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.