Exhaust Tech!

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Understanding Exhaust:
The How & Why

No exhaust system is ideal for all applications. Depending on their design and purpose, all exhaust systems compromise something to achieve something else. Before performing exhaust changes or modifications to increase performance, it is critical to determine what kind of performance you want. * Do you want the best possible low-end and mid-range power or maximum top-end power? * Will you be using an aftermarket cam with different lift, duration, timing and overlap? * Have you investigated the relationship between torque (force) and horsepower (amount of work within time)? * Do you want a cosmetic exhaust system or a performance exhaust system?
Without careful thought about these variables, an exhaust system can yield very disappointing results. On the other hand, a properly designed and tuned exhaust system that is well-matched to the engine can provide outstanding power gains.
The distinction between "maximum power" and "maximum performance" is significant beyond general discussion. Realistically, one exhaust system may not produce both maximum power and maximum performance. For a motorcycle to cover "X" distance as quickly as possible, it is not the highest peak power generated by the engine that is most critical. It is the highest average power generated across the distance that typically produces the quickest time. When comparing two horsepower curves on a dynamometer chart (assuming other factors remain constant), the curve containing the greatest average power is the one that will typically cover the distance in the least time and that curve may, or may not, contain the highest possible peak power.
In the strictest technical sense, an exhaust system cannot produce more power on its own. The potential power of an engine is determined by the proper amount of fuel available for combustion. However, the efficiency of combustion and engine pumping processes is profoundly influenced by the exhaust system. A properly designed exhaust system can reduce engine pumping losses. Therefore, the design objective for a high performance exhaust is (or should be) to reduce engine-pumping losses, and by so doing, increase volumetric efficiency. The net result of reduced pumping losses is more power available to move the motorcycle. As volumetric efficiency increases, potential fuel mileage also increases because less throttle opening is required to move the motorcycle at the same velocity.
Much controversy (and apparent confusion) surrounds the issue of exhaust "back-pressure". Many performance-minded people who are otherwise knowledgeable still cling tenaciously to the old school concept.... "You need more back-pressure for better performance."
For virtually all high performance purposes, backpressure in an exhaust system increases engine-pumping losses and decreases available engine power. It is true that some engines are mechanically tuned to "X" amount of backpressure and can show a loss of low-end torque when that backpressure is reduced. It is also true that the same engine that lost low-end torque with reduced back-pressure can be mechanically re-tuned to show an increase of low-end torque with the same reduction of back-pressure. More importantly, maximum mid-to-high RPM power will be achieved with the lowest possible backpressure. Period!
The objective of most engine modifications is to maximize the proper air and fuel flow into, and exhaust flow out of the engine. The inflow of an air/fuel mixture is a separate issue, but it is directly influenced by exhaust flow, particularly during valve overlap (when both valves are open for "X" degrees of crankshaft rotation). Gasoline requires oxygen to burn. By volume, dry, ambient air at sea level contains about 21% oxygen, 78% Nitrogen and trace amounts of Argon, CO2 and other gases. Since oxygen is only about 1/5 of air’s volume, an engine must intake 5 times more air than oxygen to get the oxygen it needs to support the combustion of fuel. If we introduce an oxygen-bearing additive such as nitrous oxide, or use an oxygen-bearing fuel such as nitromethane, we can make much more power from the same displacement because both additives bring more oxygen to the combustion chamber to support the combustion of more fuel. If we add a supercharger or turbocharger, we get more power for the same reason…. more oxygen is forced into the combustion chamber. Theoretically, in a normally aspirated state of tune without fuel or oxygen-rich additives, an engine’s maximum power potential is directly proportional with the volume of air it flows. This means that an engine of 80 cubic inches has the same maximum power potential as an engine of 100 cubic inches, if they both flow the same volume of air. In this example, the powerband characteristics of the two engines will be quite different but the peak attainable power is essentially the same.