The Subaru Turbo Exhaust System

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The Subaru Turbo Exhaust System

Post by Sabre » Sun Dec 06, 2009 4:10 pm

This is a topic I've been meaning to post up about FOREVER. So here it is, with parts stolen from everywhere :)
The above shows the 2002-2007 Turbo Subaru exhaust. From 2008 on, they went to a dual pipe design. The names of the components, however, have stayed the same.

A key component to any well tuned turbo engine is a free flowing exhaust system. Subaru's turbo models' exhaust systems are composed of four main components: the header, uppipe, downpipe (a blanket name for the downpipe and midpipe), and catback.

When selecting performance exhaust components it is important to keep in mind the materials that they are constructed from. Generally speaking, you should look for all components to be constructed from stainless steel (304 & 321), although there is an exception in the uppipe where cast iron and inconel can be used.

One common goal when upgrading any of the exhaust components is to reduce any portions that would create turbulence such as extreme bends, catalytic converters, and resonators. These portions of the exhaust system can cause choke points that reduce the power efficiency and increase the spool time of the turbo.

The above diagram holds true for the WRX, Forester, and Baja models. The turbo Legacy models have a dual exhaust, but the principles are the same.

As exhaust leaves the engines it first travels through the exhaust manifold also known as the header. Replacing the header with an aftermarket version with usually wield a power gain of ~15awhp.

An important factor to choosing an aftermarket header is whether it is of equal or unequal pipe length design. The stock Subaru header is of unequal length design -- this is what gives the car its unique boxer engine sound. Replacing the stock header with an equal length header will give the car a more traditional (Honda like) sound. The combination of an equal length header and an aftermarket turboback exhaust can even cause the car to sound more like a supped up Honda than a Subaru.

Replacing the header is usually reserved only for those looking to maximize the power output of their engine. For the average enthusiast, other modifications to their exhausts such as an aftermarket uppipe and turboback are enough to satisfy their hunger for more power.

The uppipe connects the header to the turbo. The stock uppipe on the 2002-2007 WRX, Forester XT, Legacy GT, and Baja Turbo contains a catalytic converter that restricts the flow of air. Replacing the uppipe on these models with a catless pipe shows improved turbo spool and can increase engine power as much as 10awhp. This is a popular exhaust modification as it does not require any ECU tuning and the pipe is relatively inexpensive. (~$50 - $500) The uppipe on the STI does not need to be replaced as it is already catless and no additional performance can be gained.

The downpipe is perhaps the most critical portion of a turbocharged Subaru's exhaust system - this is where many of the performance reducing, air flow bottle-necks occur. The downpipe is located directly after the turbo and in its stock form it contains two catalytic converters.

There are three distinct aftermarket versions of the downpipe: the blank plate; bellmouth; and divorced. It is under much debate as to which one of these models provides the best performance.
downpipe types.gif
Exhaust gases speed through the turbine wheel of the turbo to create boost. As the turbo reaches maximum boost, excess exhaust gases are allowed to bypass the turbo wheel and flow directly into the downpipe though the turbo's wastegate. As the gases from the turbine wheel and wastegate exit into the downpipe, turbulence is created that can cause excess back pressure. If the wastegate is opened before the turbo reaches full boost, this back pressure can cause increased spool time as well as a loss in maximum boost.

This led to the creation of different downpipes designed to decrease this turbulence. Generally, the further away from the turbo that you can combine the exhaust gases from the turbine wheel and wastegate, the more efficient the flow of gases will be through the turbo. You can see in the blank plate, the exhaust gases combine almost immediately. Turbulence is created as wastegate gases are forced to follow a "Z" pattern as they flow outward.

Next is the bellmouth design. This version of downpipe opens up the entire outlet of the turbo, allowing the gases from both the wastegate and the turbine wheel to combine at a more gradual pace. This design bottle necks only a few inches away from the turbo, forcing gases to fight for space as they make there way towards this exit. This version is a slight improvement over the blank plate design.

