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Discussion Starter #1 (Edited)
This post is intended to answer some of the questions related to the emissions control systems on your Subaru WRX/STi and what replacing them means.

What is a Catalytic Converter?
A Catalytic Converter (somtimes called a CAT) is an emissions control device. The main purpose is to reduce harmful gasses like nitrous oxide (causes acid rain), carbon monoxide (causes smog which is linked to lung problems), and unspent fuel (a pollutant). It does this by causing a chemical reaction under which the exhaust of the car is chemically changed into "safer" gasses like Nitrogen, Oxygen, water, and Carbon Dioxide. It's important to note that Catalytic Converters do not reduce greenhouse gasses. In fact, they generate them by converting carbon monoxide and unspent fuel hydrocarbons into carbon dioxide.
Wiki Link

How many does my car have and where are they?
The 02-05 WRX comes with 3 Catalytic Converters. The first is a "pre-cat" which is located in the up-pipe. This piece is between the exhaust manifold and the turbo's hotside. The second is is the downpipe and the third is in the midpipe. A note that many people consider the dowpipe and the midpipe to both be one part (even though they are two pieces on the stock car) and refer to the combined part as the downpipe (which would include two cats). Many if not most aftermarket downpipes replace both the stock downpipe and midpipe.

The 04+ STi and the 06+ WRX do not have a "pre-cat" in the up-pipe. They simply have the cats in the downpipe and midpipe like the earlier WRX.

Is it illegal to remove the catalytic converters?
Yes and no. The actual laws and punishments vary state to state. California's laws are particularly harsh. In other states, there can be little if any regulation on this.

What are the pros of removing catalytic converters?
It is often thought that removing catalytic converters will increase power. This is not always true. Modern catalytic converters are not that restrictive of exhaust gases and especially on small engine cars do not "rob" you of much if any power. The WRX's main exhaust bottleneck is NOT the catalytic converters, it's the downpipe design. However:

The 02-05 WRX folks who have a CAT in their up-pipe should be aware that is has been known to break apart (which is like setting off a shrapnel grenade 6 inches upstream of your turbo) as boost and exhaust temperature are increased (basically when you add lots of power). It is strongly recommended that the CAT be removed before switching to a larger turbo. Generally, it can be left alone before then although many recommend it for "Stage 2" cars. Note this paragraph is ONLY true for 02-05 WRX's. 06+ and ALL year STi's (04+) DO NOT HAVE CATTED UPPIPES!

For nearly everyone, the downpipe should be very high on the list of mods on a WRX. As stated above, the stock design is very restrictive. An aftermarket bellmouth downpipe is a great performance mod. This effects your cats because most of the parts out there are going to replace your stock downpipe and midpipe (which get collectively referred to as downpipe sometimes). It should be noted that aftermarket downpipes often come with or without a cat. You do not necessarily have to run "catless" if you get a downpipe. You could run one with an aftermarket cat as well. That said, CATs cost money due the to precious metals used in their construction. A catless downpipe is going to be $100-$200 cheaper than an equivalent catted one.

What are the cons of removing catalytic converters?
The main negatives are headaches with emissions. This varies from state to state. There are other problems though. Exhaust from a non-catted car is rather stinky (nitrogen dioxide is very pungent), especially to a nose accustomed to modern Cat'd vehicles. Cats also muffle some of the engine sound so a catless car will be louder. Also, you're polluting the air with toxins. The threat of acid rain and smoggy cities were virtually eliminated by the Catalytic Converter.

Will removing a cat give me a CEL?
Generally yes. However, there are numerous solutions to remove these from your car's system. The easiest is engine management. Most of the solutions out there disable all emissions related CELs so they would not show up as codes for an emissions check. There are also manual solutions. The 02-05 WRX's pre-cat can be emulated with a simple 2.2kohm resistor in place of the sensor and the rear O2 sensor can be simulated or adapted to work in a catless setup.

Will I pass emissions without a CAT?
Matters on the state. Some places will fail you simply for modifying your exhaust at all. Others will fail you because you have no cats. Others will only fail you if your ODB-II port shows an emissions related CEL. This question is really more what is involved in your emissions testing than anything else.

