Exhaust System Basics - Hot Bike Tech

The following is a brief overview of how it works: As the exhaust valve opens, gases rushing past the valve create a positive pressure wave that travels toward the open end of the header pipe. As the positive wave exits into the atmosphere, it is converted into a negative wave that moves back up the pipe toward the valve. During engine operation, these positive and negative pressure waves pulsate between the open end of the pipe and the exhaust valve. When pipe length is optimized, the negative wave will be timed to arrive at the exhaust valve during the valve overlap period. Since the negative wave reduces pressure at the valve, it helps scavenge combustion gases from the chamber.

Because pressure waves can only be timed to help exhaust scavenging over a narrow rpm band, the engine's most critical rpm range must be first determined so pipe length can be matched to the rpm band. A longer pipe length optimizes power at low rpm. Conversely, a shorter pipe length improves upper-end rpm performance, because as the pipe becomes shorter, the tuning effect has less time to enhance slow-speed engine's operation.

It is important to note that an exhaust system can only be optimized over a small rpm band, typically 1,500 to 2,000 rpm. Therefore, be sure to tune the pipe diameter and length to the engine's most critical rpm band. In the case of a drag racing engine, this band should fall between peak torque rpm and peak horsepower rpm. With due diligence, you can tune a header pipe to the middle of the rpm band to achieve the highest average power. For a lower-rpm street engine, it is normally best to tune pipe dimensions to peak torque rpm. Larger diameters and shorter length header pipes optimize high-rpm operation, while smaller diameters and longer pipe lengths favor low-end power.

Stepped Header Pipes
Various exhaust systems are designed with a stepped-header pipe. A stepped header includes the placement of pipediameter differentials in the pipe. The differentials are referred to as steps. Stepped headers are divided into two or more pipe sections (usually two or three) with each successive section at least 1/8-inch larger than the previous section. Exhaust port shape and valve size are crucial for determining the optimum size differential and location ofsteps in a header pipe.

Although a stepped header generates more low-pressure waves than a non-stepped design, its waves are weaker. Steps help maintain a higher average gas velocity over the total length of header pipe. A stepped header won't necessarily make more power than a non-stepped pipe, but it can broaden the engine's torque curve by widening the scavenging wave's effect, which increases the time of negative depression. This can result in a win-win situation: High torque at low rpm while maintaining high horsepower at high rpm. Since the engine views a stepped header as a tapered pipe, the greater the number of steps in a length of pipe, the greater the taper angle and the stronger the pressure waves. But as the pipe angle increases, the rpm band it affects gets narrower. On the otherm hand, a longer pipe with fewer steps results in a narrower angle, thereby widening the working rpm range but lessening the strength of the pressure wave. Stepped headers are most beneficial when used on large-displacement and/or high-rpm engines.