i have aem cai,dc 4-2-1 header and tanabe super racing medallion. and i am planning on getting a test pipe. what would you guys recomend the diameter of my test pipe to be with my set up. thanks for the help

 

2.25 as to match the header, or if it's a 2.5 inch exhaust maybe get it tapered up from 2.25 to 2.5.