Sorry for the dumbness of the question but what is the difference between mandrel and press bent systems??????

In short, press bend = bad. Mandrel = good..
Press bends actually 'press' the pipe to make the bend, as the name implies.
This crushes the pipe slightly, reducing the internal diameter.
It's theorised that a 2.5" press bend, flows similar to a 2" mandrel bend.
Mandrel bending I'm not sure on the actual process, but it keeps the same internal diameter through the bend.
Smoother bends = less restriction = more flow!!

thanks buddy