Usually the first thing to be replaced is the restrictive factory exhaust. You will note that tuners do not just replace the muffler with one that has a coffee can sized tip. The benefits are two fold. First, the ability of your engine to expel combusted gases increases. Increased airflow gives your engine the potential to make more power. Remember that the power your engine makes is a function of how much and how efficiently it can combust the air/fuel mixture. The second benefit of an exhaust system is the great sound they generate…in other words, your sports car doesn't sound like the family Toyota Corolla.

There are three types of exhausts or exhaust "systems". The difference lies in where the exhaust system starts.

An exhaust system has three parts...a muffler, a center pipe, and a "down" pipe (which contains the main catalyst...and connects the exhaust system to the engine/turbo).

Muffler

The muffler is the end section of the exhaust. It is what "muffles" the sound coming out of the engine. Some people prefer loud mufflers and large tail pipes...it's your choice as long as it is a high flow muffler and it delivers the look and sound you want.

"Cat-Back" (replacing from the Catalytic Converter…Back to the tailpipe)

A "cat-back" exhaust system begins at or rather connects to the factory catalytic converter. The larger than factory tubing then runs to a less restrictive muffler (that is louder and sounds more powerful) before venting exhaust gases to the atmosphere. Exhaust gases still leave the engine and pass through the low cost, restrictive factory catalytic converter. Simply put, it is typically a "Muffler" + "Center Pipe"

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Down-Pipe

The down pipe is what connects the "cat back" of the exhaust to the turbo.

In stock form, the WRX has two catalytic converters, one directly after the turbo and another one a little farther down (the factoy down pipe is actually made up of two pieces bolted together).

Also notice the difference in the opening between the factory downpipe and the aftermarket downpipe where it connects to the turbo. The factory piece has half of the turbo blocked off where the aftermarket piece has a “Bellmouth” design.

The Bellmouth design makes enough room for the wastegate and exhaust gases to pass freely on through and out the muffler. There will be a noticeable difference from changing out your downpipe, both in power and a slight sound increase as well. A high performance down pipe replaces the factory cat with a high flow cat or removes the cat completely. There are also different kinds of downpipes. You have “catted” and “non-catted”, there are also downpipes available with a divorced wastegate.

Catted downpipes will usually have one or two “high flow catalytic converters” inside the downpipe. The “high flow cat” as it’s called, is put in there to try and help out with emissions. This will also help keep sound levels down, but not by much. There isn’t much of a difference in power between catted and non-catted. It’s not a difference you will feel unless you are running high horsepower applications. In most states tampering with the vehicles exhaust may be illegal so check with your local laws before switching out your system.

Another choice you may need to make is whether to go with a “Divorced Wastegate” style or an “Open Mouth” style. The theory behind the divorced wastegate style is that, by separating the exhaust gasses and the wastegate gasses, this will smoothen out the flow of gasses that exit the turbo. Some divorced wastegate designs, will completely separate the two gasses by giving them their own tube to flow through.

The “Open Mouth” design is the most common among after market downpipes. This gives the exhaust and wastegate gasses plenty of room to flow through.

The 3” diameter of most downpipes offers more than enough room for the majority of bolt on turbos. With some turbo applications, a monstrous 3.5” diameter is used and is necessary for high horsepower applications. Put this altogether and you have a turbo back exhaust or in other words, a Down Pipe + Cat Back Exhaust = Turbo Back Exhaust. Remember that while you do gain power by removing the cats...if you remove all the catalytic converters from your vehicle, you now have an off-road only vehicle.

Turbo-Back Exhaust

A turbo back exhaust is a "Cat Back" + "Down Pipe".

A turbo back removes the factory catalytic converter and replaces it with either a straight pipe or a high flow catalytic converter. With a higher flow capability, a turbo back exhaust has greater potential for more horsepower. Typically, with all things being equal, on a properly tuned, forced induction vehicle (turbo charged) a "turbo back" exhaust will net 5-10 more horsepower than a "cat-back" exhaust. With reduced backpressure coming off the turbo, a "turbo back" exhaust will theoretically allow the turbo to spool up quicker (i.e. 100-500 rpm).
Below is Cobb’s Turbo back exhaust system. Notice the high flow cat at the end of the downpipe. (the lump in the top pipe) and the increased size in the pipe’s diameter.

