I am new to TUNING, my question is I want to cancel the speed limit on my sierra 2500HD 2008 I can't pass the 180 KM/h, and one more thing I've heard that my engine uses 4 cylinders at low speeds then uses the total 8 cylenders on high speeds is there a way to cancel it??

and finally is Gen IV V8 my engine's name or vortech V8??? thanks a lot
I know my questions might sound EEzZZ for you guys but i really need your help thanks a lot