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Old 01-19-2011, 06:12 PM   #1
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Default Exhaust operation and the back pressure myth "Or" Ask Uncle Otto about exhaust

Exhaust many times is compared to other fluids. However, it is not quite the same due to the temperatures created during combustion and the pulses an engine creates. For an exhaust system to function properly, it has to balance flow and velocity. I am writing this to cover a broad range of knowledge bases and ages that typically frequent forums and have questions about how exhaust really works. The operation and physics behind exhaust, and the common misconception that backpressure is necessary for proper engine operation is the focus of this article. The hope is, with the knowledge gained here and from other sources, that an informed decision and discussion about exhaust can carry on.

Velocity, Flow, and Temperature:
Velocity is the speed at which matter is traveling. Velocity in an exhaust system has to be understood to select the correct pipe size. The straw example is a simple but effective method to explain velocity. If a person blows through a small diameter straw, there is a great amount of speed of the gases exiting the end. However, even as the person blows as hard as they can, they are restricted by the size of the straw to the volume of gas that can be moved. Conversely, if a large diameter straw is used, the person will be able to move large quantities of gas but the velocity of the gases will be greatly reduced.
Flow is the ease at which the gases travel through a set container. The walls of the container will always cause some resistance, but this force is usually negligible. The more things inhibiting flow in the path, such as baffles or chambers, the more force is required to flow at the same velocity. If a piece of paper is held next to the end of the large diameter straw, it causes a slight resistance to the gases escaping the straw. Gases will take the path of least resistance. Since gases also compress, the molecules are forced closer to one another for a short period because it is easier to compress than escape the end. This causes the pressure to increase. Once the pressure rises and overcomes the force the paper puts on the end of the straw, the gases begin to escape out the end. This is what many call backpressure because of the pressure that is built in the area necessary to escape the container. Backpressure only causes the origin to work harder to push more gases into and out of the container.
The temperature of the gases in a container greatly affects its behavior. The cooler the gas is, the closer the molecules are together in space. The warmer the gas, the larger volume the gas occupies. This is due to the force of the molecules as they come in contact with the container and other gas molecules.

Operation in the cylinder:
On the power stroke, the piston is moved down the cylinder from the combustion. As the piston begins to move upward, the valve on the exhaust side of the head opens allowing the hot products of the reaction out. When the piston reaches the top of the chamber, it begins the downward stroke and pulls air and fuel in to repeat the cycle. Simple really, but this is where the myth of backpressure comes in. My question all who claim they need back pressure to run properly is: What does it do? The cam controls valves so no “back pressure” is needed to close the valve, the piston returns to the bottom of the cylinder via the power produced, and the chamber needs fresh air and fuel to continue the cycle. Any gases that are trapped in the cylinder are detrimental to the next combustion reaction. The chamber needs to be fully evacuated so a pressure difference is created. As the piston moves on the intake stroke, a miniature vacuum (delta pressure) is created to draw fresh air and fuel into the chamber. The ability for the pulse to maintain the correct velocity and flow, interact and aid in the other cylinder’s operation, and exit the system are the key functions of the exhaust gases.
Modern vehicles do not rely on one cylinder in an engine so the interactions with the other cylinders are critical. This is extenuated in V6, V8, and larger cylinder engines. As one cylinder is firing, another is on the opposite stroke. This creates pulses throughout the system. Using these pulses to aid in the other cylinders operation is key. As a pulse is making its way into and out of the header/manifold it creates another pocket of delta pressure behind it. This drop in pressure is used to help “pull” the next cylinder’s exhaust gases into and out of the manifold and so on. This is referred to as scavenging. This is also why V6 and V8 engines enjoy the presence of a crossover of some sort. Due the firing order on each side, the pulses are not the same all of the time. Without some way for the pulses to equal the other side’s scavenging, the scavenging effect will be slightly different on either side. This is why many times a pop or poor operation of the engine is detected without a crossover. A pop may also be audible with a crossover system but this is usually due to a rapid acceleration/deceleration and the different rates of the pulses in the system for that moment. A crossover is recommended to be installed before the muffler, close to the headers/manifold to quickly maximize scavenging.
At any time there is approximately 14.6 psi pushing on all sides of you. The atmosphere exerts this pressure at sea level due to its mass. This is the only backpressure exerted on the exhaust system. The exhaust pulses upon startup must overcome this pressure to exit the system. After startup in a properly sized system however, the pulses continue to aid the next.

