Harley exhaust choices vary almost as much as those who ride. Drag pipes, slip-fit mufflers, 2-1, staggered duals, true duals and the list goes on. An exhaust system is made for every bike, engine, style, and configuration imaginable. However, there is a big difference between performance and what looks "cool". If you are contemplating the purchase of a new Harley exhaust system you should first ask yourself a few questions.

How important is performance compared to looks?
Are you looking for performance in low to midrange or high end torque?
 
For anyone interested in what will improve performance or why those drag pipes made your ride a slug please read on.

Let's start with the basics. Without getting into a lesson in physics there are three factors that effect the performance and tuning of an exhaust system. Sonic pulse, thermal pulse, and exhaust gas velocity.

Sonic pulse. Think of this as a shock wave. This pulse travels through the exhaust and at the end of the pipe actually turns around and heads back toward the exhaust valve. The wave will also pull exhaust gas back along its path and if it reaches the open exhaust valve the gases will dilute the air/fuel mixture in the combustion chamber. Under the right (or wrong) conditions this wave will even push some of the air/fuel mixture back through the intake valve into the intake, a common occurrence with short drag pipes.

Thermal pulse. Similar to sonic pulse, this is a wave created by the hot gas exiting the exhaust valve. The thermal pulse also reverses direction at the end of the pipe but unfortunately travels at a different rate, thus reaching the combustion chamber at a different time.

Exhaust gas velocity. This is the rate at which gas travels through the exhaust system. The optimal velocity is 300 feet per minute. For the most part all stock Harley engines require a 1-3/4" diameter pipe to maintain 300 fpm. Bigger is not always better and a larger pipe will actually slow this velocity thus restricting flow.
So what does all of this mean? To best answer this question let's look at a few different exhaust designs and how they handle the above listed conditions.

Short Drag Pipes
Since the sonic and thermal pulses don't have as far to travel as in a longer system the wave is able to return faster, thus entering the combustion chamber robbing the engine of power. Drag pipes are tuned for higher rpm large displacement engines where these pulses can be overcome. Some drag pipe makers such as Cycle Shack offer special baffles to help tune their drag pipes. The exception would be the baffled drag pipes offered by Vance & Hines. The V&H cruiser pipes have that custom look but the baffles help compensate for reversion by providing just the right amount of back pressure.
 
One trick is to install anti-reversion or torque cones. When installed inside the head pipes these cones act to repel the pulse waves as they return to the engine and alter velocity as the gas passes through the narrower passage. Performance will still suffer on the lower end since there is no ability to scavenge gases as with a 2-1 or stock Harley exhaust.