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 Exhaust System Basics: Exhaust system design is critical to engine and carburetor performance. If the header pipes are too large or too small in diameter, too short or too long --- performance suffers. Stock Harley-Davidson header pipes are very near optimum in both length and diameter for the great majority of applications, including high performance engines. This is not the case with the stock Harley mufflers, however. Reduced air flow through the mufflers severely limits the performance of a completely stock Harley. To the extent that air flow is limited, so is power. It is possible to get good air flow through a reasonably quiet muffler, but not if it is small in relation to the displacement of each cylinder. The large mufflers fitted to the heavy touring Harleys generally deliver two or three more horsepower than the smaller mufflers fitted to most other models. Most factory Harley mufflers are, for styling reasons, small compared to engine size. Since they are also legally mandated to be rather quiet, they do not flow particularly well and severely limit peak power output. Besides the desire to hear the engine, this fact is a major reason for the popularity of free-flowing aftermarket mufflers and exhaust systems. The easiest and least expensive way to gain exhaust system performance is to modify the stock system. Use the factory header pipes and fit a set of small loud mufflers. Examples of such mufflers are: Harley Screamin' Eagle mufflers (not the "II" series but the older, loud ones), Cycle Shack slip-ons or other similar mufflers. Testing has shown that, in general, if it is loud and you can see through it, it will make good power. As a general rule, header pipes that are 1-3/4" in diameter, between 27 and 32 inches long and combined with small open mufflers produce excellent all around power. Many, if not all, two-into-one systems allow greater airflow and produce more top-end power than any of the small slip-on mufflers. However, these systems may not do as well in the often-used 2000 to 3000 rpm range. With no other change, a set of Screamin' Eagle slip-ons will defeat the Harley 2-into-1 system in a 50 mph roll-on. Exceptions to this are the White Bros "E" series and latest version of the SuperTrapp 2-into-1 systems. These two systems deliver more power in this range than any of the slip-on mufflers. Straight and Open Exhausts: A significant portion of tech support calls to Mikuni regarding tuning problems are caused by the use of straight, open header pipes. Straight pipes seldom perform well in the rpm range where most Harley engines spend their running life (2500 to 3500 rpm). In fact, with the exception of 1-3/4" diameter by 30" long straight pipes (stock Harley headers) running at 3800 or higher rpm, straight open pipes perform rather poorly in terms of power output and carburetion accuracy. |
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