stock input shaft?

[BSilveira]

I had my transmission rebuilt not to long ago. I wasn't to educated on transmission upgrades at the time, and now I'm a lil' concerned. The shop that did it is not too bright when it comes to diesel performance. It all started when I took it in for a 100,000 mile fluid change. They called me and said they found a broken clip in the pan. I had no choice at the time, and had to have them rebuilt it. I knew about about the BD torque converter and brought one to them to install. Right after I got it rebuilt is when I learned about billet input shafts and valve bodies. 17,000 miles later. . I now have a BD valve body and a double deep Mag-hytech pan that is going to be installed Tuesday. I'm concerned about my stock input shaft. How much power do these hold? My goal is to have 500 hp and/or 800 ft. lbs. of torque. Will it hold up?

 

[cerberusiam]

Your in the danger zone at 800 TQ and a tighter converter but a lot will depend on how you are using it and whether you modify the TC lockup strategy. If you are not towing heavy, just putting the power down on a dyno, and not locking the converter until the rpm's are up as the stock program does it will probably live for a long time. If you tow a lot and/or modify the converter lockup program then the billet shafts are a must. Aside from the pullers/racers most shaft problems are going to occur at lower rpm's when the converter is locking and holding in the high part of the TQ curve. I have a 47RH that has been holding 800+ TQ for over 8 months with no issues but I don't lock the TC until the rpm's are up and in hard pulls I am not locking it until the load is off, and, I don't drive it easy.



However, for peace of mind and a solid trans billet shafts can't be beat.

 

[BSilveira]

lock up?

I'm still learning... but what's is TC lock up? How does it work? I see the chips for it but don't know how they function. Please explain. I tow a 20 ft. race trailer loaded with race jetskis, and I'm always driving it like it's a trophy truck.

 

[cerberusiam]

TC lockup is accomplished by a clutch and apply piston in the front part of the TC. When the TC is unlocked you have a fluid coupling and an efficiency of anywhere from 70% to 90% depending on your converter. Essentially you are loosing 10% to 30% of the crank power in fluid coupling. By locking the converter you are essential running 1 to 1 crank to input shaft. All the power is being transfered to the trans. This has multiple benefits. More power is transferred to the rear wheels improving towing ability, acceleration, etc. In addition the rpm's are lower improving engine efficiency, emissions, and hopefully mpg's (its all about CAFE ratings ya know). Watch your tach the next time you are driving and when you get to cruising speed and holding a steady throttle you should feel what is almost another shift and the rpm's will drop. A stock TC usually see's about a 300 rpm drop on lockup, an aftermarket could be as little as 100 rpm's.



The stock program in the ECM basically only allows lockup in a narrow range. Any movement up or down on the accelerator more than a little will generally cause the TC to unlock and attempt to save the trans. Stock the TC only locks when you are at a steady throttle position or lightly pulling. By adding a controller to hold lockup longer and apply it sooner more of the TQ is transferred improving accleration, towing performance, etc.



The problems start when you lock the TC when the Cummins is pounding out the TQ. The trans is severely under rated for what even the older 12V engines were capable and lockiing the TC is just about like dumping the clutch on a manual. Do that enough times at peak TQ and you will needing a clutch and possibly a new trans. Same goes for the auto, you end up slipping the TC clutch, hammering the crap of internal tans parts, and generally killing things. The input shaft takes the brunt of abuse since it is the first component inline, fix it and the intermediate shaft is next, fix it and the front hub is the weak point, fix all the previous and the output shaft is in danger.



A performance built trans inlcudes the ability to handle more TQ with more and/or better clutches and higher apply pressures. with no more slip at lower r's, where the TQ is, all that gets transferred to the hard parts and failures occur.



HP is not a big deal as it is a function of rpm, but, the TQ generated will just trash the stock pieces. Not sure how well your trans is built but as a general rule if you are going anymore than 80 to 100 HP over stock you should seriously consider improved hard parts. Even at the higher power if you drive careful and don't do any lockup mods you will probably be OK. It's like any other power mod, once you feel how a 7000 lb truck accelerates with 500 HP and a locked TC it is really hard not to want to do it at will. Oo.





Now you see how slippery the slope gets?