i have a dc sports 4-1 header, and the apex'i ws exhaust. Would i need to order a cat with 2.25 in and outs? i just wanna make sure im getting the correct size here. Also i keep hearing people rave about the power increases of the jdm dc sports 4-1. Would it benefit me that much more to hold off on buying a cat for now, switch to a jdm 4-1, and then buy a hi-flow cat with a 2.5" in, 2.25 out? I mean would the 2.25" piping of the ws exhaust (which im assuming is the correct piping diameter) diminish the power increase potential of running the header with the larger collecter? (making it pointless to do) Ive heard of type R owners running jdm 4-1's with a hi-flow cat on the stock itr cat-back.. any help or thoughts would be great.

 

Going from a 2.5" collector, to a 2.25" exhaust is a bad idea. It will create reverse exhaust flow and cause backpressure.

The DC Sports 4-1 has a 2" collector, so you would want a 2" in and a 60mm out. (WS is a 60mm design).

http://www.ipsracing.com sells bolt on carsound cats, you might want to check with them and see if the have 2.0" -> 60mm designs.

 

I have installed a Magnaflow High flow cat 2.5" in & out, 2.5" pipe throughout the car. Lost very little power below 1500 rpm not enough to notice really but gained more thru the mid and upper rpms and still get 27 mpg.

 

2.5" OD is huge for the mods you have on your B18A. Are you planning to do some serious all motor or FI?

I wouldn't use 2.5" unless you are planning on making 200 - 235whp.