Thank you for this!One of the most common questions in the "Engine Modifications" section is about what parts to purchase.
Below is a diagram that gives a graphical representation of the intake/exhaust components:
There are four main types of aftermarket exhaust components you can purchase:
Like the name suggests, this will replace the exhaust after the catalytic converters. This section of the exhaust is the largest determining factor of the exhaust note/tone.
Things to consider when getting ready to purchase a catback:
Choosing an exhaust is something that needs to be done in-person. While a YouTube video can give you an idea of the various exhausts, there is no quality control to really be able to differentiate (e.g., camera quality, distance between exhaust/microphone, etc.). Where I find YouTube to be useful in deciding on a catback is to narrow down your list of choices. Once you've got a few exhausts to choose from, it's time to hit up Subaru meets; here will give you the best chance to hear how different exhausts sound. At one of our local meets, you'll usually have at least 6 different exhausts you can hear. Hearing the exhaust outside the car is well and good, but you also need to hear it in-cabin. Ask the owner to take you for a drive; on the drive, try to hit every style of driving you can (city, highway, etc.). Once you've had a chance to hear all the exhausts on your list, pick whichever one you like best.
- Style (straight / angled)
- Taper (some exhausts taper to 2.5" to mate with an OEM downpipe)
Because something like a catback is so subjective, no one can really answer a question like "What's the best catback?" Personally, I prefer a more "stealth" approach both in looks and volume. Pretty much any brand will perform the same (any difference you see by switching catbacks is minimal enough to be equated to "dyno error").
Most aftermarket downpipes replace the section of exhaust between the catback and the turbo. Only '02-'05 MYs need to worry about the few "shorty" downpipes on the market (e.g., Bosal, Oakos) which only replace the section closest to the turbo (leaving the rear cat pipe in place).
Modification of emission control devices (e.g., catalytic converters) is a violation of the Federal Clean Air Act (§203(a)(3)(A)), and therefore is illegal in every U.S. state. Some downpipes include high-flow cat(s), while others are completely catless; a downpipe which utilizes HFC(s) is just as illegal as fully catless. When determining whether or not to run a catted/catless downpipe, it is up to the end-user to research their local/state emissions laws. On vehicles with stock or mildly upgraded turbos, a HFC will not impact the performance that much (minimal enough to be equated to "dyno error"). Some owners (mostly 2.5L) have had boost creep issues with catless downpipes, and will switch to a catted exhaust to provide some backpressure; some cars have boost creep, some cars don't.
There are three styles of downpipes you can purchase:
As for which is the "best" design, you want a downpipe to be as free-flowing as possible. The flat-plate design blocks the wastegate, and is generally considered an "inferior" design on an internal wastegate turbo (this is negated when an external wastegate comes into play). The difference between a flat-plate and bellmouth/divorced downpipe has a direct correlation with the flow of the turbo. When comparing bellmouth/divorced, in theory, separating the exhaust/wastegate gases with a divorced-style downpipe is ideal, but the difference is minimal enough to be equated to "dyno error". Pretty much any brand will perform the same (any difference you see by switching downpipes is minimal enough to be equated to "dyno error").
The main thing to consider is whether or not the downpipe tapers or not. Most downpipes on the market are a full 3", but a few taper to 2.5" to mate with an OEM-style catback.
You usually need to modify the OE heatshield to fit aftermarket downpipes. If the heatshield is missing, you can usually pick them up cheap from the classified section. If you also need the bracket to attach it, you want Subaru P/N 44021AA012.
Headers replace the exhaust manifold, which connects the engine to the uppipe.
There are two styles of aftermarket headers:
Turbo Subarus come with UEL exhaust manifolds from the factory, which is what gives it the signature "Boxer Rumble". An equal length header will make a Subaru sound more like a Porsché. Strictly from an efficiency standpoint, aftermarket UEL headers will usually make smaller gains than EL headers.
- Equal-length (EL)
- Unequal-length (UEL)
The power difference between brands is minimal enough to be equated to "dyno error". There are, however, differences in the way an aftermarket header impacts the powerband (e.g., some have great top-end at the cost of low-end, some offer a huge bump in the mid-range, etc.). Aftermarket headers are sometimes prone to cracking. An alternative to aftermarket headers is porting/polishing (PnP) the OEM exhaust manifold.
This exhaust component connects the exhaust manifold to the turbo.
Any 2.0L WRX has a catalytic converter in the uppipe, and should be replaced. The following thread goes into more detail:
There are two types of uppipes:
There is no difference in performance between a solid uppipe, and one that has a flex section. Personally, I prefer uppipes with a flex section (the OEM 2.5L uppipes have a flex section). The power difference between brands is minimal enough to be equated to "dyno error".
This following only applies to 2.0L WRXs: Subaru included an EGT sensor in the OEM 2.0L uppipes as a precautionary measure to check the healthiness of the catalytic converter. Since you're going to be removing the cat, it is no longer necessary to run this (EGT should be measured in the exhaust manifold for accurate readings anyway). Reusing the stock EGT sensor is only going to run the risk of that breaking off and getting sucked into the turbo, causing damage. If the uppipe you're installing has an EGT bung, then use a bolt (M12 x 1.25) to plug the hole. There is a CEL associated with this modification that needs to be taken care of: P0546 Exhaust Gas Temperature Sensor Circuit Malfunction (High Input). This is often disabled with "StageII" maps, but if you're running the stock map, you'll need to do the "2.2 KOhm resistor mod". You can buy these at any electronics store (e.g., RadioShack), and should cost you ~$1. This resistor gets placed in the bottom connector just in front of the passenger strut tower.
Heat-wrapping or ceramic coating exhaust components is a popular modification. It serves two purposes: retaining exhaust gas temperature (EGT) for better flow, and reducing heat soak. The one downside to heat retention is that it may accelerate material fatigue/wear. Most people believe the pros outweigh the con. Before doing any type of heat retention to your exhaust component, make sure you understand the effects it may have on that parts warranty. Personally, I feel it's worthwhile to do this on any exhaust component in the engine bay (exhaust manifold / headers, uppipe), and the first half of the downpipe. You can wrap the entire downpipe if you'd like, but maintaining EGT isn't as important post-turbo (as it is for the other exhaust parts), and there no longer is the benefit of added heat soak prevention.
When possible, consider purchasing used exhaust components. You can save quite a bit of money when purchasing parts from the classified sections (and you help another Subaru owner get rid of unused parts). Exhaust parts may be shiny when purchased new, but it's not long before they've begun to discolor, so why not let someone else take the hit in depreciation? While my example is a bit extreme, it just shows how much shopping used can save: my downpipe and catback retail for ~$1000 total new; I paid $200 used.
On the subject of "eBay parts": the reason they are less expensive than other brand name parts is because the companies use inferior materials, utilize what can be equated to slave labor, and have no R&D overhead because they copy an already proven design. While the cheaper cost of parts may be good for the consumer, it does nothing but harm for the Subaru aftermarket community. Supporting this business practice leaves the companies that actually do the R&D and develop quality parts little incentive to continue if some other company can just purchase their part and copy it, and be able to sell their "own" part for a lot less. Greddy had quite a bit of financial trouble in 2009 and filed for bankruptcy; which popular, well-respected company is next? Some vendors have realized the popularity of these parts, and have begun selling and stocking them, which is also a complete disservice to the aftermarket community, since it gives the parts a more legitimate reputation. If cost is really your only consideration, then seriously consider purchasing used parts instead.