Adjusting the Power band:
The power band is described as: at X rpm produces Y horsepower/torque. This can be influenced greatly by the size and configuration of the exhaust piping. Exhaust is most effective at the point which flow and velocity are maximized. This point is when the pulse can expand to the size of the pipe without velocity being diminished. Too large of a pipe and the velocity slows, causing the pluses to create little scavenging behind them and the pulses weakly leave the system. Too small of a pipe and the flow is restricted, effectively “choking” the system.
The rpm range of operation is quite large on most motors. Idling at 700 rpm and redlining at 6000 rpm create very different environments in the pipe. At lower rpms, less volume in the system is needed. At higher rpms, more volume is needed. Since most drivers don’t do one or the other all the time, the median for normal operation is ideal. This is where the powerband comes in. By having a larger diameter system the higher rpm range may produce the best power because that is the point at which flow and velocity are optimized. However, in these systems a loss in low range power is felt due to the fact that velocity is not ideal. If the volume of the pipe is too large the pressure drop behind each pulse will not be as likely to help in the scavenging effect of the next cylinder. The same principle goes if the pipe is too short. The short pipe does not allow enough time for proper scavenging and leaves the system before it can occur. This is why things such as open manifolds and ending the system under the cab do not bode well. With a smaller system, low range power is optimized because lower rpms are most efficient at scavenging. At higher rpms, the system’s flow is not sufficient, thus less power is able to be produced.

More than just pipe:
Exhaust does more than simply get waste products out of the way. In vehicles that utilize oxygen sensors, the exhaust is the critical component to monitor air/fuel ratios. This measurement by the oxygen sensors is critical in maintaining correct a/f ratios and prevents from either lean or rich conditions. Changing the location of these sensors or how the gases interact with the sensors, needs to be accounted for by adjustments to the PCM.


Any modification to the system can have an impact on the performance of the engine. Understanding how the different segments interact with the whole is crucial when modifying a vehicle. With this knowledge, you should be able to understand how exhaust systems operate and be able to make quality decisions when choosing to modify your system.
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Old 01-20-2011, 06:56 AM   #2
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Stickied... or is it stuck?
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Old 02-05-2011, 10:49 AM   #3
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great post man, that took some time. well written!
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Old 02-05-2011, 08:29 PM   #4
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yea great post thanks for the info
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Old 03-10-2011, 09:47 AM   #5
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sooo i didn't feel a difference with my flowmaster 40 series dual cat back... should i? i didnt get it for performance tho i got it for sound, i was under the assumption that exhaust would give very minimal if any horespower increase when adding to a stock truck...
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Old 03-10-2011, 04:10 PM   #6
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Everyone's butt dynos are different. Some claim they feel differences from cat backs some don't. Chambered mufflers aren't for performance and you already knew that so no surprise from feeeling no change.
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Old 03-10-2011, 04:55 PM   #7
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Quote:
Originally Posted by otto457
Everyone's butt dynos are different. Some claim they feel differences from cat backs some don't. Chambered mufflers aren't for performance and you already knew that so no surprise from feeeling no change.
Actually I didn't know chambered mufflers don't add hp... I just figured it wasn't a performance exhaust... Learn something new everyday
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Old 03-10-2011, 11:42 PM   #8
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Thank you Otto for bringing this out of the fog.
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Old 03-11-2011, 12:03 AM   #9
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Glad I didn't mess with my pipes; just added a new muffler and dumped it out the side. Still SI/SO

Since there is less pipe in a Side-exit exhaust what kind of effect if any woud that have on power?? Although it's probably negligible.
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Old 03-11-2011, 01:04 PM   #10
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Quote:
Originally Posted by BassAckwards View Post

Since there is less pipe in a Side-exit exhaust what kind of effect if any woud that have on power?? Although it's probably negligible.
As long as there is enough pipe for the gases to travel to maintain scavenging through a certain number of revolutions, there won't be any loss. Three feet after the merger (X, Y, or H) is usually a safe length rule of thumb.
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Old 03-11-2011, 01:04 PM
 
 
 
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