Sniffer test (directly measures your emissions) - You need a cat and a decent tune to pass
ODB-II check - CEL fixes are all that is needed to pass this.
Visual Inspection - Mattering on how strict the inspector is, you may need a totally stock system to pass or just something that looks like a CAT.

Which aftermarket downpipes are available with at CAT?
Cobb, Helix, TXS, Invidia, and many others. Also, many exhaust shops can custom make something for you.
 

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Being new to the subaru i got an 05 STI i bought used. the previous owner removed the catalytic converter, and since then i have added HKS up/down pipe and HKS turbo exhaust. I want to replace the catalytic converter but a mechanic told me that i could not due to the downpipe that was installed...is it true and what could i do?
 

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Discussion Starter #5
Any decent exhaust shop should be able to cut out some pipe and weld in a CAT for you. It's not hard and is not limited to a particular type of downpipe.
 

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Will removing the cats affect gas mileage under normal driving conditions?

Can I just weld a bell mouth flange onto a stock dp to help the bottleneck but still pass smog?

(thought these questions are relevant to the topic. I can ask it in "newbie" if you want)
 

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Discussion Starter #7
Good questions, no problem. Thank you for adding to the thread!

Will removing the cats affect gas mileage under normal driving conditions?
No. Although clogged CATs can reduce gas mileage.

Can I just weld a bell mouth flange onto a stock dp to help the bottleneck but still pass smog?
The stock cat is quite high up. I'm not sure how this would work but I guess it's theoretically possible. Note that this will cause clearance issues with the heat shields and will be visible to a detailed visual inspection. Never heard of anybody doing this to be honest.
 

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I think it would be pretty difficult since most (maybe all) bellmouth downpipes are 3", so you would need to make it taper within just a few inches of piping. Also, most bellmouth downpipes reach towards the firewall before bending downpards. I don't think the stock downpipe does this. It bends right away.

I'm curious, mosc... how did you come to the conclusion that the stock downpipe cats are not restrictive? Have you (or anyone else) done any tests with them? I've often heard the opposite of what you said; that the design is not nearly as restrictive as the cats (namely, the first one).
 

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Thought this link might help some: CobbTuning.com - Exhaust Design and here is a couple pics of my stock DP. In the middle picture you can see why the stock DP design is "bad" and so restrictive. Just a small opening with the rest just being a flat piece of metal that gases are hitting against.
 

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You also have to take into consideration that the stock downpipe (and heatshields and all the other crap on it) is significantly heavier than almost any aftermarket downpipe out there. Does anybody know the actual weight of the stock downpipe? I just know mine felt significantly heavier than my aftermarket Cobb catted downpipe when I did the install. Do youself a favor, get an aftermarket downpipe - catted or not - either way, you'll gain some power and lose some weight.
 

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^^^^Do they do snuffer/sniffer tests in PA ?

If not, then you "should" be fine.....

Catless DP's are the problem children....
 

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Discussion Starter #13
Within Philly city limits, they do sniffer tests. In the burbs, they just do ODB-II. Since I'm from the area if you tell me more specifically where you live I can point you in the right direction...
 

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As a veteran smog mechanic since 1988, I should post a correction here regarding the description of a catalytic converter and how it works.

The 3 harmful exhaust gases cleaned up by a catalytic converter are 1) carbon monoxide (burned fuel), 2) hydrocarbon (raw fuel), and 3) oxides of nitrogen (NOx). NOx is not to be confused with 'nitrous oxide - NOs), a laughing gas commonly used by surgeons when having your wisdom teeth pulled. NOs is also used as a powerful additive to the air/fuel intake on very hi-performance vehicles.

