What will my 68 Dart 408 run in 1/4 mile?

Discussion in 'Mopar Racers Forum' started by 68Dart500, Jun 11, 2018.

  1. weedburner

    weedburner Well-Known Member

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    The 2-step tells me dragstrip performance is probably important to you, the gears tell me you also want to drive the car on the street a lot. I would suggest adding more clutch. Your current setup is going to bog a lot after launch due to your gearing, then if you slip that clutch long enough to make up for the lack of gear, it's going to be really hard on the organic side of that DF disc. Leaving at a low rpm will help the clutch, but you will be leaving a lot on the table at the dragstrip. I would recommend a disc similar to a Ram 900 series with a damped hub (it's the disc they use in the Powergrip HD), then add a ClutchTamer to control the hit. With that setup you will be able to bump that 2-step to 6k if you want to.

    Here's why it's hard to make radials work with a manual trans at the dragstrip...
    Radials basically need to dead hook, as they will tolerate very little wheelspeed. To help illustrate how this becomes a problem, lets imagine a car launching with no clutch slip at all- engine rpm with a dead hook on the starting line would equal "0". Lets also say the car has a constant acceleration rate in 1st gear, and the potential to reach it's 1/2 shift point of 7000 rpm at about 2 seconds into the run. If starting line rpm equals "0", and rpm 2.0 seconds in equals 7000 and acceleration rate is constant, at 0.5 seconds engine rpm would be 1750, at 1 second in engine rpm would equal 3500, and at 1.5 seconds 5250 with the tires dead hooked and no clutch slip at all. Obviously dead hooking alone is not the answer, as our engines don't make any power at zero rpm. You NEED some controlled clutch slipping to keep those radials dead hooked without dragging the engine down too far, as there's just no way any combination of spring rate and/or suspension adjustments can possibly absorb enough engine rpm over a long enough time frame to make the radials work.

    Here's how controlled clutch slip can help-
    To a point the longer a clutch slips, the more time the car/engine has to gain speed/rpm before that clutch locks up, which in turn means engine rpm does not get dragged down as far. Lets apply the above example to a magical engine that has a completely flat torque curve of 500 ftlbs from 1500 to 5500 rpm. If that engine's clutch only slips for 0.5 second, rpm gets dragged down to 1750 after launch and that engine is only making 166.6 hp at the low point of the bog. If the clutch were to slip for a full second, rpm only dips to 3500rpm which effectively doubles it's power production to 333.2 hp thru the low point of the bog. In the real world the difference would be even more dramatic, as it's pretty unlikely the engine would be making 500 ftlbs at 1750.

    When it comes time to shift, the problem for radials then becomes the fact that the rotating assembly must almost instantly shed about half of it's stored inertia energy due to the ratio change. If that excess energy is dumped into the chassis/tires all at once, there's a good chance that extra energy will be enough to knock the radials loose, resulting in far less productive instant spin rather than effectively propelling the car forward. Adding a ClutchTamer makes it possible to spread that inevitable energy transfer over a longer time period, reducing it's peak to a level that doesn't knock the radials loose. Also because the car is gaining speed during those periods of controlled clutch slip, the overall amount of energy that must be dumped due to the ratio change is also reduced.

    A ClutchTamer makes it practical to choose a clutch with plenty of torque capacity for the application, one that might otherwise grab too aggressively for a radial, then allows "dialing in" longer clutch slip as needed to raise the bog rpm without reducing that clutch's overall holding ability. You might try to accomplish the same thing with your foot, but you will find it impossible to achieve the same degree of precision no matter how well trained the foot.

    There may be other ways to make a radial work with a manual trans, but my way works pretty good.

    Grant
     
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    • yellow rose

      yellow rose Doctor of Thinkology.

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      Well there you go. Grant found the thread and gave you the straight poop.
       
    • 68Dart500

      68Dart500 Well-Known Member

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      Thank you for the explanation on all this. So it sounds like my options are either swap to a different tire with less grip, change gearing, and or get a different clutch and a clutch tamer.

