Im speaking in exhaust fitting terms lil fella sorry,Mines(1zz-fe)is custom made primarily(well was) made for semiracing specs ,as its still a daily driver ive kept the cat,ive personally made exhausts for 3SGTEs,V8s & WRXs,all differ from cat/No cats ,The point i was making its does need back pressure ,its not a huge amount enuff for u to feel the difference,but enuff to equalise itself pressure wise in the system,it does matter wat muffler u install in the rear ,a baffled or chambered muffler is recommended (cheers silva) pls dont confuse Drop CES an email mate as they will be able to help you out with specifics. :D