The WRX/STI Power Mod Path, Part 2
Yesterday, we discussed the choosing of an aftermarket catback system to the WRX or STI, not only for additional power, but also for aesthetic purposes. However, as much as we all love the boxer sound, in terms of actual measurable power, the catback alone really doesn’t do much. Thus, after the catback, the next power mod most people consider is an aftermarket downpipe. The downpipe is where the true power starts to be made, since as you can see from the photo to the right, the stock downpipe has 2 power-robbing catalytic converters, or “cats” for short. In this photo, the downpipe on the bottom is the stock downpipe, whereas the downpipe on the top is an Invidia catless downpipe. In comparing the two downpipes, you’ll also notice a few differences other than the cats: the stock downpipe is split into two pieces (some people and manufacturers only refer to the top-most section as the downpipe, and the second section as the midpipe), and the flange at the top that connects to your turbo has a flat plate. These two differences are clear examples of how different companies prefer to approach creating an aftermarket downpipe.
Before we go further however, we must make a disclaimer: removal of the stock catalytic converters is for off-road use only, and if caught using a catless system on the street, you may be cited and fined. In other words, you do have to pay to play, so make sure you’re well aware of this, especially in states like California where emissions laws are very strict and may involve impounding your car if violated. Now that that’s out of the way, most aftermarket downpipes have the following features that are geared as improvements over the stock downpipe:
- Removal of all cats or replacing 1 or 2 cats with high-flow performance cats
- Replace the flat flange at the turbo side with a “bellmouth” or “divorced wastegate” flange
- Change from a 2-piece pipe to a 1-piece pipe
Of course, just the same as with catbacks, not every downpipe is equal, and this is even more clear with the different designs out there. No matter the design though, the main thing that really should be considered is fitment. Some downpipe have problems fitting due to hanger location, but others simply have issues with fitting because they’re made to mate to their own proprietary exhaust system, so they might require a separate pipe for it to connect to the aftermarket catback of your choice. On top of this, most aftermarket downpipes will not bolt on directly to the stock catback for 02-07 WRXs and STIs, however this is completely the opposite for 08-09 WRXs and STIs, which don’t have any problem bolting to the stock catback. Why would you want to bolt onto the stock catback you might ask? Well, although you do sacrifice power by sticking with the stock catback (since it creates a bottleneck), you do get a significant power increase over stock and you don’t attract any unneeded attention from the authorities.
Once you’ve sorted out fitment issues, the next thing to consider is going catless or staying green-friendly with a high-flow cat. It has been argued that a good high-flow cat will not hamper power at levels below 400 whp, but in our experience, this has not been the case whatsoever. Moreover, running catless helps with throttle response and faster turbo spool since there’s no back pressure in the exhaust system to slow down the exhaust flow. It has also been argued that running catless creates a bad exhaust smell in the cabin, but while this may be the case when a catless downpipe is initially installed, the smell does go away over time, so long as the downpipe was installed properly with no leaks. And, since catted downpipes are typically almost twice as expensive as catless downpipes, unless you absolutely need a catted downpipe for emissions or racing restrictions, we always recommend a catless downpipe. Catless? But what about the check engine light? Well, certain forms of engine management will turn off this check engine light for you, but we will get into that when we talk about engine management in a future part of this blog series.
After deciding on whether to go catless or catted, the next big decision is in regards to the downpipe’s flange at the turbo side. Here are a few examples of the available options, other than stock downpipe’s flat plate design. First up is the “bellmouth” design, which looks like this on the TiTek downpipe:
Next up are two variations of the “divorced wastegate” design. The idea behind the divorced wastegate is that a stock style turbo has exhaust gases the escape from the turbo’s internal wastegate, and since that might cause turbulence when mixed in with the ordinary exhaust gases that exit the turbo, it’s better for these gases to exit out separately so that the exhaust flow is not interrupted. The first one is Invidia’s version, which is essentially just a flat divider plate that diverts the gases, but the gases still go down the same exact pipe further along:
In contrast, here is Element Tuning’s take on the divorced wastegate design. This downpipe actually features a completely separate pipe for the wastegate gases, which then meets up later on with the rest of the piping. The theory behind this is that it’s a true divorced wastegate and the extra smaller piping creates a venturi effect which makes the wastegate gases not only flow faster, but also create a suction effect that makes the rest of the exhaust gases flow faster as well:
In our experience, we definitely recommend either of the above styles. It’s best to stay away from the stock downpipe’s flat plate style, which can be found on a few downpipe brands out there. However, between a bellmouth and Invidia’s divorced wastegate style, we have not seen or felt a significant difference in power. On the other hand, Element Tuning’s divorced wastegate style has proven to produce more power over the other styles.
Finally, you’ll notice that Element Tuning’s downpipe is a 2-piece design, whereas the Invidia design is a 1-piece. Generally speaking, a 1-piece tends to be better when avoid exhaust leaks, but a 2-piece design gives the option to change from a catless race pipe to a catted pipe at your discretion. Not every 2-piece downpipe has an option for a catted downpipe however, so it’s best to do your research (as always) before purchasing. Nevertheless, if you have any further questions, again please don’t hesitate to contact us. For the next blogs, we’ll change gears a little bit and highlight some products and feature a customer car, but next week we’ll come back to the power mod path with a discussion about intakes and engine management.