The primary purpose of an aftermarket downpipe is to remove or replace the stock catalytic converter with a better flowing unit. It also increases the exhaust diameter for better flow.
HP gain is 15-20HP. This figure is highly debated as different manufacturers use different dynos with different cars with different levels of mods. It also varies because some downpipes use one high flow cat, while others are catless and actually extend far enough back to eliminate the 3rd cat as well.
Which manufacturer is best? This topic is highly debated. There have been no reported consistent "bad" downpipes on the market. Obviously, there may have been bad pipes sold, but not enough to report as "bad" overall.
Who manufactures downpipes? The NASIOC user fliz has kindly started a downpipe chart that lists all the downpipe manufacturers and lots of comparative data. fliz's downpipe chart.
What downpipe metal material is best? Downpipes are made from mild steel and stainless steel (304 & 321). There is no irrefutable evidence that one material is better than the other. Obviously, corrosion levels are higher with mild steel (coated or otherwise). Article on exhaust materials.
Which downpipe construction method is best? Downpipes have 4 main construction methods:
1. Blank plate: Identical to stock construction with the wastegate portion completely covered.
2. Bellmouth: Completely open design.
3. Split bellmouth: Similar to bellmouth only with a divider inserted to separate the wastegate.
4. Divorced or Twin Dump: Separate exhaust and wastegate piping that connect further downstream.
There is no irrefutable evidence that one design is better than the other. The thought process is that the greater the separation there is between the wastegate gases and exhaust gases, the smoother the overall exhaust flow.
Which downpipe construction method is least expensive? Generally, the blank plate design is the least expensive. The other designs, depending on manufacturer, are all around the same general price range.
Can I gut my downpipe like people do their uppipe? Yes. Scoobymods.com directions. Others have used a 2 foot section of bent rebar and a hammer and reported the de-catting process takes 30 minutes total if the scoobymods directions seem too labor intensive.
Can I use an STi downpipe? Yes. There is very little difference between the STi and WRX downpipes though. Using the STi downpipe is fine, but do not consider it a significant upgrade. Differences in HP between the two are likely to be very tiny (5HP or less is a guess) and not worthwhile.
What other differences are there with downpipes? The other differences are as follows:
1. High flow cat or catless.
2. Overall pipe diameter. Sizes range from 2.5, 3.0, and 3.5 inches. 3.0 and 3.5 are generally only necessary for higher (300-400+ HP) applications.
3. Length of downpipe, some are stock length and others are longer to remove the 3rd cat.
4. Propietary length. Some have manufacturers have unique lengths or are JDM length which requires manufacturer specific length or JDM specific length components after the downpipe.
5. Flanges. Some are manufactured with a donut gasket and some with a flat flange, the end user needs to research what bolts up to what before purchasing.
Which downpipe has the best gains? There is no irrefutable evidence that any downpipe has better gains than another. The consensus, if there is one, is they are all within 5HP or less, gain wise, of each other.
Are there any downsides to downpipes? There have not been significant amounts of problems with downpipes. There have been some reports of wastegate issues with split bellmouth and divorced or twin dump downpipes. These type of downpipes have a "splitter piece" that juts into the turbo to separate the exhaust from the wastegate. Sometimes this piece is too long and requires some slight cutting or grinding for a perfect fit. The telltale sign of this occurance is less than full boost or irratic boost upon installation. As with any aftermarket part, even "perfect" ones, sometimes you run into a rogue issue. From research, this is not a widespread issue at all, only an advisory if this situation should occur.
I have an 06 WRX, is the downpipe the same? No. The 2006 WRX has a unique exhaust in that it's downpipe is both the downpipe and catpipe sections of the "older" exhaust. This means you must use a "long" downpipe to bolt up to the rest of your exhaust system. A full TBE will fit fine, but when replacing just the downpipe, you must use a long downpipe. Check with your Vendor for other possible fitment issues prior to ordering.
Where do I buy a downpipe? Every Subaru/Import performance store sells downpipes. For purchasing, support your local economy or the NASIOC Vendors.
What is some good background information about exhaust pipe fitment?
Generally speaking, there are two types of catbacks:
Type 1: It has a sliding flange designed to bolt up to the stock downpipe.
Type 2: It has a flat flange designed to bolt up to 3" downpipes.
Generally speaking, there are two types of downpipes:
Type 1: It is designed to bolt up to the stock catback and other type 1 catbacks.
Type 2: It is designed to bolt up to only type 2 catbacks.
Type 1 catbacks do not bolt up well to type 2 downpipes, and type 2 catbacks do not bolt up well to type 1 downpipes.
Type 1 catbacks and downpipes taper to 2.5" where they meet.
Type 2 catbacks and downpipes are full 3".
Type 2 downpipes are usually less expensive than type 1 downpipes.
Type 1 and 2 catbacks are relatively similar in price.
All 2.5" catbacks are type 1.
Some 3" catbacks are type 1, and some are type 2.
What questions should I ask of the retailer or manufacturer before purchasing?