      Yeah I have a long drive to get to events because I live about 45 minutes outside of the cities so car shows and cruises are typically a bit of a drive just to get to. I almost went with 3.23 gears but decided I should at least get 3.55's. I'm not sure if I'll actually use the 2 step but I got my 6AL2 on sale so it was the same price as a normal 6AL so I figured why not have that option.

      I just talked to a tech guy at Mickey Thompson and he said I really should go with a bias ply so I think to start off I'm going to switch to the 28x12.5-15 Sportsman Pro bias ply tire.
       
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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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        Or you are a streeter, you can just do what I do; namely ignore the ET and be happy with big-speed. If you spend enough money you can eventually whittle your ET down.

        Assuming 3450 race weight and 485hp, your W/P is 7.11 and the Speed that goes with that is about 122 mph.
        Now an unlimited SS chassis might turn that into a low 11/high 10.......
        but a streeter might leave .5 second or more in the 60ft, and the 3.55 might lose another .4 to the traps, so your probably looking at a very low 12. Then factor in a street tune, and maybe lo to mid 12's could be in the cards for you. If you short-shift, or you over-rev, or you just can't shift that beast worth xxxx, well then there's that.....
        But if you can make it hook, well then, 11s are back on the table.
        _____________________________________
        But I'll tell ya, if I saw something close to 120mph on my slip, I'd call R done, and go get me some track-food, cuz I ain't spending all summer whittling the ET down. I'm quite content to know that my chassis sucks,lol. Heck I was happy with a 93 Eighth, and a 2.2plus 60ft..............
         
        Last edited: Jun 12, 2018
      • mbaird

        mbaird mbaird FABO Gold Member

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        What AJ said..... If it is 90% Street use maximize that aspect. Hell you can always buy a second set of Bias ply slicks for special occasions. And swapping gears is easy on 8 3/4s. Come track day throw a set of 4.30s in and run the slicks.
         
      • weedburner

        weedburner Well-Known Member

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        You guys give up way too easy!

        Gears/rims/tires... maybe $1000?
        Clutch disc/ClutchTamer… maybe $500?

        With gears/rims/tires, you are probably also going to need better shocks to control the hit on the chassis. With the clutch disc/ClutchTamer route, you can drive thru the gate when you visit the track without even changing tires.

        One of my NMRA Factory Stock guys sent me a note last nite, just got back from winning $9700 over the course of the weekend. Saturday he won $4700 in the rain delayed last couple rounds of the Maple Grove race, Sunday he took home another $5000 check for his Super Nats win. With the air, it was running 10.2's @ 129. He's running an off the shelf SPEC 10.5" diaphragm clutch with the hit controlled by a ClutchTamer.

        "DA got terrible and lost a bunch of power, with a turn of the dial I went from 1.45 60' to a 1.38"- Dan Ryntz

        Grant
         
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        • mbaird

          mbaird mbaird FABO Gold Member

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          Now I feel kinda stupid for spending in excess of 1000$ on the Mcleod RSX clutch and 20# SFI flywheel that was delivered yesterday....
          Lol
           
        • yellow rose

          yellow rose Doctor of Thinkology.

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          That wouldn't have been my pick for a clutch. The FW is a touch heavy but should be fine.
           
        • mbaird

          mbaird mbaird FABO Gold Member

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          It would not be my choice either.... it was a RST that I ordered...lol
          I had a brain fart....
           
        • 68Dart500

          68Dart500 Well-Known Member

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          Well i checked with Summit and they said I could return my MT drag radials so I ordered some MT Sportsman Pro bias ply. Bopefully that'll help ease the harshness of my launch a little. I really don't want to swap my clutch right now and I just setup my rear end .

          It's not going to be ideal for drag racing but it'll be fine hopefully for the 3 or 4 times I go this year.
           
        • yellow rose

          yellow rose Doctor of Thinkology.

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          I suspect you can probably *MAYBE* use the disc you have now with a clutch tamer. I suspect (again...guessing here) that it's the disc Grant was a bit worried about. You may be able to use the disc you have and just not get 100 runs out of the disc.

