8. A reverse cone is fitted to a collector to augment, extend and smooth out the peak horsepower band. A reverse cone can improve top-end power without compromising low-end torque. This pipe includes heat shields to retain the show-chrome look.
A cam with a long overlap can benefit from a pipe design that produces a wide exhaust-scavenging wave because a greater portion of the overlap period is effectively covered. A stepped header is an excellent design for this application as long as the pipe taper is not too steep. Conversely, if the scavenging wave is too narrow, the overlap period will not be covered effectively and performance will be reduced. Increasing the amount of time the scavenging wave covers the overlap period tends to broaden the tuned rpm band. Combiningm a relatively wide scavenging wave with a short overlap or generating a wider wave with a long overlap are two ways to broaden the rpm band over which the exhaust system is effective.
V-Twin engine builders have various philosophies for stepped headers. Some favor equal length sections for a stepped pipe while others prefer unequal lengths. Still others achieve good results by making the first section 10 to 12-inches long, and then splitting the remainder of the header into equal length sections. Some tuners base the length of the first stepped section on valve diameter. Moreover, the location of the bend in the header immediately after exiting the port and the design of the bike's frame can dictate the length of the first step.
Mufflers
Most stock Harley mufflers are relatively small for aesthetic reasons, and include a healthy dose of internal baffling to satisfy EPA noise limitations. As a result, airflow is restricted and so is power. Large, touring model Harleys ship with large volume mufflers and a crossover pipe. This large interconnected design produces a few extra ponies over the Dyna/Softail models with smaller volume mufflers.
Adequate muffler volume for the engine displacement is important for keeping exhaust backpressure low at high rpm. Generally, muffler volume should be roughly 10 times the cylinder volume to make good high rpm power. However, horsepower is also a factor because the more horsepower an engine makes, the more exhaust flow it generates. In other words, as engine airflow increases exhaust gas volumeor also increases. With increased exhaust gas volume, muffler airflow and volume must also be increased. For example, assume we have two 113 cubic inch engines, one producing 100 horsepower and another producing 125 horsepower. A muffler that barely provides adequate airflow for the 100 horsepower engine will surely be inadequate for making good top-end power with the 125 horsepower engine.
Large engines require a muffler with a large main body, free-flowing baffle and unobstructed exit. Since large mufflers do not look aesthetically pleasing on a V-Twin, it is difficult to make a pipe for a large-displacement engine that satisfies both aesthetics and performance. The easiest and least costly method for improving exhaust system performance is to modify the system. Most 2-into-2 systems are tunable by modifying the internal baffles. Modifying baffles, especially those used in small-volume mufflers, can improve power. Increasing the number and/or size of holes in the baffles or shortening the baffles reduces backpressure and helps top-end power. However, remember that increasing flow too much can reduce bottom-end torque. Furthermore, tunable 2-into-1 systems offer a clear advantage over non-tunable collector systems, especially if engine displacement is large.
2-Into-1 Collector Systems
A 2-into-1 exhaust system terminates both header pipes into a tapered merge collector, which includes baffling that acts as a muffler. Some collector systems for the V-Twin are designed with replaceable baffles, which allows the tuner to select a baffle offering the optimum amount of airflow and backpressure for the application. Since collector systems tend to increase torque below peak torque rpm, they typically improve low and midrange power. Highrpm race engines, such as Top Fuel motors, generally do not use a collector because the engines do not drop below peak torque rpm.