ive been looking round on other forum and ive read a few posts the say if you fit a header with a 2.5" collector on a stock motor you will lose power.

is this right ??

 

no - if you have an intake and exhaust - it will help

 

stock or not if you get bigger any of the exhausto parts (header, collector, cat-back) ur motor will gain some power at high rpm but will lose some at the lower. The bigger the part the higher u will need to rev to get power. About ludes 2,5" is the best bet.