Question on exhaust system piping size

[Chappy]

If I am going to be using a header with a 2.5" collector can I still use a 2.25" cat and exhaust? Or can I use a 2.5" cat with a either 2.25" or 60cm exhaust? Or should I use a 2.5" cat and a 2.5" exhaust?
Sorry about all the questions, I just want to make sure that I do this correctly.

 

[TegSox]

You want piping size to remain the same or increase as exhaust flow moves along the exhaust system.
You want the header collector to equal or be bigger than the diameter of the cat, and you want the diameter of your exhaust piping to equal or be bigger than the cat.
Decreasing size at any point creates exhaust reversion, that is, small amounts of exhaust gets sucked back into the combustion chamber through the exhaust valve and fouls the incoming air charge. As you know, smoke doesn't ignite and it takes up space that clean air could be occupying.

Are you sure you want a 2.5 inch exhaust? What are your planned engine mods and powerband area?

 

[Chappy]

Right now I will only have I/H/C/E, but I plan on doing more to my engine. (once I get enough money). Like getting a new TB, IM, Valve springs, Cams stuff like that. Also I got a really good deal on the header with the 2.5" collector.

I plan on building the powerband around the mid to upper rpms (yeah I know it changed, but upon doing more research I think thats what I want.)

 

[import810]

get a full 2.5" exhaust like hytech,smsp, T1R, kteller, custom(like me), or magnaflow.

It should be good if you getting at least 170hp to the wheels

 

[MoJoJoJo]

do a search...this subject has been covered so many times

 

[TegSox]

I'd recommend a 2.5 inch exhaust to someone only if they plan on pushing WHP to 190 or more.
A GSR with a properly sized I/H/C/E setup can get you into the 160-165 WHP area, that's with a 2.38 inch exhaust, which should be used in such a setup.