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I have an 03 civic ex automatic. I currently have a cat-back installed and I'm not trying to swap the engine and all of that stuff, I'm not racing, it's more for show. But I want it a little louder and a bit more power so I'm going to put 4-2-1 header on. I was also looking into the high flow catalytic converter, but I was told I needed to keep the converter stock, because I need the back pressure. I don't really understand what that means, so I was wondering if someone could explain it to me and give me their opinion of whether or not to change it or keep it stock.Thanks

 
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Header will do absolute zero for you performance wise especially since you have an EX model and thus a better flow than the lower end models. Usually lower end models like dx lx will swap to an EX header, but there is no reason to change if you already have one unless you just want your engine bay to look pretty. Even then you need an expensive one to make sure it fits right ect ect... As for the back flow thats important to in order to keep your accel strong, after having switched my setup to a SRI and 2 1/2 inch catback and open muffler I think I messed up my backflow and the car bogs at low RPMs now. Fixing that with a nice silent restricting muffler here in a week because I cant stand the fart cannon anymore.

If im wrong on any of this then pls someone corret me :P

Edit, btw I dont think the hi flow cat will offer any performance increases eaither, I have only heard people say the oposite and that it can throw a CEL. Otherwise ill get one to.

 
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yeah the cat has a mesh screen in it that restricts airflow and keeps a bit of pressure in the exhaust system. gives the car better low end power below 4k rpms. on such a small motor, you dont want huge free flowing exhaust pipes.

 
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Hi if someone can help me, i like to know if the cat converter is really really necessary, since mine has been lost, and a couple of days the check engine turns on, the computer reads that the O2 sensor and the iat could be recieving more voltage than needed, could that be for the missing cat ?
thanks

 
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A missing cat will cause a check engine light. It's also bad for the environment.

 
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And you cabin will be stinky without a cat lol

 
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gearbox where were you a couple weeks ago when another poster got in an argument about this lol


gearbox is right.

header does not help, unless you force more air in through the intake side (supercharger)