1. Will this unit mate up to the rest of my exhaust? Some downpipes will fit the stock exhaust, or must be fitted to a larger exhaust.
2. Will this unit throw a CEL? Some do, some don't, so find out beforehand.
3. Does it come with new gaskets? It is always a good idea to replace exhaust gaskets.
4. Does it come with a CEL fix if needed and is it the right one? In the case of the electrical fix, there are two unique fixes, one for the 02/03 and one for the 04. The 05+ does not currently have an electrical fix.
How hard is it to install a downpipe? Allow around one-two hours for install time. Professional installation, depending on your area, is around $75. This is one vehicle modification that is very simple and can be successfully accomplished by even the greenest shade tree mechanic.
How do I install a downpipe? Refer to the downpipe manufacturer's instructions. For downpipes without instructions, below are links to some of the better known downpipe installation instructions:
Keep in mind that the short black rubber hose on the passenger side side of your fire wall is the AC drain line. It's best to ensure that it is routed away from your new downpipe to ensure it properly drains and does not melt onto your downpipe. It can drip onto your downpipe and this causes no reported problems if you choose to cut it short and allow it to drain on the downpipe itself. Though not the best route, this is mentioned as a possibility.
What about ceramic or other heat wrap coating? Though there are pros and cons to heat retention treatments with exhaust components, the general consensus with headers is that heat retention benefits outweigh the liabilities.
Increased heat retention, which is beneficial for exhaust flow.
Reduces underhood temps, which decreases IC heatsoak.
Additional lead time, in the case of coatings.
Increased heat retention, which in some cases, can accelerate metal fatigue, especially at weld joints.
No proven HP increase.
If you plan on using a coating, ensure you are aware of what impact this will have on your header warranty if applicable. Coating or wrapping your header can void exhaust component warranties. If using a ceramic-type coating, ensure it's working heat range is above the known EGT. EGTs can be as high as 1400-1600 degrees in the extremes. Some normal coatings are not rated to this temperature level.
What types of ceramic or other heat wrap coating are available? The two most popular manufacturers of heat wrap are Thermo Tec and DEi. The two most popular coating manufacturers are Jet Hot and Swain Tech Coatings. Wrapping is a DIY project with a shorter life span than the professional only coating with a longer life span.
Excellent coating thread
.pdf file on coatings
Car Craft magazine's testing of Jet Hot
What is the CEL fix? It stops the rear CEL code P0420 Catalyst System Efficiency Below Threshold from appearing, this code being caused by the lack of a catalytic converter in the downpipe. This is usually fixed with a Pony Express MIL Eliminator or similar fix. Ensure you obtain the correct type for your model year as there are two types: 02/03 and 04. The 05+ does not currently have an electrical fix. Another fix is the mechanical fix, which works on all models. ***NOTE*** There are two CEL fixes for the WRX, the other one is the 2.2 KOhm 1/2 watt resistor. It is used to combat the CEL code P0546 = Exhaust Gas Temperature Sensor Circuit Malfunction (High Input) in the uppipe. This notice, though not important to this thread, acknowledges there are two "CEL fixes" and defines them individually to avoid possible confusion.
Which is the best CEL fix, mechanical or electrical? Both work fine, though personnel living in states with OBDII testing prefer the mechanical fix. The reason for this is during state mandated OBDII testing, the electrical fix can show up as a malfunctioning sensor. For some states, this one malfunctioning sensor is fine, for others, you will have to fix it and return. In the end, the mechanical fix is cheaper and easier for most users.
Can I drive around with a P0420? Yes. This is not a code that will throw your car into limp mode, so you can drive normally with it until you get a CEL fix. It's never wise to do so though as a CEL is easily seen by police who may then do a "safety inspection" as well as cause issues during any local emissions testing.
Should I reset my ECU after this mod? It is never a bad idea to perform an ECU reset after a mod. The traditional route is to disconnect the negative battery terminal, press the brake pedal for a few seconds to bleed the system of charge, and reattach. Some use the more advanced Vishnu Reset.
What about emissions problems like emissions tests and legality? Using a catless downpipe is a violation of section 203 of the Clean Air Act. Each state has statutes that cover this subject as well. As far as passing state emissions tests, as long as you still have one cat in your system, you should be fine unless you run into a sharp-eyed and knowledgeable inspector who knows there should be two in the main exhaust. The exhaust should be properly warmed prior to any testing to ensure it is operating efficiently. Another smart move is to find someone in your local area with the same set up to compare notes. Seek out locals via the NASIOC Chapters or NASIOC Regional Areas. A nice article on cat testing can be found here.
Hey man it is not really a dumb question... Just some misdirected, tasteless humor
There will be some loss in power because of the cat. If you are racing, and every HP counts, then cattless.
If your car is your daily driver, and the loss of about 5hp (That number is highly debated) doesn't bother you, I would say catted.
I am in California and cattless would not pass smog inspection, and IMO, I am not going to swap a DP to goto a smog test. (although I think a car has to be 8 years old before we are required to get a somg test) Depending on where you are your local laws it could be different.