Exhaust System Basics - Hot Bike Tech

Optimum collector design requires cut-and-try testing, so there is no one size fits all solution. However, in general, the longer and smaller the diameter of the merge collector, the lower the point where peak torque rpm will occur. A long slow-tapering collector effectively spreads the reflected wave over a wider rpm band. Spreading the wave essentially tricks the engine into thinking the header pipes are longer than they actually are. On the other hand, a short and wide-angle collector creates the strongest return pressure wave but works over a smaller rpm range. Additionally, the merge angles of the header pipes and the internal design of the collector can have an influence on power production.

Reversion
A slow-flowing port typically allows excessive amounts of exhaust gases to backup in the port and re-enter the combustion chamber (called reversion). The exhaust gases dilute the intake charge and ruin carburetion and throttle response. An anti-reversionary (AR) flange is often installed in a header pipe where the pipe intersects with the exhaust port. This feature helps when the exhaust port is inefficient and slow flowing. Essentially, an AR flange shrouds the port, thereby catching much of the back-flowing exhaust, which improves performance, particularly at low rpm. Some exhaust ports are designed with a miss-match at the bottom of the exhaust header pipe. The miss-match functions in the same fashion as an AR flange by reducing exhaust backflow. The mismatch creates a ledge at the bottom of the port that stops reversionary exhaust gases from backing into the combustion chamber.

Final Thoughts
Key exhaust system considerations include pipe diameter and length. Both pipe variables should be optimized based on engine displacement, rpm band, cam timing, and application. If a collector is used, its diameter and length also must be considered. Bike weight, gear ratios, and the number of gears along with the application also enter into the equation for optimum exhaust design.

Header diameter (inside diameter) is typically the most important factor in exhaust system design because it sets the torque curve. Increasing diameter generally improves top-end power at the expense of low-end torque. Changing pipe length will move the torque curve either up or down the rpm scale. A shorter pipe favors top-end horsepower while a longer pipe caters toward low-end torque. Some engine builders have found great power numbers using an exhaust system that appears contradictory to known rules: long large-diameter or short small-diameter header pipes. However, these designs can offer the best of both worlds-great low end torque and top-end horsepower. Straight pipes typically improve power above roughly 4,000 rpm, which is great for an engine that never drops below this rpm. However, at low rpm, straight pipes generally create big dips in the torque curve, reduce throttle response, and make jetting difficult.

Exhaust systems for '06 and all '07-up models have bungs welded into the pipes for EFI O2 sensors, which are required for maintaining EPA compliance. For these models, be sure to buy a pipe with bungs. Since a performance exhaust system increases airflow through the engine, the carburetor jetting or EFI fuel map must be altered to ensure a proper air/fuel mixture. Since some years and models of bikes may have warranty implications if the exhaust system is changed, be sure to check your warranty status before buying an exhaust upgrade

Finally, consider the exhaust system an integral part of components that regulates airflow through the engine. For optimum performance, it is important the exhaust and induction systems, camshaft and ignition timing be tuned together as a complete system. If a component is replaced or modified, the engine must be retuned for best performance.