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http://www.cobbtuning.com/info/?ID=3222Pre-turbo Exhaust
The exhaust system before the turbo and the turbo itself have a greater effect on backpressure than the exhaust behind it. You want the least restriction after the turbo as possible for both top end power and quick spool-up. Careful attention has to be paid to keep velocity high before the turbo and in the exhaust housing of the turbo to spool the turbo up as quickly as possible while not choking off the exhaust gasses on the top end.
The header can be simpler in some ways than a non-turbo header. Bigger dividends can be had by getting the exhaust gasses to the turbo with the least amount of restriction, highest velocity, and the most heat rather than worrying about a tuned equal length design. It would be optimal to make an equal length header, but the packaging of the WRX make a tuned equal length header difficult to design. This helps explain why we usually get near identical results from a factory header when compared to the aftermarket ones we have tested thus far.
The factory header gets the gasses to the turbo as quickly as possible and goes a 
job of keeping the heat in. Aftermarket headers tend to take a longer path and loose quite a bit of heat in the process. Also, most have a poor collector design that is s a byproduct of the unique packaging of the WRX. In this case a
collector does play a more important role than the length of the pipes. If all of the gasses ram together at a steep angle it causes a lot of turbulence, creates backpressure, slows velocity, and tends to make a mess of things before the turbo, which is the worst spot for inefficiency on a turbo charged car.
A header design would be something like a 4 into 1 design that uses castings or thermal coatings as much as possible to keep heat in and would also get the gasses to the turbo as quickly as possible. The collector would have to be longer than the ones I have seen and the transitions nice and smooth. The main problem comes from trying to make it package well into the constraints of the turbo Subaru. A well designed header would likely require the header to up-pipe connection to have different flange points than factory, so it would not be as easy to sell in header and up-pipe pieces.
The WRX is a hard car to design a proper header for, to say the least. It is very hard to improve what the factory has already done.The up-pipes duty is to get the collected gasses from the header up to the turbo.
The best size is the smallest that does not create excessive backpressure for the intended use. Again, the goal is to keep the gasses moving as quickly as possible while flowing enough gasses to make the desired power. There also needs to be a small amount of flex in the system to avoid cracking, warping, and blown out gaskets. The exhaust before the turbo has a lot of heat differential from one point to the next and adding to that is the fact that different metals have different expansion. This leads to a system that wants to twist, pull and push quite a bit. Without some give, something has to go. The gaskets and welds are usually the first victims. One problem lies in getting flex without having a flex section that is prone to cracking, splitting, and leaking it's self. It is not wise to cure a problem with a part that causes the exact same problem. That would be like sun screen that causes skin cancer.
Turbo Exhaust Housing
With turbos there even more factors than just the design of the exhaust side of the turbo that go into a
turbo for your application. However since we are talking about exhaust theory here I will only talk about the exhaust section of the turbo.
The size and design of the exhaust housing plays a major roll in the spool-up characteristics of the turbo and its ultimate power potential. There has to be a balance met if you want to have the quickest spooling turbo for your power goals. If you go with too large of an exhaust housing you greatly increase lag. Too small of exhaust housing and you severely limit the amount of boost and top end power you can make. You can only push so much gas volume through a small housing without having negative side effects. Adding to the complication is that each pound of boost created makes a ratio of backpressure before the turbo. It is different for each turbo, the amount of boost you are running, the size of the motor, RPM, and load on the motor. Once you start trying to push too much through the exhaust section of a turbo (running too much boost for the turbo) you start making a huge ratio of backpressure, and it only gets higher the more boost you run. This not only limits the amount of power you can make, but makes EGT go up, hinders the motor's ability to get the burnt gasses out of the motor, and makes the car more prone to detonation. This is also a big cause for failed pre-cats in the up-pipe. Choose too large of an exhaust housing for the application and it takes the turbo too long to spool, effecting torque production. The best way to make
torque on a turbo motor is to spool up the turbo as quickly as possible. Also, who cares how big your turbo is, or what power it can theoretically produce if you can never spool it up or if it falls out of the powerband every time you shift. I have heard of WRXs that theoretically make enough power to run in the 11's in the quarter mile actually run a 14 in real life because of mismatched parts. Bigger is not always better.
Adding another factor is the design of the exhaust wheel. It has to have
aerodynamic properties or it is inefficient. A more efficient wheel design means that you will make more power and/or less lag.
Post Turbo Exhaust
The main performance goal of a post turbo exhaust is to create the least amount of backpressure possible. There are a lot of factors that affect this.
Turbulence is one main factor. If the gasses are all stagnating and/or running into protrusions or running into each other it creates more backpressure than a well designed system. The more laminar (smooth and straight) the gas flow, the more the system can flow for a given pipe diameter. Steep angles and abrupt pipe diameter chances should be avoided.
The methods of collecting the outlet gasses and the wastegate gasses add another part of the equation to change. It would be optimal not to join the outlet from the turbo and the wastegate together, but the real world messes with our fun. Just dumping the wastegate to atmosphere is great for a racecar, but not a street car. So a street exhaust should combine them to get all of the gasses through the same cat and muffler system.
Some of the turbo outlet designs include: flanges with a simple pipe, bell mouths, divorced wastegate, and split bell mouths You also have castings and formed piping to choose from. Which one works best is also determined by quite a few different factors and how well they are designed and manufactured.
