CM 870 435 "ST"? Torque ratings?

[Craag]

On quickserve my engine serial number (79092458) shows ISX 435 ST? Looking around the web I found that to mean smart torque? What exactly, or rather, how exactly does that work and where do you find peak ratings at rpm? I understand there are 2 different ratings but how or when does the truck decide where it's peaking at?

I pull heavy, usually gross around 120000 lbs, 15 spd and 3:90 gears. Every once in a while at low rpm (950-1150) and splitting out of a low gear it seems like the truck just takes off. Other times its just seems like normal, hard work. not sure if its just the nature of the beast with an old truck, something Im inadvertently doing wrong, or right when it does seem to go.

When researching the torque online a chart from cummins gave values on 2 other 435 ISX motors but both were not ST they were V and something else? Dealer told me its a 453 but I assume previous owner had that HP bumped but would that have also affected torque?

Thanks!