The twin turbo acert motors are dogs, both the 13 and the 15, but then again an ISX isnt much to brag about either, the older 3406s and N14s just had plain raw power and low end grunt.
Ditto...dad's fuel transport semi was a '97 daycab eagle with an N14 Celect Plus...whenever I'd ride along he'd always mumble "damn this thing pulls good." (We'd only drive it a couple times throughout the summer to move equipment between properties) It takes a lot to impress him too lol.