Finally, there is the Divorced wastegate design. This design actually allows the wastegate and turbine wheel gases to flow through separate pipes that run parallel to one another. The gases are then combined further down the pipe to minimize the amount of turbulence created. Generally, the further away from the turbo the gases are combined, the less effect the turbulence has on the performance of the turbo.

After stating the difference in downpipe designs, it is important to understand how the turbulence created by gases from the wastegate is minimized with Subaru's wastegate design. This increased spool time and power loss was prevalent in systems with a spring actuated wastegate. The spring would be set to open the wastegate at a preset level of boost, but the exhaust pressure would actually begin to open the wastegate when boost levels were as low as 50% of maximum.

Subaru cars on the other hand, utilize an electronically controlled wastegate. Signals from the cars ECU (Electronic Control Unit – essentially the car’s electronic brain) tell the wastegate when to open. This allows the wastegate to potentially stay closed till the maximum target boost is achieved.

By opening the wastegate as late as possible, Subaru is able to minimize boost loss and decrease spool time associated with internal wastegate turbos. In this respect, the downpipe type does not play a huge role in getting extra power out of your turbo engine. For the daily driver, a blank plate or bell mouth downpipe will perform just fine. Keep in mind that the stock downpipe is of the blank plate design (sure you could argue that Subaru went with this design to save some cash on materials, but the design has been tested to work efficiently with the car).

For those that are looking to squeeze out every ounce of performance from your engine or take your car to the track, the divorced design could theoretically get you those extra milliseconds that may mean the difference between winning and loosing.

The stock downpipe has a diameter of 2.25 inches. You’ll want to look for a downpipe that uses a full 3 inch pipe if possible. Make sure to check on the inlet size of your catback (discussed below) in order for it to fit properly.

Finally, if you are keeping the car on public roads you’ll want to choose a pipe with a metal-substrate catalytic converter. The metal cat will flow better than the stock ceramic ones and in most cases the performance loss is negligible. If you are looking at building a car strictly for the track, you'll want to go with a downpipe that deletes both cats.

The final section of the exhaust is known as the Catback section. As its name states, it is the section of the exhaust system that lies after the two stock catalytic converters in the downpipe. In its stock form, the catback portion of the exhaust is composed of two different sections, the rear exhaust pipe and the muffler or axelback section.

Stock, the rear exhaust pipe is made from 2.25" piping and contains a resonator which helps reduce exhaust noise. This unfortunately causes turbulence and unnecessary back pressure in the exhaust gases as they exit the car. Most aftermarket Catback Exhausts replace this section with a straight pipe, to improve gas flow.

Like the downpipe, to get the maximum performance you’ll want to use a full 3 inch pipe.

The axelback portion contains the car's muffler. When fitting the car with an aftermarket system, a less restrictive muffler is usually used to increase exhaust flow.

Replacing the catback exhaust section on a Subaru will usually wield an increase of power by ~10awhp and the exhaust volume by ~10-15db. It is important to keep volume in mind when you make your decision - are you looking to keep the stock volume for a stealthy sound or are you looking to set off some car alarms?

The combination of a downpipe and the catback section is known as the turboback. Replacing the turboback section of your turbo charged subaru will yield the greatest performance of any exhaust modification. Replacing the stock turboback with a catless 3 inch pipe can result in gains of ~70awhp.

The pipe diameter of a turboback should follow the rules for the downpipe and catback sections, but does not need to remain constant to be efficient. In some cases, drivers have found that very little performance is lost with combining a 2.5 inch catback with a 3 inch downpipe. This combo will even help to hush the noise a bit.

Legal Note:
Please keep in mind that removing any catalytic converters from your vehicle is a violation of section 203 of the Clean Air Act and should not be performed on cars that travel public roads.
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Sabre (Julian)
92.5% Stock 04 STI
Good choice putting $4,000 rims on your 1990 Honda Civic. That's like Betty White going out and getting her tits done.

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