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i wanna know if i need a down pipe for better performance cuz i have headers and soon to get tanabe catback. will down pipe make difference? will i pass smog check with or with out downpipe?

 
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You will not pass a smog check because the down pipe eliminates your cat. Just a little side note. A down pipe on our cars is pointless unless you are planning on running a turbo or race gas.

 
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I haven't seen a down pipe with out a Cat attached, or shortened to allow for a Cat.

some down pipes have performance Cats on them,
Other are shorter so you have to bolt/weld a Cat on.

A downpipe will definatly help,
If you have Intake, headers and Exhuast, with stock down and cat.
Then you Get air really quick,
Send it nicely through the headers to the down pipe,
It slows down at the Down Pipe,
and almost Hits a Dead Stop at the Cat.
But then can procede quickly out the muffler.

so a 2.25" down pipe with performance cat, would complete the air flow.

 
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yes the downpipe is better to get more wast out.

The only thing that would happen if you do decide to take off the cat or use a high flow cat is that your engine light will turn on because the system will detect that you are not burning all the gas that is give to the car. you would need to get a O2 simulator. i have not found one that will work with the 7th gen civics. IF any one can find a DIY on how to make one or one that works that would be great. I know that you could use a 100k 1/2 volt resistor and connect it on the black wire but i don't know which other wire to connect it to. but should work just like the O2 simulator and cost less. i am still looking into this.