          I'd contact Grant and ask him for sure. It may be a disc change or maybe use what you have and change it out after you beat on it for a year.
           
        • RustyRatRod

          RustyRatRod Just another dumbass. FABO Gold Member

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          17.40s @ 56 MPH.
           
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          • flyfish

            flyfish C8H18+N2O = :-D

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            I didn't read the whole thing (sorry if this was touched on already), but if you are looking for a more dragrace oriented street slick, try Hoosier Quick Time Pro's. They hook WAY better than the MT Sprotsman Pro's (I've used both), and they last longer too. The setup I used them on was a sprayed 360 I ran in Colorado at Bandimere Speedway (5800', DA was usually 9000+ feet), ran high 11's on the MT tires, and 11.1's on the Hoosier QTP's, no other changes. I was spraying a 180 shot out of the hole with a 727 manual valvebody auto...so probably similar shock to the tire as hitting it with a clutch. Just food for thought.

            By the way, my guesstimate would be you run high to mid 12's the first time out. There is 11 second potential for sure, but it will probably take some tweaking.
             
          • roccodart440

            roccodart440 Well-Known Member

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            Try them tubleless first. You may be pleasantly surprised. I run MT sportsman pros tubeless all the time and have for years. One time I had one tire needed to be tubed.
             
          • skep419

            skep419 It's only money, you can always make more

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            12.75@112
             
          • famous bob

            famous bob mopar misfit

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            I ran a hemi for 14 yrs w/ drum brakes, and it was a hell of a lot faster than low elevens , just be sure all is right w/ the brake system .
             
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            • 68Dart500

              68Dart500 Well-Known Member

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              How often do you have to fill them up? They Sportsman Pro's are on there way and I'll be running them tubeless (or at least I'll try).
               
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              • roccodart440

                roccodart440 Well-Known Member

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                Once a year when I take it out in the spring

                Or after I air down for the strip. I run 18 or 19 at the strip. 25 on the street. 40 up front.

                60391970-DSC07017.jpg
                 
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                • 68Dart500

                  68Dart500 Well-Known Member

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                  That looks sweet! Is your Dart mini tubbed or fully tubbed? What size are you running?
                   
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                  • roccodart440

                    roccodart440 Well-Known Member

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                    Mini tubbed.

                    28x12.50 on 11.25" wheels
                     
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                    • 68Dart500

                      68Dart500 Well-Known Member

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                      Well here she is with the same tires as you but on 15x10's with 7.25 back spacing. Might need to trim the front of the rear wheel well opening or move the axle back a half inch. I've only got about 5/16 to 3/8 clearance on the front corner :/.

                      20180614_180120.jpg

                      20180614_175642.jpg

                      20180614_174856.jpg
                       
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                      • j par

                        j par Well-hung Member

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                        If it's a known fact that it will help which I'm not sure of cuz I don't know these tires at all it would be good to do it now. This just happened with a customer of mine with an older Trans Am. He wanted a different size tire and when they're put on they were the wrong size. Like a size too small instead of a size bigger lol the tire company had no problem exchanging them as we didn't drive on them.
                         
                      • 604B1duster

                        604B1duster Well-Known Member

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                        Not sure if it’s been mentioned, but you can drill a new hole in each spring pad on the rear. Drill them 1/2 inch forward of the holes there to move the rear back, and if all is well take it back out and plug up the original holes.
                        Half inch and you might get away without changing the driveshaft.
                        Those tires look great on your car
                         
                        Last edited: Jun 14, 2018
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                        • roccodart440

                          roccodart440 Well-Known Member

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                          THe tire is close. I'd move it back a tad.
                           
                        • j par

                          j par Well-hung Member

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                          I moved mine back. I took some half inch aluminum stock and made plates to go in between the front of the rear springs. I just bought longer grade 8 bolts to go through. There was easily a half inch to play with on the slip yoke. Well a picture is worth a thousand words, let me run out to the garage real quick...
                          IMG_20180615_071421.jpg
                           
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