • K.O. SWINGER

                            K.O. SWINGER Meeting in the alley since 1976

                            Messages:
                            2,227
                            Likes Received:
                            1877
                            Joined:
                            Nov 18, 2010
                            Location:
                            oregon
                            Local Time:
                            5:29 AM
                            I will say that going over 130 and an a body is terrifying! Emphasis on aerodynamics. I was doing about 130 in my dart and went from the center of the road to one lane in a swooping motion by no means quick, air got underneath the front and well my life flash before my eyes I could feel my car leaving the runway I managed to getback into my lane and thank God I'm still on this planet.
                             
                            • Thanks! Thanks! x 1
                            • Agree Agree x 1
                            • StreetSleeper

                              StreetSleeper Member

                              Messages:
                              22
                              Likes Received:
                              12
                              Joined:
                              Jul 21, 2015
                              Location:
                              32202
                              Local Time:
                              5:29 AM
                              I looked down to change the radio at 160 and had shifted lanes in a blink.
                              new bushings and a tight front end was quintessential to safety, my friend with a 70's charger said his would lift up at 140mph-aerodynamics and that giant air scoop being all the physics stopping him at 140. Not the same with my plymouth though i was very excited
                               
                              • Agree Agree x 1
                              • K.O. SWINGER

                                K.O. SWINGER Meeting in the alley since 1976

                                Messages:
                                2,227
                                Likes Received:
                                1877
                                Joined:
                                Nov 18, 2010
                                Location:
                                oregon
                                Local Time:
                                5:29 AM
                                Ok I do think a cross wind was a major culprit because I have had my Swinger over 100 many times and it as vary true at that speed.I also think l had my torsion bars turned up at that time.
                                 
                                • Like Like x 1
                                1. This site uses cookies to help personalize content, tailor your experience and to keep you logged in if you register.
                                  By continuing to use this site, you are consenting to our use of cookies.