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It seems to me that in an exhaust system there are three main points of constriction....at the exhaust manifold, the cat and the muffler (and the muffler isn't major). I see/hear about so many guys using cat-back systems but leaving OEM cat. Wouldn't any hp gains be minimal? I would think performance wise if you're going to leave the OEM cat on there just a hi-flow muffler would be very, very nearly the same.

With a 1.7l motor you just aren't talking about that much flow to begin with. I see these catbacks advertising 2.5" pipes. I understand theoretically about "breathing easier" but you have to be pretty much at the point of diminishing returns. Even if say 2.25" pipes make a difference (and I'm guessing they probably do around 1-2hp) why not just oem style pipe and save a bunch of cheese?

I would think that using a good header, stock downtube, high flow cat and muffler would be about as good as you can get.....of course it won't be shiny but it should outperform a set up w/header & catback & OEM cat and it would be a couple hundred less $$.

No disrespect to anyone who has a catback or buys them mainly for the sound/look aspect....this is just something I was thinking about. Am I wrong?

 
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well i believe you are right about the OEM cat. It is very constictive, but i also know that you need a cat if you want to stay legal, that is why most people buy a hi-flow cat.

about the header, in not so sure. what good does it to have a large tube header and a small tube downpipe, won't that constrict more than before?
and then, wont the same go for the downpipe to the exhaust system?
This is why i got a racing header, and will soon be replacing the OEM cat for a hiflow one and a better exhaust system.

Just a opinion/question.....

 
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i would think that you want to keep the piping larger, but without going too large. you don't want to lose all the back pressure or your torque will plummet.
you don't want to have one size pipe, then have the next pipe have a smaller diameter, that would make the hot air constrict and basically nullify the larger pipe where it originated.
for example, say you have an aftermarket catalytic converter that is a set diameter, lets say 2.25". then if you use a custom catback piping of a diameter of 2.0", it's going to compress the air and defeat the purpose of the increased air-flow of the previous pipe.
you want to keep the exhaust free flowing with as little resistance/restriction as possible for free flow, but without going so large that it loses the back pressure.
just my thoughts. i could be wrong.

 
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or.. buy a catback, and lose the cat all together.. like meeee. adds lots of performance.. at least it feels a lot quicker to me and the people that ride with me.

 
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buy a kamakaze header and eliminate the cat......throw the stock exhaust back on for inspection purposes if neccesary.

 
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i was thinking of going with a long-tube header design (yes even though i live in CA). and just swapping back to stock cat for smog checks.
for the EX, isn't there a better design than kamakazi? wasn't the jackson racing header really good?

 
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You'll of course be better off performance wise if you lose the cat all together, but lets just say for the sake of discussion that you want your set up legal.....

I'm glad skunk2ner brought up torque loss....you definitely don't want to go to large with your pipes. I just see so many set ups with catbacks with large pipes and OEM cats that I wonder how much of a difference (if any) does the slightly larger pipe make? For normal induction I gotta think 2.5" has got to be about it before you lose torque. It seems to me that for the money you're spending on a cat back you aren't getting much of any performance upgrade.

I think a performance exhaust should have a header to help get all the exhaust gasses out of the block...this will increase hp and gas mileage but after that it seems a lot of people then spend $400+ on a catback and keep their stock cat. It seems to me you could get a hi flow cat from magna flow or wherever a hi flow muffler, spend something like $150 total and actually acheive higher performance gains albeit w/o the shiny bits.

 
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high flow header + high flow cat/ditch the cat + catback exhaust with high flow muffler = a complete exhaust

it should be no other way if you're looking for somekind of performance from exhaust.

 
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I wanted to lose my cat, but now we have emissions inspections. It is crazy, hooking the car up too all these fuking machines and what not. Mine is due in August, I hope I pass.