Motorcycle Exhaust

If you insist on using drag pipes on your bike, there is something you can do to improve the low and mid range power produced by the engine. Even with the improvement listed here, the streetable engine power is not going to match power output of a good 2-1 or 2-2 exhaust system. Motorcycle Performance Guide does not recommend drag pipes or porker 2" pipes for serious street engines, but the performance fix listed here will improve the power of your drag pipes.

If it is loud, it must be fast?
The first item to get modified or changed on most new Harley-Davidson motorcycles is the exhaust system. Getting the proper Harley sound always seems to require increasing the decibel level out the exhaust, with many riders installing drag pipes as the exhaust system for the proper sound. The rider often believes that by reducing back pressure in the exhaust system the engine will also increased power. This is wrong. As a resulting of changing a stock exhaust system to Drag Pipes, most engines promptly lose 5-10% of the power the engine produced.

By properly re-jetting the carburetor and adding a free flowing air cleaner to an engine with drag pipes, the maximum horsepower produced will improve over the stock engine. But there is a difference between usable power and maximum horsepower. The maximum horsepower of two engines may be similar, but the horsepower torque curves may be different. The area under the horsepower and torque curves defines the "power" the engine produces. The more area that is under the curve, the better the power.

A typical drag pipe produces a horsepower curve that initially rises very slow. As the RPMs start to rise above mid-range power, the curve begins to rise at increasing rate until maximum horsepower is achieved. Once RPMs have passed maximum horsepower, the curve drops of rapidly.

The horsepower curve of a typical 2-2 pipe like the Cycle Shack Slash Cuts produces a curve that may actually be closer to a straight line from low RPMs up through the rpm that maximum horsepower is produced. Once maximum horsepower is achieved, the curve drops at a relatively mild rate.

The horsepower curve of a typical 2-1 pipe like the SuperTrapp starts off slightly lower than the 2-2 pipe, but rises at a rapid rate in the mid rpm ranges. As the rpm range approaches maximum horsepower, the curve flattens out. Once maximum horsepower is achieved, the curve drops of rapidly.