When is it need and at what pressure would a setup need to be at?? How do you go about calculating something like that.

 

I've always been told after 300hp or when you go with an after market pump.

 

you only really need it if you start to max out your injector duty cycle. you increase the pressure a little to richen it up so you're still within the safe limits of your injectors. you don't want to exceed 90% duty cycle of the injectors. to figure out which injectors you need, you'll need to set a hp goal and search which injectors meet those numbers. each manufacturer rates their injectors by how much hp they can support. hope that helps