Turbo back exhausts range from $900-1,500 where as "cat-back" exhausts range from $450-750. The bottom line is that you are paying between $30-60 per pony…not bad when all things are considered.

Furthermore, a "turbo-back" exhaust provides you with a good foundation if/when you plan to pump up the ponies even higher than the typical "stage I".

Up-Pipe

The up-pipe is what transfers exhaust gasses from the header to the turbo. The reason behind changing out your up-pipe is because in the 02-05 WRX models and 05 and up Legacy GT models, there is a “pre-cat” in the up-pipe that is quite restrictive. By changing that out with a cattless up-pipe, you remove that restrictive catalytic converter and free up some torque in the process. It is also important to know that if you keep the factory up-pipe in, increase power, and increase boost, overtime, you run the risk of breaking apart the catalytic converter and sending pieces into your turbo, which is never a good thing (unless your looking for a reason for a turbo upgrade ;-) ). Below is a picture of APS’ "flex" up-pipe:

An up-pipe is good for approx. 9 hp and is a worthwhile upgrade. This helps the turbo spool faster and gives the car a little more “aggressive” sound. The downside (especially to tree-huggers) is you are removing one more of the catalytic converters and will trigger a check engine light (though we can't condone removing a cat on a street car, the check engine light is easiliy fixed with a Cobb Access Port or an O2 Boss/CEL Spacer)

It is also good to know that if you own a 06 and up WRX, or any STi, there is NO “pre-cat” in the up-pipe like the 02-05 WRX models. This makes life a little easier for you as there is no necessity to change out the up-pipe. If you have ever installed an up-pipe, you know it’s no walk in the park. It is said by many that the up-pipe install is the hardest “bolt on” install on the WRX. Some STi owners as well as 06-07 owners will change out the up-pipe to get every ounce of power they can out of their little boxer engine. Aftermarket up-pipes usually will have a slightly increased diameter pipe helping the turbo to spool quicker giving you more torque, or that feeling of being pushed back in your seat we all love.

Headers

Headers (also known as the Exhaust manifold) are a worth while upgrade that will help bring out more horsepower and torque. Headers have a slight increase in diameter over that will help flow more of the exhaust gasses over stock. There are a couple different kinds of headers that you’ve probably seen such as: One Piece, Two Piece, Equal Length and Un-Equal length. One Piece means that the Header and the up-pipe come as one piece, you can’t separate them. “Two piece” means the exact opposite. Either the Header will come by itself, or it will come with an up-pipe that will have to be bolted to it for installation. For example: The Invidia Header unit comes as one solid piece. Notice how the up-pipe is welded on so it is “One Piece”, and when you look at the APS unit, it has the Header and the up-pipe as two separate pieces. Some “Two Piece” units have to be bought with the right up-pipe because it will not bolt up to any other up-pipe.

“Equal Length” and “Un-Equal Length” are pretty much self explanatory. “Equal Length” means that both lengths of pipe that come off the block are of equal length such as the PERRIN header, and yup you guessed it “Un-Equal” mans that the length of pipe that comes off the block are un-equal.

The unequal length header is what gives you the “Boxer Rumble” we love. I think PERRIN gives a good description of what happens: “Because of the unequal nature of the OEM header, the exhaust pulses are paired together causing that lump sound. The PERRIN header changes the note of the exhaust because of it equal length design. The sound pulses from the exhaust are equalized and spread out, causing the smoother, more refined sound coming from the exhaust”. Headers are usually good for about 10-15hp and will help you out in your top end.

Invidia "One Piece" Header APS "Two Piece" Header

PERRIN "Equal Length" Header Borla "Un-Equal Length" Header

APSTwo Piece Equal Length Header installed on a STi