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:
Catback Exhaust
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:
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").
Downpipes
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:
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
Headers replace the exhaust manifold, which connects the engine to the uppipe.
There are two styles of aftermarket headers:
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.
Uppipes
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:
http://www.clubwrx.net/forums/engin...n-02-05-wrx-owners-importance-uppipe-mod.html
There are two types of uppipes:
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.
Miscellaneous
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.
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:
- Catbacks
- Downpipes
- Headers
- Uppipes
Catback Exhaust
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:
- Style (straight / angled)
- Volume/tone
- 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").
Downpipes
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:
- Bellmouth
- Divorced
- Flat-plate
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
Headers replace the exhaust manifold, which connects the engine to the uppipe.
There are two styles of aftermarket 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.
Uppipes
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:
http://www.clubwrx.net/forums/engin...n-02-05-wrx-owners-importance-uppipe-mod.html
There are two types of uppipes:
- Flex-joint
- Solid
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.
Miscellaneous
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.