When ET and MPH Are Not a Match?

Discussion in 'Mopar Performance Issues' started by dibbons, Jul 22, 2018.

  1. dibbons

    dibbons Well-Known Member

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    There is a formula to determine what ET one should be running based on MPH in the quarter mile. ET = 1353/MPH or MPH = 1353/ET

    my mph was 108 so it should run 12.52 but my best was 12.72
    my mph was 104 so it should run 13.00 but my best was 13.29

    When I punch my MPH into the formula, I find my ET is .2 or .3 slower than indicated by the math. I wonder where I am losing time? Thank you.
     
  2. Abodybomber

    Abodybomber Breaking street machines , since 1983.....:)

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    What combination ?
     
  3. dibbons

    dibbons Well-Known Member

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    .060 over 273 (282 CID) A833, 10 inch slicks, 4.56 sure-grip, super stock springs/pinion snubber, bolt-on frame connectors, 3140#
     
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    • Fisher

      Fisher Old Guy with a Cool car.

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      Back in the day my 3680 lb BBC camaro ran 12.26@108 first time out, we started the day at a 12.75. so tuning and tire pressure and 13 passes later we found half a second. Next trip to the track, i had swapped cam and carb, and gears from 4.10 to 4.56 we ran 12.02 @112 .
       
    • Fisher

      Fisher Old Guy with a Cool car.

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      But only got 3 passes the second time up some dummy hit the gaurdrail so the repair took almost all day.
       
    • Charrlie_S

      Charrlie_S Well-Known Member

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      On the shift(s)?
      PS: That formula doesn't work at all for my cars.
       
    • fklskv

      fklskv Well-Known Member

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      60ft time, shift points, carb jetting, timing. You name it, its there somewhere.

      Are you lifting and shifting? That would do it.
       
    • replicaracer43

      replicaracer43 Old school member

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      Post the whole time slip and we can help
       
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      • Dartnut

        Dartnut I'm H.R. Shovin'stuff

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        What is your 60 ft time?
        A lot can be told by that, (tire spin, etc...........)
         
      • dibbons

        dibbons Well-Known Member

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        Sorry, can't find the time slips.
         
      • replicaracer43

        replicaracer43 Old school member

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        So what do you want us to use? Our crystal ball? You have given us basically zero information and then asked for help.
         
      • fklskv

        fklskv Well-Known Member

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        I have a magic 8 ball I use.
         
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        • brian6pac

          brian6pac Well-Known Member

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          lower the gear to 4.88, rev the motor higher between shifts, make sure you got a higher RPM through the traps. That should lower your E.T.
           
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          • brian6pac

            brian6pac Well-Known Member

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            Oh yea I forgot one thing loose 100 lbs that always helps
             
          • General Disarray

            General Disarray Well-Known Member

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            MPH and ET do not go hand-in-hand. It is possible to run faster MPH and a slower ET than the guy you raced.
             
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            • crackedback

              crackedback FABO Gold Member FABO Gold Member

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              chassis isn't working and 60' is weak.

              The typical issue with most every car at the track.
               
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              • mderoy340

                mderoy340 Well-Known Member

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                I have the opposite issue, et hp calc is always higher than mph hp using Wallace Racing HP calculator. I think the aero of my brick has something to do with it.
                 
              • replicaracer43

                replicaracer43 Old school member

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                Horsepower cannot usually be properly calculated from ET. Horsepower is figured from WEIGHT, and MPH
                 
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                • Bobzilla

                  Bobzilla Well-Known Member

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                  The formula is based on perfect input data, and the mph through the traps is calculated buy how quickly you go through the timing beams at the end of track. That is why the numbers are not perfect and some guys are higher and some lower.
                  You can see this if you get a few good runs with no tire spin and really close shift points, as a base line. Next get a couple runs on the same day but leave the starting line softer and use the same shift points.
                  You'll see a difference in mph, usually only 1 or 2 mph, but you'll know the horsepower has not changed.
                   
                • JBurch

                  JBurch Well-Known Member

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                  Some calculators post results based on "ideals", ideally tuned engine, ideally tuned suspension

                  ET is a function of how well the chassis performs, and that shows up in your 60 foot time, the lower the time, the better your chassis works, shocks, springs, converter comes into play here also if automatic, clutch if you run a standard, it's how hard you hit the tires to plant them, to get the car moving. As a leaf spring car, you want the body to rise in a slow controlled fashion; you do not want the car to look like it's hopping down the track like a rabbit, that is when the suspension is coming to the end of shock travel and unloading the suspension. You want to think Sir Isaac here; for every action there is an opposite and equal reaction; as the body rises, it is pushing down on the tires, control it.

                  MPH is purely a function of horse power; it takes so much power, to move so much mass, at some velocity. I won't say it's simple Physics, but it is Physics just the same.

                  Your 12.72 ET on a 108 MPH is not way out of line, what was/is the elevation, what was the temperature?? These are factors not accounted for.
                   
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                  • 383Scampman

                    383Scampman Well-Known Member

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                    Agree with JBurch . there is a formula in the MP books called "Minimum et for a given speed". What they are saying e.g. If your car runs ,say, 115-116 mph and your et is 12.50 , you have a problem . Based on a chassis that is 100% efficient .
                     
                  • yellow rose

                    yellow rose Doctor of Thinkology.

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                    MPH is horsepower.
                    ET is bite.

                    If the MPH is high and the ET is slow, you have a bite/chassis issue.

                    If the ET is quick and the MPH is slow you are down on power and you have an ignition/fuel/tuning issue.
                     
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                    • Fisher

                      Fisher Old Guy with a Cool car.

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                      Converter, shocks, springs, gear ratio, tire hook, all equal 60' 330' 660' . You get the car working great and those numbers will be where they should be. The ET will be a result of those. MPH is HP and weight. jet up the carb, till the mph dropped off them go back 2 sizes for that day. increase Squirter size, pump shot, air pressure to get the 60' where it needs to be. same applies when it falls off go back 2. I was speaking to a fellow mopar man who said his distributor clamp loosened off and he lost 15 degrees of timing... Check your timing, You have lots to cover, hp just doesnt go away on its own, unless your nitrous bottle is empty? LOLOL
                       
                    • Locomotion

                      Locomotion Well-Known Member

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                      As Fisher elaborated on, carb tune-up has a lot to do with it. Squirters & pump cam (duration) for 60', which mainly helps ET, and jetting for MPH. Just be careful that too much of one isn't hiding the need for the other. Converter is also a big factor. Gotta optimize everything.

                      They can be helpful, but I don't put too much "absolute" faith in formulas because there are too many variables. My car never adds up right. I've been 11.14 @ almost 118mph with a 1.47 60' @ 3,530 lbs. Running the numbers every which way don't match. NHRA/IHRA "Stock" type engines have a lot of limitations (cam, ports, valves, intake, compression, etc.). So emphasis leans toward getting the car off the line as efficiently as possible with what you have to work with.
                       
                      Last edited: Jul 29, 2018
                    • 1Fast340

                      1Fast340 Well-Known Member

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                      As has been pointed out most of the time formulas dont match up real good,but the most comon missmatch of numbers is the one in your example and that is due to having the power to get alot of MPH but not the chassi to make full use of it to get a low ET,most of it can probably be found in the 60foot.
                       
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