I have purchased a RB header, and I have a Magflow high flow cat. I know thaty I will need to do some fabrication to fit, but will going from a 2 in. od on the collector of the header to the 2.5 in od on the cat, would that cause a disturbance in the exhaust flow, and cause too much pressure? Any help.

 

i'm not sure but i think it would help, cause the cat is a restriction, and if you make it bigger its less of a restriction

 

It will help, and if u have your sleeve's still in you will need a little back pressure to actuate them, all's you will need is a decreaser piece that goes from 2" inside diameter, to 2.5" inside diameter. Unless the header bolts right up to your high flow.

 

I have the same setup except using a Catco cat. There was a slight neck down at the flange to the header but its ok.