Flange w/Simple Pipe - The only advantages to this design are cost and simplicity. The pipe does not have to be formed and the flange is simple therefore reducing cost. The labor to weld the pipe to the flange is easy and therefore less costly as well. That is the main factor that make it desirable to the factory and why it is used on the stock exhaust. The wastegate gasses joining the turbo gasses right at the turbo outlet does create turbulence in the worst spot post turbo and reduces flow, thus not making it as desirable for performance as other designs.
BellMouth - This method is much closer to optimal for joining the gasses from the outlets. There is more room for them to join and if the transition is done properly it can flow very well into the main piping. It packages very well and does not have a lot of complexity, making for less to break. We have gotten the best results from this type of downpipe so far. Boost response has been the best out of the outlet designs we have tuned on, it is easy to put a wideband oxygen sensor bung into. We have also had the fewest problems with this design.
Split Bell Mouth - This design separates the gasses in the beginning of the turbo outlet and joins them at the rear of the bell mouth section. It works well and has some of the advantages of the bell mouth and some of the advantages of the divorced wastegate designs. The main deterrent for this is the cost and complexity of adding the splitter. I am a fan of keeping things as simple as possible while still making the product work well.
Divorced Wastegate - Keeping the gasses from the turbo outlet and wastegate separate until farther back in the system is an attempt to combine the advantages of not collecting the gasses and the real world. Combining them far back is closer to optimal than collecting them closer to the outlets. It is also critical to power production and spool-up to join the pipes smoothly and avoid turbulence. The disadvantages are that you add a lot of cost and complexity. You have big temperature differences on each pipe and that makes for a system that can crack. Putting in flex or expansion joints helps, but adds even further complexity and yet another part to fail. With all of the exhaust systems we have tuned with on the dyno we have seen that it is generally harder to bring boost on as quickly with these types of systems as compared to the bell mouth type systems. Perhaps it helps the wastegate function too well. Also, we have had a few situations where the splitter caused problems allowing the wastegate to function properly by not allowing it to open to its full extent, or even open at all. That caused either boost spiking, or no control over boost what so ever. Since the wastegate could not function the turbo ran as if it did even not have one, and the poor turbo just ran whatever boost it could make uncontrolled. The fix was not hard, but the least amount of stuff to go wrong the better. I know that I would not be happy having to pay for someone to install the exhaust only to have another place diagnose the problem, remove the exhaust, repair the part, and re-install the exhaust.
Cast Outlets - Castings have the advantage of keeping a lot of heat in the exhaust as well as freedom with design. You can basically make it almost any shape you want. The disadvantages are more weight and cost. Cast iron pieces can weigh a ton and that is a valid concern for many people. The casting form that the piece is made in is also very expensive and depending on complexity can range from a couple of thousand dollars to well up in the tens of thousands.
Formed Piping -Forming pipe has almost as much design freedom as a casting with less expense and less weight. The only disadvantage lies in if it is not done properly. Poor forming can look bad and effect flow by having creases and crimped spots. You can also get the piping too thin if you try to stretch the metal too far. If done improperly you can also make the metal brittle and it will usually happen where the metal is the thinnest.
Remember, you will only flow as well as the greatest restriction. If you have a poor cat or muffler design then it will choke the flow no matter how
the rest of the system is designed. Fortunately straight through mufflers and newer high flow cats flow very well. Having a cat is not only
for the environment, but we have seen very little power difference in levels in excess of over 350 h.p. Why be dirty when you can make just as much power while keeping tree hugging hippies happy? Also, a cat tends to quiet things down a little.
Pipe diameter does have an effect on flow rates as well, but again it is not the major factor in most cases. 2.5" may flow enough for 300-350 h.p. without being a restriction. 3" is usually capable of flowing 500-600 h.p. before becoming a restriction. This is assuming that you have designed the rest of the system up to par. There are also full 3.5" systems and those that start out at 4" and taper down. Unless you are making over 500-600 h.p. anything over 3" is a case of diminishing returns and in most cases has no advantage. There is more to gain going from 2.5" up to 3" than there is going from 3" to 3.5". A 3" system will not loose torque compared to a 2.5" system if designed properly. In fact if designed properly 3" may be capable of making better low end torque than 2.5". Again, since the way to make the most torque with a turbo exhaust is to get the turbo to spool-up as quickly as possible, it should be the main goal of the entire exhaust system and
flow after the turbo is one way to achieve it. We use 3" as we want our system to flow enough to be capable of coping with a customer's changing goals. Properly designed we can offer it to the big power crowd while still appeasing the low end torque club.
The only reason to reduce the size towards the end of the pipe is for packaging, cost, and noise reasons. Tapering the diameter does not make more power, torque, or bring on boost faster. However having smaller pipe towards the end has less effect that having smaller piping at the beginning. In other words a system that has 3" pipe for the majority, and necks down to 2.5" at the end will flow enough for more power than a complete 2.5" system. The further downstream you neck down the exhaust the better
..if you decide to neck it down.
Attracting unwanted attention and not hearing your stereo or you passenger would make for an exhaust system great for a racecar, but poor for the average Joe. I like hearing the exhaust myself, but there are times I want to listen to the radio or go on a date without screaming at my passenger. Law enforcement and your neighbors do not appreciate loud exhausts either, even if you do.
