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Modifying stock exhaust


GT_Red

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I was wondering if anyone has just modified their stock exhaust? What is the diameter? I was thinking it should be 2.25 or 2.5", and I was also thinking about just modifying the middle muffler to an long resonator and replacing the stock cats with high flow cats...any thoughts?
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How many cats are on the exhaust? I see uppipes downpipes, 2 section this and that...I am a little bit confused...at times it seems as if there are 3 cats on the thing. If I remove a cat on the uppipe...or downpipe...how does it affect emissions?
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Depends on the year. 05-06 it's three, 07-09 it's two.

 

If you have three, the first one is before the turbo in the uppipe. It's only used for cold-start emissions really, so that wont hurt you at all to remove. Also it's so hard to get to, no one will EVER check.

 

On the downpipe, you might as well just get an aftermarket one. After you modify the stock one with high flow cats, and maybe a belmouth, you'll be at the cost of a nice aftermarket downpipe. If you wanted to do it on the cheap and legal, you can get a CNT catted downpipe (cheap, but fairly good) and add a second cat to it.

 

Also, the stock system is 2.5" to the Y in the back. It flows farly well stock, so modding the exhaust wont do much for you unless you get a BIG turbo. You're looking sub 5HP on the stock turbo after the downpipe is done.

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Also, the stock system is 2.5" to the Y in the back. It flows farly well stock, so modding the exhaust wont do much for you unless you get a BIG turbo. You're looking sub 5HP on the stock turbo after the downpipe is done.

 

If you have a 3" dp then the stock exhaust is still going to be a restriction. IMO, a fully open 3" all the way back with a straight through resonator and muffler will add more than 5hp, especially towards the top end.

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Here you go...

 

The catalytic converter utc-pyro is referring to is labeled "Pre-catalytic converter". Most after market down pipes replace BOTH sections with cats (labeled here as down pipe and mid pipe), unless they are specifically called a "shorty" down pipe.

 

Cheers!

551040297_LGTintakeandexhaustmap.jpg.25e4d2044c2a6543bde76b9076404fe9.jpg

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Excellent information I appreciate it. So I will probably go with a catless up-pipe and what about replacing the middle muffler? I want more sound from my exhaust, at this point all I hear is whine and rattle from the engine...its too quiet :)
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You don't need to replace the UP unless you have an '05, that was the only year for the UP cat.

 

Most after market cat back setups will definitely give you more sound. Some have a resonator (like stock but much higher flowing) and the mufflers will let out that boxer rumble. To really let out the sound though you need to get the 2 DP cats out of the way. Of course catless is loudest but a single High Flow Cat in the DP won't hurt the sound or power much. ;)

 

Cheers!

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You don't need to replace the UP unless you have an '05, that was the only year for the UP cat.

 

I have an '06, there was a cat in the up-pipe.

 

OP, change out the up-pipe, get a NEW (or used aftermarket) downpipe with at least one cat in it. You car will pass the ODBII test still. If you have visual, put on another cat or the heat shield from one. If you're in Cali, you're screwed ether way by CARB. Just swap on your stock one downpipe for emissions.

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I have an '06, there was a cat in the up-pipe.

 

OP, change out the up-pipe, get a NEW (or used aftermarket) downpipe with at least one cat in it. You car will pass the ODBII test still. If you have visual, put on another cat or the heat shield from one. If you're in Cali, you're screwed ether way by CARB. Just swap on your stock one downpipe for emissions.

Oh yeah, sorry I forgot it's '05 and '06 with the UP cat. :redface::p;)

 

GT_Red - Do you know if your state does OBDII checks or sniffer test or visual inspections? Or a combination of the above?

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If you just want more sound, get a cat back. That way you dont have to fuss with tuning and such.

 

I am going to remove my resonator, and mufflers from my stock exhaust soon, to make a straight pipe. Not sure how it will turn out, but I will post with pictures and sound clips.

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I am from AZ, and we do OBDII emissions, the sniffer is mainly for the old cars without access ports. So if only 1 cat will do the job then I am in. Sounds like all that is needed is a catless up and catted down pipe and thats it? The exhaust is a sufficient 2.5" from what I understand, but is it 2.5" through the mufflers too?
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I am from AZ, and we do OBDII emissions, the sniffer is mainly for the old cars without access ports. So if only 1 cat will do the job then I am in. Sounds like all that is needed is a catless up and catted down pipe and thats it? The exhaust is a sufficient 2.5" from what I understand, but is it 2.5" through the mufflers too?

 

 

Yep, that's it. Car will pass as long as they don't visual it. Also it's by far the best bang for the buck.

 

Stock is 2.5" to the Y, then necks down to something like 1.75" to the cans. Cans are at most 1.5" on the internal pipe leading out.

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