This shows why the Black Mac flows better than the chrome Mac.  The chrome Mac uses a core made from one long continuous tube with an inside diameter that’s about 1” (in way of the louvers).  The core for the Black Mac is two-piece.  The major portion of the core is about 1.75” OD and the inside diameter in way of the louvers is about 1.4”.  There’s a lot more room for the hot gas to flow through.

Then positioned the rod and tapped it in place.  Gave it a nice glob of the Extreme Heat putty and smoothed it off.

To provide clearance for back-filling with additional wool packing, dress down the original end cap remnant on the core.  File or grind it down until only about 1/8” remains.  Just enough for the new end cap to bear against and hold the core in place.  Red marks the area in question.  I haven’t started filing on this one yet.  Once done, it should only stick out from the core about 1/16” to 1/8” on the radius.

To keep the wool packing from migrating through the louvers, wrap the core with a layer of stainless-steel screen.

Wrap the core with the wool packing and tie the packing in place with baling wire.  One layer is all you can wrap at this point.  If you try to wrap more than one-layer, you can’t get the core back in.

Wrap the wool packing with the stainless-steel screen.  This outer layer of screen protects the packing when you slide the core back into the muffler body.

Tape off the edge of the screen on the exit side of the core.  Leave a dog-ear on the end of the tape so you can remove it later.  Insert the core into the muffler.  The tape allows you to back-fill more wool packing without snagging on the screen.