Doesn't critical speed decrease with increased wall thickness? Those huge diameter shafts are excellent at handling all the torque, but its the RPM thats known to kill them.

I've already planned to set aside like 7-800 dollars for the custom two piece drive shaft... I don't want to play the guessing game with my drivetrain anymore.

 

just buy a stock driveshaft and install it. mines been 12.7 @ 104 5 times back to back on DRs. plus its been in countless 100-125mph races. just saying...why spend 7-800.00 on a "custom" driveshaft when the stock one is enough. just get it balanced. if you wanna spend the money by all means please do. id rather save the money and put it towards go fast parts.

 

I definitely considered that, don't get me wrong. But finding the right used shaft and shortening/lengthening and rebalancing would be almost as much as the custom shaft.

What truck did you get your dshaft from, and what sort of ujoints does it have?

 

Is a tune required for a 4L80/85 swap in general or do I just need to repin the harness? Setup in question would be a 4l85 behind a 4BTA cummins 4 cylinder using a factory 2006 PCM from a 4.3 v6.

 

several threads on it already . . . if it was a gaser, but I have no idea on the cummins you might try dieselplace dot com

tune or segment swap
new harness or repin the orginal