Last part of the engine that needs some tender loving care is the XR500 exhaust system. The pipes are rusty, the Supertrapp muffler which came with the Basket Case is covered with exhaust grime and soot and its disks are all clogged with solid carbon deposits. Not a pretty picture but it’s salvageable.
Let’s start with the header pipes. Over the years, I’ve experimented with a long list of rust eating chemicals but none deliver on the promises so I don’t have any recommendations. At the end of the day what works is an old fashion metal brush, sandpaper and the all mighty patience factor.
To paint the header pipes, I’m using a very high temperature resistant flat black silica/ceramic VHT header pain. It can supposedly withstand temperatures of up to 2000F (1093 Celsius). We’ll find out if this is for real or just yet another claim that only a marketing department can dream up.
Now on to the Supertrapp performance muffler. The Supertrapp design has been around for ages (40 years) without major changes. What differentiates it from other mufflers is how the back pressure can be tuned by changing the number of disks in its diffuser system i.e. by adding or subtracting discs, one can tune the powerband and sound of a motorcycle in just a few minutes and with basic hand tools.
Removing discs decreases the exhaust opening and increases backpressure. This effectively decreases the powerband to create more low-end torque. It also decreases exhaust tone and enriches carburetion.
Adding discs increases the exhaust outlet and decreases back pressure. This widens the powerband at the top end. It also increases exhaust tone and leans out carburetion.
Sound energy in SuperTrapp mufflers is reduced in two ways: it is absorbed through a perforated core inside the body of the muffler, and it is diffused 360º radially as the exhaust passes through the diffuser discs. The number of discs used determines how much total sound energy will be absorbed and diffused.
An interesting tidbit of related information is that the human ear is more sensitive to higher frequencies; this means that we perceive higher frequencies sounds as louder. So now you know — focus on reducing these higher frequencies to stay on friendly terms with your neighbors! Or not.
Another tidbit of information and popular misconception: “The more free-flowing an exhaust system is, the more power it will make”. Not quite. Some backpressure (2-3 PSI) is necessary to achieve maximum power and best usable powerband. Removing the muffler altogether thus is not the greatest idea. Supertrapp suggests to start with 6-8 disks and then add or remove disks according to tuning needs. Our muffler includes 8 disks.
New packing material from FMF is fitted around the center core.
This is it for the engine parts – everything is ready for prime time. We’ll now focus our attention on the frame and suspension elements.