XR500 exhaust system gets a facelift

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.

XR500-60a
Exhaust pipes as they came off the bike.  Interesting amounts of rust.

XR500-61a
A set of metal brushes, sandpaper and lots of patience gives the pipes a new life.

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.

XR500-113a
Painted header pipes.

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.

XR500-114a
Supertrapp’s disk diffuser system.

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.

XR500-85a
Disassembled and cleaned Supertrapp muffler.

XR500-111a
New packing material from FMF is fitted around the center core.

XR500-115a
Exhaust system is overhauled and ready to be mounted.

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.

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8 Responses to XR500 exhaust system gets a facelift

  1. H. Fritzer says:

    Quelle patience pour redonner vie à cette “vieille tante”. Papi et moi-même sont impressionnés et attendons avec impatience la prochaine séquence!

    PS
    Dommage que l’on ne puisse pas faire pareil pour les humains:-):-)

  2. fabien says:

    mon cher mike!

    te voila parti pour une passion qui va occuper la majeure partie de ton temps a l’horizon de ta retraite…
    je suis particulierement interessé a te voir chevaucher la bête tres prochainement.
    bon courage, et beaucoup de plaisir!

  3. Francisco Gonzalez says:

    HI mike!, greetings from colombia south america, im myself making a XR 500R custom rebuild, and it has been a pain in the …., as you can imagine there’s no pieces available in my country for this bike, so i had to import all of them from USA. Well, now im looking for the right exhaust, and i choose supertrapp, but when i look in their web page, there’s no references for the XR 500 R 1981. Which supertrapp muffler should i buy, any references?

    • Mike says:

      My understanding is that the Supertrapp muffler for the XR500 is discontinued. You’ll need to find a second hand one if you wish to run this exhaust on your XR.

      Sent from my iPhone

      • Francisco Gonzalez says:

        Thank your for the answer mike, i really appreciate your good will… Any case do you know the references from this exhaust, Inlet diameter or disk diameter measure… I tried searching for them on ebay, but there’s no information on which fits the Xr 500 81. Please excuse me for all this asking stuff and my bad english. I’ll send a photo when the work is finished, BTW.. Great (and expensive) work you got on this XR.

      • Mike says:

        I’ll be back in Colorado where the bike is in early July and will make all the measurements for you then. Ok?

  4. Craig says:

    Hi Mike,

    The Mid Pipe, was that a piece that came with the supertrapp muffler?

    • Mike says:

      The mid pipe was part of the pile of parts I bought at the start of this project. Looking at an OEM Honda pipe (via pictures), I’m pretty sure that this mid pipe is part of the Supertrapp muffler kit as the OEM Honda muffler is one piece, including what would be the mid pipe in the case of the Supertrapp.

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