Tom Keith (aka "Knobbylips") has come up with a modification for stock late model EXC mufflers that looked very promising to me. He has posted several messages about it at KTMTalk, and when I expressed interest, he offered to do my muffler for a "pre-production" test. Here is a link to that thread:
Here is what he will do when you ship him your muffler:
Trim a portion off the front of the spark arrestor. This does nothing to impair it's function.
Re-drill the can and move the center bulkhead rearward that same distance.
Cut the core and modify it. See the above pic.
Reassemble with Silent Sport Premium muffler packing and ship back to you.
Optional: install a stainless down-turned 45 degree elbow on the rear cap. Re-plating, if desired, will be up to the owner.
Cost is $100 for the basic mod, and $125 to include the elbow. This includes a $25 repack with premium packing and return ground shipping.
I didn't especially want the elbow, since I didn't like the look and was concerned about damage to both the muffler and myself in the event of a fall, so I had him leave off that part. He turned the muffler around very quickly and I had it back within 4 days of his receiving it (shipped from CA to WA).
With nearly a full pound of packing due to the added internal volume, I expected the muffler to be quieter than stock. I am a little bit disappointed in the results, but only because I may have been overly optimistic. The bike is noticeably quieter at all power levels, with a good reduction in "bark" when you whack open the throttle. I can't detect any negative effects on the power delivery (but this bike has so much more power than I can use, it isn't really a factor for me). I think the elbow is the key to this mod, and I hope Tom will do some testing using a sound meter on a modified muffler without the elbow, and then with the elbow. If I could get another 3 dB sound reduction, I may send him my end cap for that additional mod.
I am glad I did this, and I hope Tom is successful in his endeavor. Less Sound, More Ground.
If you are a KTMTalk member (and you should be), you can contact Tom by sending him a Personal Message at:
Knobbylips Contact Info (Members must be logged in)
Muffler 101:
Silent Sport Product Information (pdf) Note: 03/03/08: I changed this link to the stored file on my server, since the original file seems to have vanished from Silent Sport's Home Page.
It's VERY important to use the
correct amount of packing - this pic shows the required amount for this muffler. For a 4-stroke, Silent Sport recommends 220 grams per
liter of can volume. The above article shows exactly how to measure,
calculate and then pack the can. Here is the calculation section:
With the dimensions of the modified can, I came up with a fill weight of 430 grams. That's convenient, because you can buy the packing in 450 gram bags, eliminating the need for a digital postal scale to weigh the stuff.
I used a plate and postal scale to measure out the
bulk packing I had on hand - I just tallied the weights until I reached my 1
pound fill goal.
Here is one source:
Area P :: No Limits - Silent Sport page
It's important to tamp down the packing tightly, but still allow a little "loft". The packing should have about an inch of spring -back when done, before installing and riveting the muffler base. It will expand to fill the residual space in the base cap when you start the engine.
To ensure correct tightness, you can weigh out 25% of the packing (about 110 grams in this case) and tamp it down to fill a quarter of the available space in the can, checking with a tape measure. Repeat for each quarter of the packing until the can is nearly full (less an inch when pressed down with the tamping tool). A large wooden paint stir-stick works very well for tamping. Throughout this process, hold the rear mount of the can securely clamped in a padded bench vise, with the can in a vertical position.
Silent Sport also makes some important points:
The muffler will get quieter after the first run.
A warm muffler will be as much as 1.5 dB quieter than a cold one.
A muffler with an inadequate amount of packing will be significantly louder than one with the correct (calculated) amount. Somewhere I have seen a graph demonstrating this - maybe someone can send me a link or email me the image?
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