The 3 harmful exhaust gases are 'converted' into carbon dioxide before shooting out the tailpipe. Carbon dioxide is the make up of nitrogen, oxygen, and water. It the same gas as what are human lungs expel. The conversion action is done by heat reaction from precious elements housed within the exhaust passage way of the catalytic converter.
well coming from chemists stand point, nitrous oxide is N2O not NOS. NOS is a name brand for nitrous oxide for car power (Nitrous Oxide System), theres no sulfur that comes out of your exhaust . the CAT breaks down heavier gases like nitrogen dioxide (NO2) into nitrous oxide (N2O). so your car does produce laughing gas if you have a CAT.

also carbon dioxide is not a make up of nitrogen, oxygen and water. its made up of one carbon molecule and two Oxygen molecules. There’s no Nitrogen in CO2 or Hydrogen.

the CAT oxidizes the hydrocarbons into water and CO2. a hydrocarbon is 1 to at least 60 carbons connected to each other with hydrogen surrounding on all sides. carbon makes 4 bonds so the ends will have 3 hydrogens and the middles will have 2. thats where you get your hydrogen to make water.
 

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well coming from chemists stand point, nitrous oxide is N2O not NOS. NOS is a name brand for nitrous oxide for car power (Nitrous Oxide System), theres no sulfur that comes out of your exhaust . the CAT breaks down heavier gases like nitrogen dioxide (NO2) into nitrous oxide (N2O). so your car does produce laughing gas if you have a CAT.

also carbon dioxide is not a make up of nitrogen, oxygen and water. its made up of one carbon molecule and two Oxygen molecules. There’s no Nitrogen in CO2 or Hydrogen.

the CAT oxidizes the hydrocarbons into water and CO2. a hydrocarbon is 1 to at least 60 carbons connected to each other with hydrogen surrounding on all sides. carbon makes 4 bonds so the ends will have 3 hydrogens and the middles will have 2. thats where you get your hydrogen to make water.
I think that is the most intelligent thing I have ever seen you post...lol
 

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Discussion Starter #19
As a veteran smog mechanic since 1988, I should post a correction here regarding the description of a catalytic converter and how it works.

The 3 harmful exhaust gases cleaned up by a catalytic converter are 1) carbon monoxide (burned fuel), 2) hydrocarbon (raw fuel), and 3) oxides of nitrogen (NOx). NOx is not to be confused with 'nitrous oxide - NOs), a laughing gas commonly used by surgeons when having your wisdom teeth pulled. NOs is also used as a powerful additive to the air/fuel intake on very hi-performance vehicles.

The 3 harmful exhaust gases are 'converted' into carbon dioxide before shooting out the tailpipe. Carbon dioxide is the make up of nitrogen, oxygen, and water. It the same gas as what are human lungs expel. The conversion action is done by heat reaction from precious elements housed within the exhaust passage way of the catalytic converter.
Well, there's some correct info in here but there's also some complete falsehoods as synolimit pointed out. Wow.

Anyway, since we're into a more technical discussion on catalytic converters, I feel compelled to make my usual whine about reductions in carbon monoxide (smog) and hydrocarbons being converted into a major cause of global warming (CO2) emissions. Cars make up somewhere around 20% of all CO2 emissions and some of that is brought to you by the wonderful catalytic converter. Course, most of that CO2 was made in the engine not in the cat but it still makes for good debate about which emission is really worse.
 

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while there are lots of bad things that come out of an exhaust like ozone and the gases we already named, the CO2 im wondering if its really that bad? not all the NO2 and CO will get oxidized in the CAT forming N2O and CO2 so no matter what the exhaust is still bad. what im getting at is i heard a speech from a total Dbag that seemed to not know crap about the enviornment but is a Dr. in meteorology. among is rambling that there is no global warming, the ozone layer is fine, are earth is better then ever, (told you he's a Dbag) he said one thing that made me think. they say CO2 is very very bad! but havnt we been told since we were 5 that ALL plants USE CO2 and give us oxygen?? so in a way the more CO2 we make the more oxygen we get back, thats if we stop cutting down all are rain forests and stuff. now the CO2 we make could be over whelming are plants and the extra just heats the planet that takes a whole lot more to talk about how it does this, but i thought that was a little good point the Dr. made. not sure how good it was though.
 
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