The foundation to all Engine Upgrades
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 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
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"
The down pipe is what connects the "cat back" of the exhaust to
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”
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.
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".
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
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 (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
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