Why is it so necessary to upgrade the fuel system in a modified car, I know that sounds dumb, but I mean like, some people say avoid a modified car if the fuel system hasnt been modified too, why is that?

 

well if you have a turbo and are running big boost levels with bigger injectors then you will need a more powerfull fuelpump to feed the bigger need for fuel.
The stock fuel rails aren't as good for turbo or supercharger applications but for a na car they are fine.

I am trying to post you the fc3s pro because it is better than my little paragraph
go here http://www.fc3spro.com/main.html
look for tech section and search for upgrade fuel rails

it will explain why it is a good idea in certain applications

for some reason it isn't loading now for me.

 

If you have a TII the first thing you should upgrade is the fuel system.

Making more power usually means getting more air into the engine, you have to inject more fuel to compensate for the added air. Too much air, not enough fuel = bad. Stock fuel systems simply cannot keep up with the demand for more fuel.