The primary purpose of an aftermarket header on a turbocharged Subaru is to remove or replace the stock exhaust manifold with a better flowing unit.
An Agency Power Stainless Steel Header for a EJ257.
HP gain is 15HP and 20TQ. This figure is highly debated as different manufacturers use different dynos with different cars with different levels of mods. It also varies because some headers incorporate an uppipe into their design. This makes it nearly impossible to compare the gains of a header without an uppipe vs. a header with an uppipe.
Which manufacturer is best? This topic is highly debated. There have been no reported consistent “bad” headers on the market. Obviously, there may have been bad pipes sold, but not enough to report as “bad” overall.
What differences are there with headers? The main difference is exhaust piping length. Headers are made to be unequal length or equal length. Unequal means the piping on the driver’s side of the engine will be longer than the passenger’s side. Equal means the piping length is equidistant from the engine outlet to the header outlet for all piping.
Buying a aftermarket downpipe is a typical modification which removes or replaces 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.
I have an 06/07 WRX, is the downpipe the same? No. The 06/07 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.
I have an 08/09 WRX, is the downpipe the same? No. The 08/09 WRX has a unique exhaust in that it’s downpipe is the same, fitment wise, as the Legacy GT. So if you have one of these models, you must ensure you state your model/year to your vendor or specifically request the “Legacy GT downpipe” for your car in order to assure proper fitment.
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.
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.
What does the stock intake look like? To gain a perspective on how aftermarket intakes function, the stock intake features will be shown and discussed for further clarity.
This is the stock intake as viewed from the driver’s side.
The “U” shaped assembly and the tank that connects to it on the left is commonly referred to as the snorkel, silencer, or resonator. It is actually hidden in the fender.
The primary purpose of an aftermarket intercooler hose is to replace the stock intercooler hose with a better flowing unit. Airflow testing indicates 14.2% better flowing capacity than the OEM piece. It can also decrease intake temperatures, increase reliability, and dress up the engine compartment.
HP gain is 5HP. This figure is highly debated as different manufacturers use different dynos with different cars with different levels of mods.
Which manufacturer is best? This topic is highly debated. There have been no reported consistent “bad” intercooler hoses on the market. Obviously, there may have been bad hoses sold, but not enough to report as “bad” overall. The current manufacturers of intercooler hoses that fit the OEM intercooler are:
What intercooler hose material is best?Intercooler hoses are made from silicone and cast alloy. There is no irrefutable evidence that one material is better than the other. Obviously, heat soak levels are higher with the cast alloy model.
Which intercooler hose construction method is best?
There is no irrefutable evidence that one design is better than the other. With regard to the number of silicone plys, here is a breakdown:
The primary purpose of a top mount intercooler is to reduce post turbo air temperature prior to entering the combustion chamber via the throttle body.
HP gain is around 15HP. This figure can vary as many TMICs replace the restrictive OEM piping and results can be further enhanced with post installation tuning. This is one modification that is extremely difficult to put a traditional HP figure on as results truly vary from car to car based on tuning and turbo output in terms of CFM.
How much HP can my stock TMIC hold? 300WHP on the WRX and 400WHP on the STi are attainable. That does not mean those power levels are 100% efficient, but that those power levels are attainable with their OEM TMICs. As discussed below though, TMICs are not meant to be HP rated, but rather CFM related, but this is an FAQ so the HP figures are given as a good “bad” answer.
TMIC basics on a Subaru WRX/STi: 04-07 STi OEM TMIC
Which manufacturer is best? This topic is highly debated. There have been no reported consistent “bad” TMICs on the market. Obviously, there may have been bad TMICs sold, but not enough to report as “bad” overall.
I have a 2008/2009 model, any differences? Yes. Your engine bay has an entirely new layout vs. the 2002-2007 models.
TMIC basics on a Subaru WRX/STi: A 2008 STi stock TMIC
The primary purpose of an aftermarket turbo is to increase the performance over the stock unit. This can be accomplished by choosing a turbo with better spool, more flow, or a combination suited to the end user’s needs.
Subaru WRX/STI turbo charger basics: TD04 turbo and a VF39 turbo side by side
Common Terms:
Boost threshold- the lowest RPM at which a turbo will generate positive manifold pressure at maximum engine load.
Turbo lag- the time between hitting the throttle and the turbo providing full boost.
Recommended Reading: Maximum Boost by Corky Bell is considered by many to be THE publication for turbocharger information.
What is the best turbo? There is no best turbo. Generally speaking, aftermarket turbos fall into these generic categories:
a. Turbos with a little more top end power
b. Turbos with a lot more top end power
c. Quicker spooling turbos
What do all the names and numbers of turbos mean?This link sorts many of them out nicely.
