my question is,
what is the standard size zorst for 98-00 cars , ie is it 2.5in or other,
+what will fit what concerning diamiter, can a 3in down pipe fit a centre section of 2.5 ect,
just looking to fully decat car and have been offerd a 2.5 and a 3in in diferent sectoins, what to do ?

 

standard is 2 1/2" most common makes of backbox and centre will bolt on ie scoobysport, h&s, magnex scorpion etc. revolution is 3" and i think hks is.
aftermarket downpipes are normally 3" tapering to 2 1/2 at the flange. open neck design is considered the better system. you can get 3" downpipes with 3" flange but you need a complete 3" system to mate up to it. hope that helps
ken