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**BoostedFC** · 09-28-2010, 09:02 AM

Flow Volume & Flow Velocity One of the biggest issues with exhaust systems, is the relationship between gas flow volume and gas flow velocity (which also applies to the intake track). An engine needs the highest flow velocity possible for quick throttle response and torque throughout the low-to-mid range portion of the power band. The same engine also needs the highest flow volume possible throughout the mid-to-high range portion of the powerband for maximum performance. This is where a fundamental conflict arises. For "X" amount of exhaust pressure at an exhaust valve, a smaller diameter exhaust pipe will provide higher flow velocity than a larger diameter pipe. Unfortunately, the laws of physics will not allow that same small diameter pipe to flow sufficient volume to realize maximum possible power at higher RPM. If we install a larger diameter pipe, we will have enough flow volume for maximum power at mid-to-high RPM, but the flow velocity will decrease and low-to-mid range throttle response and torque will suffer. This is the primary paradox of exhaust flow dynamics and the solution is usually a design compromise that produces an acceptable amount of throttle response, torque and horsepower across the entire powerband.
A very common mistake made by some performance people is the selection of an exhaust system with pipes that are too large in diameter for their engine's state of tune. Bigger is not necessarily better and is often worse. Equal Length Exhaust
The effectiveness of equal length exhaust is widely debated. Assuming that an exhaust system is otherwise properly designed, equal length pipes offer some benefits that are not present with unequal length pipes. These benefits are smoother engine operation, tuning simplicity and increased low-to-mid range torque.
If the pipes are not equal length, both inertial scavenging and wave scavenging will vary among engine cylinders, often dramatically. This, in turn, causes different tuning requirements for different cylinders. These variations affect air/fuel mixtures and timing requirements, and can make it very difficult to achieve optimal tuning. Equal length pipes eliminate these exhaust-induced difficulties. "Tuning", in the context used here, does not mean installing new sparkplugs and an air filter. It means configuring a combination of mechanical components to maximum efficiency for a specific purpose and it can not be overemphasized that such tuning is the path to superior performance with a combination of parts that must work together in a complimentary manner.
In an exhaust system that is properly designed for it’s application, equal length pipes are generally more efficient. The lengths of both the primary and main section of pipes strongly influence the location of the torque peak(s) within the powerband. In street and track performance engines with longer pipes typically produce more low-to-mid range torque than shorter pipes and it is torque that moves a motorcycle. The question is... Where in the powerband do you want to maximize the torque? * Longer pipes tend to increase power below the engine’s torque peak and shorter pipes tend to increase power above the torque peak. * Large diameter pipes tend to limit low-range power and increase high range power. * Small diameter pipes tend to increase low-range power and to some degree limit high-range power. * "Balance" or "equalizer" chambers between the exhaust pipes tend to flatten the torque peak(s) and widen the powerband.
Among the more astute and responsible exhaust builders, it is more-or-less understood that pipe length variations should not exceed 1" to be considered equal. Even this standard can result in a 2" difference if one pipe is an inch short and another pipe is an inch long.

09-28-2010, 09:02 AM	#2
BoostedFC Full Access Member Join Date: Sep 2010 Location: Fredericksburg, Va Age: 43 Posts: 785	Continued Flow Volume & Flow Velocity One of the biggest issues with exhaust systems, is the relationship between gas flow volume and gas flow velocity (which also applies to the intake track). An engine needs the highest flow velocity possible for quick throttle response and torque throughout the low-to-mid range portion of the power band. The same engine also needs the highest flow volume possible throughout the mid-to-high range portion of the powerband for maximum performance. This is where a fundamental conflict arises. For "X" amount of exhaust pressure at an exhaust valve, a smaller diameter exhaust pipe will provide higher flow velocity than a larger diameter pipe. Unfortunately, the laws of physics will not allow that same small diameter pipe to flow sufficient volume to realize maximum possible power at higher RPM. If we install a larger diameter pipe, we will have enough flow volume for maximum power at mid-to-high RPM, but the flow velocity will decrease and low-to-mid range throttle response and torque will suffer. This is the primary paradox of exhaust flow dynamics and the solution is usually a design compromise that produces an acceptable amount of throttle response, torque and horsepower across the entire powerband. A very common mistake made by some performance people is the selection of an exhaust system with pipes that are too large in diameter for their engine's state of tune. Bigger is not necessarily better and is often worse. Equal Length Exhaust The effectiveness of equal length exhaust is widely debated. Assuming that an exhaust system is otherwise properly designed, equal length pipes offer some benefits that are not present with unequal length pipes. These benefits are smoother engine operation, tuning simplicity and increased low-to-mid range torque. If the pipes are not equal length, both inertial scavenging and wave scavenging will vary among engine cylinders, often dramatically. This, in turn, causes different tuning requirements for different cylinders. These variations affect air/fuel mixtures and timing requirements, and can make it very difficult to achieve optimal tuning. Equal length pipes eliminate these exhaust-induced difficulties. "Tuning", in the context used here, does not mean installing new sparkplugs and an air filter. It means configuring a combination of mechanical components to maximum efficiency for a specific purpose and it can not be overemphasized that such tuning is the path to superior performance with a combination of parts that must work together in a complimentary manner. In an exhaust system that is properly designed for it’s application, equal length pipes are generally more efficient. The lengths of both the primary and main section of pipes strongly influence the location of the torque peak(s) within the powerband. In street and track performance engines with longer pipes typically produce more low-to-mid range torque than shorter pipes and it is torque that moves a motorcycle. The question is... Where in the powerband do you want to maximize the torque? * Longer pipes tend to increase power below the engine’s torque peak and shorter pipes tend to increase power above the torque peak. * Large diameter pipes tend to limit low-range power and increase high range power. * Small diameter pipes tend to increase low-range power and to some degree limit high-range power. * "Balance" or "equalizer" chambers between the exhaust pipes tend to flatten the torque peak(s) and widen the powerband. Among the more astute and responsible exhaust builders, it is more-or-less understood that pipe length variations should not exceed 1" to be considered equal. Even this standard can result in a 2" difference if one pipe is an inch short and another pipe is an inch long.