Worth the read...


source-http://www.harley-riders-guide.com/harley-exhaust.html

Your Harley exhaust system is an important part of the components that control airflow through your engine. Other important components include the induction system, cams, and heads. To get the best performance, these components must be tuned together, through a given rpm range. If one of these is changed or modified, the group of components must be retuned to get the maximum performance.
It's important to remember that a louder exhaust system does not always mean better performance. Dyno tests have proven that there is no direct, proportional relationship between sound levels and power. Some quieter aftermarket mufflers actually make more power than some louder ones.

An optimized exhaust is designed to achieve balanced pressure between the engine's intake and exhaust within a given rpm range. The range of 2,000 to 4,000 rpm is usually best for street use. The range of 4,500 to 5,000 rpm and higher is more important for racing applications. No Harley exhaust system performs efficiently through the full rpm range. Knowing this, priorities have to be determined and some compromises have to be made.

HERE'S HOW IT WORKS
The exhaust valve opens, gases go past the valve and create a positive pressure wave that travels out the header pipe. As this positive wave exits, it is converted into a negative wave which moves back up the pipe toward the valve. These positive and negative pressure waves pulsate between the open end of the pipe and the exhaust valve. With the right pipe length, the negative wave will arrive at the exhaust valve during the valve overlap. The negative wave reduces pressure at the valve and helps scavenge combustion gases from the chamber.




Pressure waves can only be timed over a narrow rpm range. The rpm range should be determined so pipe length can be matched to the rpm band. Shorter pipe length improves higher rpm performance. Longer pipe length improves lower rpm performance.

When considering a Harley exhaust, remember there are sometimes tradeoffs between looks and performance. Exhaust system design has a big impact on engine performance. Header pipe diameter, bend radius, pipe length, muffler volume and design of the baffle effect performance.

If the pipe is too long or too short or if the diameter is too big or too small, performance will suffer. Performance also suffers when airflow is restricted by the mufflers. Bigger engines generate more exhaust and need bigger higher-flowing mufflers, but looks can often suffer with increased muffler size.
Harley-Davidson engineers know which header pipes work best in terms of deminsions, so most of the time a set of slip-on mufflers will give you a good performance boost, without the added expense of changing the header pipes.


Here's a look at different types of Harley-Davidson exhaust.


TRUE DUALS
True dual exhaust systems use a separate pipe for each cylinder. This design provides a balanced flow for both cylinders.

TWO-INTO-ONE
The two-into-one design merges both header pipes into a tapered collector. Two-into-one exhaust systems tend to increase low and midrange power.

STRAIGHT PIPES
Two-into-two straight pipes can improve top-end performance but low and midrange power suffers. Throttle response below 3,800 rpm is usually poor. These pipes are also very loud for street use.

STEPPED HEADERS
Stepped headers are divided into two or more sections. Each section is a minimum of 1/8 inch larger in diameter than the previous section. Stepped headers perform better on large-displacement and or higher rpm engines.

SLIP-ON MUFFLERS
Slip-on mufflers are an excellent choice since your keep your stock Harley exhaust pipes while adding improved performance and better sound. Since the original pipes are kept, velocity and lower end torque is retained. Slip-on mufflers increase flow compared to factory mufflers which are restrictive because of EPA regulations. Screamin Eagle slip-ons are a popular choice for Harley riders, especially for touring models.

THINGS TO CONSIDER WHEN BUYING AN EXHAUST SYSTEM
Price
Performance
Warranty
Sound
Configuration (2-into-1, 2-into-2, true duals etc.)
Apperance (anodized, chrome, etc.)
Quality of the finish


Buying a Harley exhaust can be a confusing experience. There's a huge selection of exhaust systems to choose from. There are two-into-one, true duals, large and small diameter headers, long and short headers, stepped headers etc. Most Harley riders want power from 2,000 to around 4,000 rpm range. There are a lot of exhausts systems that will give you good performance in this rpm range. There are reliable dyno charts available for you to reference before buying your new exhaust.

Exhaust systems for all 2007s and up models have bungs welded into the pipes for EFI O2 sensors, which are required for EPA compliance. Make sure you buy a pipe with bungs. A performance exhaust requires an EFI fuel remap to insure proper air/fuel mixture.