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I have been arguing this with a buddy of mine for a while now...
With my Exhaust setup i will most likely be removing my cat entirely with a DC Sports Header, now here is the thing, i have had people say you need "Back Pressure" I Disagree.. from my understanding of it, is all you need in a modern Fuel Injected engine, is proper "Exhaust Velocity" the idea behind it is that it will assist in "Pulling" the exhaust gasses from the cylinder when its correct, rather then the engine having to "Push" the gasses out.



Lets hear what you guys think.

 
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Without enough backpressure you will lose a lot on the bottom end. There is definitely a balance between backpressure and high flow.

 
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https://www.civicforums.com/forums/2...t-diy-faq.html

Triz talks about it a little about half way down his post here and I've also read about it elsewhere (Hayne's Modification manual or something if I remember right.) I can also speak from personal butt dyno experience. I felt like I lost a little oomph in the low end when I opened up my exhaust.

 
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its a common misconception. exhaust velocity is important yes, but mainly at the head and header. on a lower displacement motor like the civic, adding a huge 3 inch exhaust may drop the torque and increase hp but we are only talking a few whp difference. on larger motors, a free flowing exhaust makes more power, usually a much bigger amount like 10-20whp. its not even worth considering this debate on a civic because you wont feel the difference. on real cars with power, the stock exhaust ends up being too restrictive to meet noise standards and that is where you start losing power. on a civic the stock exhaust is already well capable of doing its job. if you need proof, dyno your car before and after adding exhaust. you will see at most a 2-4whp change, which could even be the result of standard dyno error due to different test conditions.

 
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Don't remove your cat. It adds like a 1/2 hp and pollutes the allready polluted air even more. Get a Random Technology hi-flow cat. You also loose valuable back pressure with out a cat, great for turbo not for n/a.

 
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I'm not 100% on not having a cat, but the factory one is going, due to the header I want (I also live in a heavily forrested area, not that's its an argue net to run with ought a cat but it does help a bit).
I get a cat it will probably be a Magnaflow, I have never heard of random technology before, will have to look at them.

As for the other comments, I'm not even neccisarily meaning this discussion is 100% civic. I'm talking in general here.

Quite personally? I don't know why you would put a 3" pipe on a car with this low of displacement... My max is 2 1/4" but would prefer 2"...

 
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haha it all depends on how its optimized and tuned.

my 1 liter bike has 3" piping and dual exhausts...

 
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Well heres my input, if this helps, i have a full ebay stainless exhaust from the header back, with no power gain i figured why pay so much for the dc sports header. So i have a 4-1 ebay header, with a catless downpipe (instead of cat it has a small resonator in it), then a full 3" n1 style catback. Yes i am running 3" exhaust. Honestly if i run it straight piped i can tell a difference, but if im running it with the canister muffler, plus the resonator thats in my catback the car feels great and doesnt sound half bad either, but thats just me. I had custom flanges made so instead of a 3 piece catback like it was i made it a 4 piece catback and now i can unbolt the canister and bolt up a piece of 3" straight pipe with a 4" tip or a bullet muffler with a 3.5" tip. Things you do when you get bored.