Uncorking the Turbo!
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.