I know this question has been covered numerous times, however, when I purchased my car I had no real plan for any sort of bolt ons besides rims, tint, and a catback. Before I even knew what a "dsm" was I got a 2.5 in. catback with a 2.5 in/out magnflow muffler. Fast forward a year later and I have replaced the motor along with a Big T-28 turbo, just bought ECMLink, injectors, fuel pump, etc, and am back to my exhaust. Will I be creating detrimental backpressure once I start increasing the boost. I know that I should have done it the "right" way the first time but I did not and here I am. My understanding is that my downpipe "should" be replaced making me feel that I should replace both the downpipe and catback at this point. Some say that I will be fine with my current setup and some disagree. Opinions? Thanks

Its not too bad. Most JDM catback options like the Apexi N1 for the FWD DSM's have a resonator which internal diameter is down to 1.5" or so. The AWD N1's are full 3", dunno why they decided to bottle up the FWD's. 3" would be better, but the 2.5 isnt that bad.

check it out:
Index of /eclipsetech/partsthatsuck

^ Thanks for that info. I guess Im not as much worried about my 2.5 catback but I worry that my stock downpipe is the restriction. Maybe Im just being paranoid though since Im really not pushing any mentionable power. Thanks again.

I don't know if you care about emissions and such but you could also look into getting a high flow-cat or perhaps a test pipe which should free up the flow a bit more for you.

In regards to the Magnaflow (love the muffles, I run 2 on my setup), do you have the straight through race version or a chambered muffler? If it's the chambered one you'll want to get a straight through race design.

LiQUiDx said:

In regards to the Magnaflow (love the muffles, I run 2 on my setup), do you have the straight through race version or a chambered muffler? If it's the chambered one you'll want to get a straight through race design.

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It is chambered unfortunately, thanks

Big T-28 GSX said:

It is chambered unfortunately, thanks

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If you have the big t-28 still 2.5" is good enough. your down pipe is a restriction though. Change the dp and im sure you will feel the difference right away. For most cars if you replace everything from the head down its like night and day. but idk on our cars. the most I've replaced on my gsx is a cat back.

I don't think the 2.5 is restricting anything really at those power levels, I've been running a 2.5 inch down pipe and test pipe leading to my thermal 3" catback for a tad over 10 years and have made a best on my last turbo of 438hp, and 396 lb/ft torque on the turbo alone at 30psi (it'swas a T3/T4 57 trim) And with a shot of nitrous on top of it I pulled off my all time best of 503whp (no torque reading though it was a free dyno pull at a shop during a promational thing and they weren't hoooking up a tach line or anything, just measuring you against' miles per hour) exhaust temps were never high usually about 1500 or so after a 3rd gear pull.

Anyway, my point is EVERYTHING i've read on the subject says 2.5 doesn't become a restriction untill you've passed the 500whp mark. The main reason i'm just this weekl making myself a 3" SS down pipe is that with my new turbo i've been exceeding 500hp

I'm basing my new turbo's estimated horsepower to be in excess of the 500 mark using the calculation of the AFR's compared to the amount of duty cycle with your injector size and fuel pressure factored in, my math yielded a conservative 546hp, the numbers were 12.8 :1 AFR, 1000cc injectors running at 100% IDC and a base fuel pressure of 43psi

So i think you'll be just fine on 2.5 untill you are also getting to or passing 500whp

Thanks for the replies

This is the system I run with my FP BT-28:FP Exhaust Manifold,S.S.Tubular Recirculated O2 Housing,Borla 3inch Downpipe,OBX High-Flow 2 1/2inch S.S.Cat,SR-S 3inch S.S. Cat-Back Exhaust.

He is runing a STOCK downpipe, yes the stock downpipe is a restriction. Buy a aftermarket dp and you are good to hi

I agree that you will need to replace the stock downpipe. When I was in your position a while back with a 2.5" catback and looking to replace my downpipe (when I did my turbo swap) I started this thread, which you might find helpful: http://www.dsmtuners.com/forums/bolt-tech/361863-3-downpipe-2-5-exhaust.html. The reason I was asking about the 3" downpipe was because they are more prevalent and thus cheaper. In the end though I went with a full 3" exhaust.

9!'clipseDOHC said:

I agree that you will need to replace the stock downpipe. When I was in your position a while back with a 2.5" catback and looking to replace my downpipe (when I did my turbo swap) I started this thread, which you might find helpful: http://www.dsmtuners.com/forums/bolt-tech/361863-3-downpipe-2-5-exhaust.html. The reason I was asking about the 3" downpipe was because they are more prevalent and thus cheaper. In the end though I went with a full 3" exhaust.

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This is the exact response I was looking for and why I would go to a full 3 in. because 2.5 in dp is more than its worth and I didnt want the bottleneck.Thanks

Hmm, well definitely want rid of the stock down pipe. But the fact that 3" is cheaper than 2.5 when it comes to buying pre-made down pieps is something i didn't know about. And in that case go ahead and get the 3". Sometimes i overlook a lot of things like market price on pre-made parts like intercooler setups and exhaust parts because i make all my own stuff and make all that stuff when it comes to my customers as well just because i know i'm saving them money either way from the get go.