Timing
Timing is controlled in the pcm, if you want to dial it back for juice or the like I would suggest an aftermarket ignition like the Malloy 685 which has a retard function, window switch and other good features for the bottle.
MSD makes stuff for this too.
Really it would be best if you told us why you want to do this and we could much more easily help you figure out what is the best way for you.
My answers were based on a nitrous guess, if boost look for a box with boost referencing for the retard.





