Rishi
Still waiting on some shims!
Exhaust Design, Turbo and Naturally Aspirated
by Josh Tenny
Exhaust design is one of the areas of most confusion and can be one of the areas of greatest gain if done properly. There are many factors to consider when designing an exhaust system. Complicating things is the fact that what works perfect for one application may not be even remotely close to optimal for another. Making it even harder to choose the proper exhaust components is the fact that different "tuners" say different things. I have heard so much info myself it is hard to sort through and determine what is marketing and what is correct and truly useful and information. But keep in mind that perception and marketing do play a roll when looking at it from a company's point of view. What good is any product if it does not have enough value for people to buy, or if they do not understand its true advantages. We want to offer a technically superior product and letting our marketing explain why.
One thing I do want to explain before I get too far into this is backpressure. I hear "You need backpressure to make torque." all the time. And it even comes from "tuners" as well as customers. This is flat out not true. Before I arrived into the company of true tuners I fell into the same trap myself. The reality is that backpressure is the enemy. You want to keep it as low as possible. What you do want is to keep velocity up. However doing the things that keep velocity high involves slightly more backpressure under some conditions. You want to keep the gasses moving as quickly as possible to make both good torque and top end power. The perfect exhaust system would keep the gasses moving as fast as they did coming out of the cylinder and have zero backpressure. However this is impossible to achieve in the real world.
Turbo System Exhaust
Is bigger better all throughout the powerband or does it loose bottom end torque?
Should I use 2.5", 3.0", or 3.5" for XYZ power?
Does reducing the pipe diameter towards the end of the exhaust help power?
What is the best turbo outlet design?
How does a cat-less exhaust perform compared to a high flow cat?
We get these types of questions all day every day. We get many questions, but most center on the size of the turbo-back system alone and disregard any other parameters. I wish it were that simple, but unfortunately it is not. Post turbo pipe diameter is only one small parameter in many that determine how well the exhaust performs.
Pre-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 good 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 good 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 good header design would be something like a 4 into 1 design that uses castings or good 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 good 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 good 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 good aerodynamic properties or it is inefficient. A more efficient wheel design means that you will make more power and/or less lag.
by Josh Tenny
Exhaust design is one of the areas of most confusion and can be one of the areas of greatest gain if done properly. There are many factors to consider when designing an exhaust system. Complicating things is the fact that what works perfect for one application may not be even remotely close to optimal for another. Making it even harder to choose the proper exhaust components is the fact that different "tuners" say different things. I have heard so much info myself it is hard to sort through and determine what is marketing and what is correct and truly useful and information. But keep in mind that perception and marketing do play a roll when looking at it from a company's point of view. What good is any product if it does not have enough value for people to buy, or if they do not understand its true advantages. We want to offer a technically superior product and letting our marketing explain why.
One thing I do want to explain before I get too far into this is backpressure. I hear "You need backpressure to make torque." all the time. And it even comes from "tuners" as well as customers. This is flat out not true. Before I arrived into the company of true tuners I fell into the same trap myself. The reality is that backpressure is the enemy. You want to keep it as low as possible. What you do want is to keep velocity up. However doing the things that keep velocity high involves slightly more backpressure under some conditions. You want to keep the gasses moving as quickly as possible to make both good torque and top end power. The perfect exhaust system would keep the gasses moving as fast as they did coming out of the cylinder and have zero backpressure. However this is impossible to achieve in the real world.
Turbo System Exhaust
Is bigger better all throughout the powerband or does it loose bottom end torque?
Should I use 2.5", 3.0", or 3.5" for XYZ power?
Does reducing the pipe diameter towards the end of the exhaust help power?
What is the best turbo outlet design?
How does a cat-less exhaust perform compared to a high flow cat?
We get these types of questions all day every day. We get many questions, but most center on the size of the turbo-back system alone and disregard any other parameters. I wish it were that simple, but unfortunately it is not. Post turbo pipe diameter is only one small parameter in many that determine how well the exhaust performs.
Pre-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 good 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 good 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 good header design would be something like a 4 into 1 design that uses castings or good 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 good 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 good 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 good aerodynamic properties or it is inefficient. A more efficient wheel design means that you will make more power and/or less lag.