What supporting upgrades are required for aftermarket turbos? At a minimum, aftermarket turbos require a fuel pump, injectors, and engine management for safe operation.
What is my stock turbo?
2002-2008 WRX TD04-13T
2004-2005 STI VF-39
2006-2007 STI VF-43
2008 STI VF-48
Is there a turbo upgrade that does not require other upgrades? Yes. A ported and polished (P&P) stock turbo is an easy upgrade over the stock unit. Though there are many turbos that may be used for short periods of time with a boost controller, it is generally unwise to bolt on an aftermarket turbo with a boost controller.
What is the best turbo with a little more top end? The most widely used turbos meeting this criteria are the VF30/VF34 and the 16G.
What is the best turbo with a lot more top end? The most widely used turbos meeting this criteria are the VF22, 18G, 20G, FP Green and it’s clones.
What is the best turbo with quicker spool? The most widely used turbo meeting this criteria is a P&P stock turbo.
What makes a good autocross type event turbo? The big thing to look for in a good performer for autocross use would be quick spool and more than stock flow. The TD04, TD05-16G, VF34, VF22, VF39, 16G, and 18G can all be considered good autocross turbos, but their particular suitability depends on the type of events where the car is generally run.
During the consultation with your Vendor, discuss in depth the course length, speeds seen, gears used, and other local venue particulars to assist in determining what best suits your needs. A word of caution….before modifying or changing your turbo, be aware that this will have an effect on what class your vehicle can legally run.
The primary purpose of an aftermarket exhaust is to remove or replace the stock catalytic converter(s) with a better flowing unit. It also increases the exhaust diameter for better flow. The term “aftermarket exhaust” can be broken down into the three main types:
1. Turbo back exhaust (TBE) covers items 1-4 as described below.
2. Cat back exhaust (CBE) covers items 3&4 as described below.
3. Axle back exhaust (ABE) covers item 4 as described below.
Keep in mind that the above information is catered to the 2002-2005 WRX and is likewise applicable to all years of the STi, though there are slight variations. For example, the STi (all years) has a catless uppipe and the 2006+ WRX has a one piece downpipe/midpipe.
HP gain is dependent on the type of exhaust chosen. TBE gains are 20-30HP. CBE gains are 5-15HP. ABE gains are 5-10HP. These figures are highly debated as different manufacturers use different dynos with different cars with different levels of mods. It also varies because exhausts use different amounts of catalytic converters or are catless.
What about overall fitment between the WRX and STi or different model years? All OEM or aftermarket exhausts designed for 02-07 WRX/STi will fit either the 02+ WRX sedan/wagon or 04+ STi. Occasionally, you will have a slight fitment issue with some models (moreso if your car has the optional rear differential cover) that usually get be fixed via hanger adjustment, longer exhaust hangers etc. 06+ models have fitment issues with some exhausts (mainly the muffler portion) due to the new rear diffuser.
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 midpipe 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.
You can easily install the upgraded STI top-mount intercooler in your garage using basic tools.
1. Disconnect your battery and make sure you have all the necessary tools. You need a flat bladed screwdriver, a 10mm wrench, and a 12mm wrench. Consider the picture before you in the engine compartment. The parts you’re looking for are centered on top of the engine. You need the new intercooler, an STI Y-pipe and inlet elbow and a new gasket for the bypass or blow-off valve. You also need the STi air conditioning line bracket for the firewall.
This is the stock WRX engine with the stock intercooler on top.
2. Two 12mm bolts that hold the intercooler to its mounting bracket on either side of the cooler. On the passenger side, note that there’s an open hole in the bracket—this hole is there to accommodate the STI intercooler. Three 10mm bolts hold a metal tube to the front of the intercooler, and two 12mm bolts that attach the bypass valve. Underneath the intercooler on the passenger side, there’s a hose clamp that holds the Y-pipe to the turbo, and there’s another hose clamp in front that holds the intercooler to the throttle body.
Note the extra bolt-hole Subaru thoughtfully placed in just the right spot for an STI intercooler.You need to undo all these bolts along the front of the intercooler.
3. Loosen the hose clamps first (and hopefully the one under the intercooler is oriented to make this easy) and then undo all the bolts. The intercooler should lift out and take the Y-pipe with it. Leave the bypass valve on the intake manifold—you don’t need to totally disconnect it. You might need to wiggle it loose, but don’t pull too hard.
Carefully lift out the intercooler and the hoses should come free if you’ve loosened the clamps.
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