Rishi
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Exhaust Design, Turbo and Naturally Aspirated
by Josh Tenny
Non-turbo Exhaust
Designing a non-turbo exhaust system is quite a bit different, most noticeably in the header section. The primary goal of getting the exhaust out with the most velocity and with the least amount of backpressure is still the same, but that is about where the similarities end. The real world also steps in and throws in the same requirements like noise, environmental concerns, and packaging into the mix, which can also compromise power production.
Header
The header has the greatest effect on the power band and ultimate power production of a non-turbo car. There are MANY factors that go into a properly designed header. One factor is the way you join the pipes together. The two possible configurations for a 4 cylinder are 4-2-1 and 4-1. Basically a 4-2-1 design joins two primaries together into a secondary pipe, and then joins the two secondaries together. A 4-1 design joins all four at the same time. Both have advantages, but the 4-1 design allows the gas pulses to interact in a way that makes the best torque. Here is a simple drawing to help visualize the two different styles.
Primary Pipe Diameter - Smaller diameters keep velocity higher with smaller exhaust volumes. The more exhaust you are trying to push out the larger the primaries need be. The volume of gasses that you need to flow depends on displacement, RPM, and load. The more displacement you have per cylinder the larger the primaries need to be. The same is true for RPM, the more RPM you will be turning, the more diameter you will need as you will be pushing out a lot of volume over time. Higher loads on the motor also create a higher volume of gasses. As with every other variable there is a balance to be kept. If you are not flowing enough gasses for the pipe diameter (pipes are too big) the gasses will loose their velocity If the gasses get too slow you loose torque, and if you go way to large you can even loose top end power as well. Get it right and you get the best of both worlds, good low end torque and good top end power.
Primary Pipe Length - This has a huge effect on the powerband. Generally longer primaries make better low end while shorter lengths move the powerband up in the RPM range. The length affects the powerband by timing when pressure waves reach the cylinder. To put it as simply as possible, the pressure wave comes out of the cylinder and travels down the primary pipe until it hits the collector. There it gets reflected back down the primary pipe as a negative wave. When it hits the cylinder it helps pull more exhaust gasses out of the cylinder and pull more air in to the cylinder. Since power is made by mixing air and fuel and then exploding it, more air and fuel make more power. This effect is known as scavenging and is one of the main goals of a well designed header. Equal length primaries help each exhaust pulse pull the one behind it. This helps create a suction in a sense. Instead of just relying on the pressure of the exhaust stroke of the motor to get the spent gasses out, the suction of the pulse in front of it helps pull it out. One factor some header designers forget when trying to design an equal length header for the Subaru is that the length of the exhaust port is effectively part of the header and needs to be accounted for. Complicating this is the fact that the exhaust ports on the Subaru are not the same lengths. Not accounting for this effects power production. Below is a simple drawing of the port layout. You can see that Cylinder A has a longer path than Cylinder B.
You can see how getting the length correct is where most of the time is spent during testing.
Collector Type -The collector merges all of the primary pipes together. There are designs ranging from cheap and simple to incredibly complex and costly. If you just joined the pipes in the simplest possible way you would have something that resembled the picture on the right.
The dead space in the middle of all of the pipes would cause a lot of turbulence and hinder flow. Eliminating the dead space is the main advantage of the merge collector. The image on the right is an example of a way that you can form the pipes to make a cross pattern in the center. This is a more cost effective way to make the pipes join smoothly. Not quite as elegant as the merge collector, but still very good. You can see that the dead space in the center is virtually eliminated.
The bad daddy of all collectors is the merge collector. It is from Burns Stainless and is one of the finest collectors you can buy. Note how all of the pipes are joined in a smooth way to avoid turbulence.
by Josh Tenny
Non-turbo Exhaust
Designing a non-turbo exhaust system is quite a bit different, most noticeably in the header section. The primary goal of getting the exhaust out with the most velocity and with the least amount of backpressure is still the same, but that is about where the similarities end. The real world also steps in and throws in the same requirements like noise, environmental concerns, and packaging into the mix, which can also compromise power production.
Header
The header has the greatest effect on the power band and ultimate power production of a non-turbo car. There are MANY factors that go into a properly designed header. One factor is the way you join the pipes together. The two possible configurations for a 4 cylinder are 4-2-1 and 4-1. Basically a 4-2-1 design joins two primaries together into a secondary pipe, and then joins the two secondaries together. A 4-1 design joins all four at the same time. Both have advantages, but the 4-1 design allows the gas pulses to interact in a way that makes the best torque. Here is a simple drawing to help visualize the two different styles.
Primary Pipe Diameter - Smaller diameters keep velocity higher with smaller exhaust volumes. The more exhaust you are trying to push out the larger the primaries need be. The volume of gasses that you need to flow depends on displacement, RPM, and load. The more displacement you have per cylinder the larger the primaries need to be. The same is true for RPM, the more RPM you will be turning, the more diameter you will need as you will be pushing out a lot of volume over time. Higher loads on the motor also create a higher volume of gasses. As with every other variable there is a balance to be kept. If you are not flowing enough gasses for the pipe diameter (pipes are too big) the gasses will loose their velocity If the gasses get too slow you loose torque, and if you go way to large you can even loose top end power as well. Get it right and you get the best of both worlds, good low end torque and good top end power.
Primary Pipe Length - This has a huge effect on the powerband. Generally longer primaries make better low end while shorter lengths move the powerband up in the RPM range. The length affects the powerband by timing when pressure waves reach the cylinder. To put it as simply as possible, the pressure wave comes out of the cylinder and travels down the primary pipe until it hits the collector. There it gets reflected back down the primary pipe as a negative wave. When it hits the cylinder it helps pull more exhaust gasses out of the cylinder and pull more air in to the cylinder. Since power is made by mixing air and fuel and then exploding it, more air and fuel make more power. This effect is known as scavenging and is one of the main goals of a well designed header. Equal length primaries help each exhaust pulse pull the one behind it. This helps create a suction in a sense. Instead of just relying on the pressure of the exhaust stroke of the motor to get the spent gasses out, the suction of the pulse in front of it helps pull it out. One factor some header designers forget when trying to design an equal length header for the Subaru is that the length of the exhaust port is effectively part of the header and needs to be accounted for. Complicating this is the fact that the exhaust ports on the Subaru are not the same lengths. Not accounting for this effects power production. Below is a simple drawing of the port layout. You can see that Cylinder A has a longer path than Cylinder B.
You can see how getting the length correct is where most of the time is spent during testing.
Collector Type -The collector merges all of the primary pipes together. There are designs ranging from cheap and simple to incredibly complex and costly. If you just joined the pipes in the simplest possible way you would have something that resembled the picture on the right.
The dead space in the middle of all of the pipes would cause a lot of turbulence and hinder flow. Eliminating the dead space is the main advantage of the merge collector. The image on the right is an example of a way that you can form the pipes to make a cross pattern in the center. This is a more cost effective way to make the pipes join smoothly. Not quite as elegant as the merge collector, but still very good. You can see that the dead space in the center is virtually eliminated.
The bad daddy of all collectors is the merge collector. It is from Burns Stainless and is one of the finest collectors you can buy. Note how all of the pipes are joined in a smooth way to avoid turbulence.
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