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.
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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.
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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