Running Riot

So I got all my parts. ARP head studs, Mugen head gasket Skunk2 pro cam gears. SO I'll have them all in within a few weeks but my question now is this. I had a DCH with a 2.25 collector and a GSR cat back. Ordinarily, the GSR would suffice, however, with this set up, not only am I bumping up compression and hp but I'm supplementing my already installed pro series cams. So, I've read the exhaust theory links and other exhaust theory threads about exhaust pulse and using too big of a pipe on NA engines etc. I have a 1.6 DOHC and the 2" i have is good enough but now that i'm throwing all these power adders under the hood, there has to be a point where using a larger pipe diameter becomes necessary and my question is after my parts are installed i'll have

B16a
JDM P30 pistons
Mugen head gasket
Skunk2 pro1 cams
S2 pro cam gears (set +1 +1)
ITR manifold
Spoon 70mm TB
DCH 2.25 collector.
Compression at about 10.9-11.0

Is it worth picking up an ITR 2.5 header and going 2.5 all the way or should I get the ITR header and 2.25 all the way? Any suggestions?

TRIZ

iTrader: (14)

I'd use a race header. Run your setup with your current exahust, feel it out. Bolt on the ITR exhaust and feel it out. Stick with which one you like more, you can always sell the parts you dont want to use.

ronaldo9

iTrader: (6)

2.25 all the way IMO. N/A still need to utilize some back pressure for tq.

Running Riot

The +1/+1 setting made 10 hp on a GSR with the same cams and similar IM BEFORE 5500rpm, definately going for a wide powerband. The 2.25 collector and gsr cat back I got on there now I think is choking my cams. When I ran 2.5 previously, the transition to vtec was smooth, like constant acceleration. Now i can feel the vtec spike.

Think of it this way, a big dog is chained to a fence, the dog tries to run after someone and takes off qucikly gets to the end of the chain but still tries to keep going. (thats my car before vtec) then the chain break and the dog takes off full speed (after 5500)

previously it was just like the dog was unchained however it took it a while the get started.

My powerband shifted way too high with the 2.5 and 2.5 was hella loud and raspy, thats what prompted me to get the gsr and now the gsr feels like its choking my engine (not to mention i avg 4 less mpg and 8 mpg less on the highway) and from what i've read I couldn't tell definitively if 2.25 was teh right choice or 2.5 again and just deal with the rasp.

Running Riot

So I had the shop put the 2.5 axle back portion of my magnaflow back on and wow, SO MUCH MORE BETTER! Maybe not a whole lot of power but definitely smoother power delivery. The VTEC spike is pretty much gone and the car doesn't feel like it is topping out at 7000+ like it used to. Thanks for the input guys. Appreciate it.