...but I have a 1031 with that 205 and it is one noisy sob and I Dont like the vibration it puts out. At only 30,000 miles it shouldn't need a rebuild yet but you never know til you open it up...
Well, a 30 year old truck that drove on average 1000 miles a year or about 2.75 miles a day, probably spent most of its time sitting. That's like taking the truck to the post office or grocery store every day. Probably run for shorter terms and longer terms (nature of average), and that was probably really abusive. I'd guess that M1031 may never have been in 4WD, or at best in 4WD less times than you have fingers (and even then it was probably only for training). Dis-use is the enemy of long-life, things need to move frequently to lube all surfaces, and seals will dry out crack and leak if not warmed up and splashed with lube. You probably need a rebuild - the 205 is a helical cut box, so it should be quieter than a straight cut gearbox (gears slide past each other, rather than "slapping" together).
...I know the np205 is strong, but I wonder how it would hold up behind a duramax...
Input torque rating on the NP205 is 1100 foot-pounds. If you take your peak torque value from the engine rating (not the HP value peak torque and HP are usually at different RPM vales), you will probably find that that number multiplied by the low-gear ratio and the torque converter ratio, the resulting number is probably higher than 1100 - but you'll find that you will likely only ever hit that peak torque number if you pop a manual transmission while the front of your truck is pushed up against a rock or wall (and you are keeping the Allison auto...). By the way, I'm sure you know that doing something like that is stupid
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I'm putting a Cummins 6BT diesel in the XM1027, that is being up-tuned to 352HP at the flywheel (@ 2500RPM, only 740 ft-lbs) - peak torque is developed on that engine around 1750RPM @ 815 ft-lbs (only about 270HP). Now couple that with the Eaton Fuller transmission's 7.05 first gear ratio (and toss out any mechanical losses) and I'm looking at roughly 5,750ft-lbs. The reality is, you would only push the gas pedal hard enough to see that torque if your tire and wheel was stuck between two rocks and seized (not going to move no matter what). If your truck is moving, you won't actually develop that much torque, and your not going to push the pedal so hard that your truck spins out and crashes into a ditch full of the innocent baby children and fluffy endangered bunnies, catching fire and destroying the world for all eternity... right?
At freeway speeds up-hill on gravel or sand with a headwind is where you will see the need for the most power - higher gears increase speed at the cost of torque - my same setup in 6th gear ( 0.78 ) will only provide a peak of 635ft-lbs of torque to the NP205.
It'll hold.