8 3/4 Zero Lash for Racing

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  1. moparcrazy

    moparcrazy Well-Known Member

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    Has anyone Ran a zero lash on a 489 case or the 742 for drag racing?? Car weighs 3630 with me in it, and I know I'm taxing the center section, housing is the Fab 8.75 so I know that's Not gonna be an issue. engine makes 700TQ.
     
    Last edited: Aug 10, 2018
  2. hangn0ut

    hangn0ut Well-Known Member

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    Gotta have some clearance or it will just burn itself up.
     
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    • dvw

      dvw FBBO Gold Member FBBO Gold Member

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      When you start to put power to it the teeth bend. That will tighten up the backlash. At 700lb/ft you will break it. Just a matter of how many pases it will take.
      Doug
       
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      • 67belvedere/225

        67belvedere/225 FBBO Gold Member FBBO Gold Member

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        why would you want no backlash? anything that has no play or flex will just break.
         
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        • moparcrazy

          moparcrazy Well-Known Member

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          What would be an acceptable back lash, I've been told to pre-load the suspension holding the foot break and accelerator, then apply the trans-brake so not to shock the ring gear when trans-brake is released.
           
          Last edited: Aug 10, 2018
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          • gtxracer

            gtxracer FBBO Gold Member FBBO Gold Member

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            In my experience, releasing the trans brake button will shock the drivetrain plenty.:thumbsup:
            Bob :moparsmiley:
             
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            • dvw

              dvw FBBO Gold Member FBBO Gold Member

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              I run .007"-.008". Trans brake makes your time table even sooner.
              Doug
               
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              • 5.7 hemi

                5.7 hemi FBBO Gold Member FBBO Gold Member

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                That much torque on an 8 3/4 will break it. Get a Dana 60, problem solved.
                 
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                • LemonWedge

                  LemonWedge Well-Known Member

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                  No less than .007 is what Dr Diff has advised me. I set mine up at .009/.010 and it’s given me good results (knock-knock).

                  At 3630lbs; 700 lb/ft with a brake, you’re well into the red-zone with an 8-3/4.
                   
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                  • 67belvedere/225

                    67belvedere/225 FBBO Gold Member FBBO Gold Member

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                    i like em at .010 my self , and checked on 3 places on the crown , thats how the old guy thought me when i worked at GM
                     
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                    • dvw

                      dvw FBBO Gold Member FBBO Gold Member

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                      I've found that the pinion depth is far more important than a back lash number. The wider you can make the tooth contact pattern the better. If this takes a deeper pinion setting with back lash in the ball park, (.007" .011") go for it.
                      Doug
                       
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                      • 67belvedere/225

                        67belvedere/225 FBBO Gold Member FBBO Gold Member

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                        I SAY GO DEEP !
                         
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                        • 5.7 hemi

                          5.7 hemi FBBO Gold Member FBBO Gold Member

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                          Don’t go too deep! Lol! You’ll burn up the gears and shear off teeth.
                           
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                          • moparcrazy

                            moparcrazy Well-Known Member

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                            Got gears today, from US Gear !
                            IMG_2157.JPG IMG_2158.JPG IMG_2159.JPG
                             
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                            • dvw

                              dvw FBBO Gold Member FBBO Gold Member

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                              Unless the pattern covers a large portion of the ring gear face on the drive side, you can experiment. Yes it's time consuming. Especially with a Dana. Driving the pinion deeper then resetting the backlash can result in a very wide pattern across the drive face of the ring gear tooth. Instead of just saying "good enough" keep going until it's the best you can make it.
                              Doug
                               
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                              • David Stillie

                                David Stillie Well-Known Member

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                                My Dart ran 8.90's with an 8 3/4. Never broke the rear end. I had a billet cap on drivers side and would replace the gears every 300 passes. Set up with .008 backlash and lots of preload on the carrier bearings. This is what worked for me.
                                 
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                                • gtxracer

                                  gtxracer FBBO Gold Member FBBO Gold Member

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                                  David, the rear end housing in my GTX is original as well. Not as fast as yours, only a 10.34 best but never had and issue with breakage. I use 3 different third members that have all aftermarket parts and it works for me.:thumbsup:
                                  Bob :moparsmiley:
                                   
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                                  • dvw

                                    dvw FBBO Gold Member FBBO Gold Member

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                                    What ratio? How much did the car weigh? Was the housing braced?
                                    Doug
                                     
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                                    • David Stillie

                                      David Stillie Well-Known Member

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                                      It weighed 3000 lbs. The housing was braced. The ratio was 3.91 for some of the time but mostly used 3.23. 742 case.
                                      Turbocharged 5.7. Trans brake too.
                                      1.23 60 foot
                                       
                                    • Cranky

                                      Cranky Moderator Staff Member FBBO Gold Member

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                                      Weight and power is the killer of any kind of parts. Beef up one area and the breakage will move to the next weakest point. Throw a stick in front of an 8 3/4 and start launching at 6k in a 3400 lb car and you're taking a chance :D

                                      Setting the pinion deeper will usually create some gear whine on decel but being a few thousands or so deeper than than recommended probably won't make enough of different in sound to be noticeable. When I set one up and I'm a tad deeper, I'll go with it vs being a tad shallow. Other words, I'll shoot for being deeper if it checks out to be shallow. One was .006 deeper than marked and it ran quiet.

                                      Moparcrazy, where did you get the idea to run 0 BL? Everything has to have some clearance to live. And that spec sheet is just the opposite from the info I have for setting BL....unless there's new thinking or new materials that are being used to manufacture gears? I never have given much if any extra BL for drag racing but have gone with .010-.012 for dirt and oval track stuff with .008 being the number I shoot at for the street. But one thing the remains the same is to check BL in at least 3 different positions. It's very rare that you will find a carrier that has perfect runout and that goes the same for the ring gear and they can stack up against you. Once I rotated the ring on the carrier to see if things would improve and it did. Having .007 in one spot and .012 in another didn't make much sense to me so I checked the runout on the mounting face of the carrier and it was out .005 and rotating the ring gave an acceptable reading checking in 4 positions....
                                       
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