REPOST: HRC "Pee Trap" Stealth Exhaust Mod

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  • Keyser_Zoze

Posted August 04, 2001 - 07:33 AM


Just wanted to repost this old article as I have done this mod and will cover it in the upcoming article. I still need to test the Pro-Circuit T4 with quiet core insert.

As cut & pasted from Yahoo Groups 650R board.


So late last week there were a bunch of posts about the merits of riding a quieter bike
while not sacrificing too much power. This was my goal exactly, but I wan't ready to cut
up the stock tip because I wanted something to fall back on if needed.

So, with a little help from a friend that works with auto exhaust tuning/testing, I set out to modify the hop-up tip to get it from
about 96dBa to less than 94dBa, which is the requirement that the Michigan DNR and
USFS, as well as the CCC will test to.

Here's the written description of the modification - I'm trying to weasel a digital cam totake pictures, but haven't had any luck yet.

Go to your local home improvement store and buy a 1 1/2' chromed-brass p-trap kit
(typically used for kitchen sinks). (I found one that was "heavy duty" (17 ga. wall
thickness)) Make sure that the straight section of tube with the 90 degree bend in it has at least 7 inches of straight before the bend (not counting any silly ends that you'll cut off).

Cut the end with the 90 degree turn such that it looks like an exhaust tip turn-down, and
not a sink pipe. In the end opposite the
bend, drill 3/16" holes thru the pipe spaced about an inch apart on the last 5" of pipe
(again, the 5" furthest from the 90 degree bend.) Repeat this driling process around the
pipe so that you end up with about 30 holes in that last 5" of pipe.

Take the pipe and load it into the hop-up tip (test fit). It'll fit a little sloppy, because the ID of the tip is about 1/8" larger than the OD of the pipe. Load the (drain) pipe into the jaws of your vice
with the swiss cheese end pointing straight up. Slide the hop up tip on top of it. Adjust
the position of the pipe to make the end that looks like swiss cheese is within 2" of bottoming on the end cap (the piece that goes inside the muffler and is attached to the
S/A Screen) of the hop-up tip. Make sure the pipe is centered in the hop-up tip, and
turned the right way so that drain pipe will point down when installed in the Pig. Get
everything just right, 'cuz it's time for
the modern day wonder-- J-B Weld! Give the inside of the hop up tip a squirt of brake
cleaner (or other suitable enviro-friendly
solvent), mix up a batch of J-B Weld, and smear it on the pipe where it will come into
contact with the outlet of the hop up tip, then set the tip onto the pipe and let it dry
over nite.

The result is a pretty clean looking exhaust tip with a turn-down that flows pretty close
to the unmodified hop up tip (I didn't need
to change any carb settings), tests at just under 91 dBa, cost less than $20, and took
less than 45 minutes of time to make.

Sure, there are other solutions out there (Bill, the Baja Baffle is nice (my buddy's got one on his WR-tests at 93 dBa)), but my wife won't ask about the Home Depot receipt and she'd surely notice another box of motorcycle parts from the UPS guy! And I still have the stock tip to fall back on when the CCC says I have to have a tip that says "arrestor" on it even though the DNR and USFS told me that's not a requirement!

If anyone wants more details while I'm trying to find a camera, give me a message off list
and I'll fill you in as best I can.


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