Since the inside diameter or the core is only about 1.4” (actually only about 1” in way of the louvers), inserting some sort of baffle inside the core will dramatically restrict flow.  My goal is to retain good flow while reducing noise.  The diameter of the muffler body is also small, and it’s tapered.  There’s not a lot of room to incorporate a free-flowing baffle.  The only option seems to be surrounding the core with wool packing to absorb the acoustic energy.

The mufflers are welded together, and they are manufactured from heavy-gauge steel.  Any mods will require cutting the muffler open.  Cutting requires touching up the exterior coating, so I chose the Black Mac as the most likely candidate for surgery.  I can touch up the cuts with VHT Flame Proof Paint.

I had never run the motorcycle with the Black Mac installed, nor had I flow tested the Black Mac, so I needed to do some baseline tests.  On the test bench, I immediately saw that the chrome Mac and the Black Mac are different animals.  While the chrome Mac pulls 32-1/2” H2O on the bench, the Black Mac only pulls 26” H2O.  I repeated the tests over & over.  Same results.  Why is the Black Mac less restrictive?  I thought they were identical.

I set them side by side.  The Black Mac is about one-inch shorter.  Hmmmm?  I would find out why the Black Mac flowed better when I cut it open.

Cutting up the muffler requires a lot of crude work.  I didn’t want to damage the flame-spray coating, so I needed a way to hold the work piece without marring the finish.  A piece of aluminum scrap served the purpose.  Once bolted to the muffler, you simply grab the aluminum in a vice.  Solid as a rock.

And then lobbed off the reverse cone with a hacksaw.

We’re talkin a BIG hole saw, like 2-1/2”.  It’s gonna want to bite and walk.  A metal plug with a drive-fit was in order.  Set it up with a ¼” pilot hole in the center and drive it into the exit pipe.  It must be tight.

These Ryobi drills are powerful.  When that saw bites, it’s gonna try and rip your hands off.  Make sure the muffler is secure in the vice.  You don’t wanna see it flying across the garage.

You will need to make an end cap to hold everything in place when you put it back together.  I cut one out of a piece of 3/16” steel plate.  The OD is 3” and ID is 1.62”.  There are four notches to accommodate screws.  I used 10-32 stainless steel screws on mine.

I used this drill extension with a carbide burr and a grinding stone.  The burr to plunge through the weld, and the stone to smooth off the remnant after the core was out of the way.  I turned them with a drill.  A high-speed grinder runs too fast; you have no control.