https://forum.classicmotorworks.com/index.php/topic,14714.msg156993.html#msg156993Look for Arizoni's post on July 1, 2012. I followed his instructions. Here's what I saved in a document from my files from Arizoni's posts:
Harley Muffler Drill out"After riding it a bit you might want to think about drilling the internal plug to free up the breathing.
I used a 3/8 inch "Aircraft length" drill bit and drilled one hole thru the plug. These aircraft length drill bits are about a foot long so reaching the plug to drill it was no problem.
Adding the hole thru the internal plug resulted in a noticeable gain in acceleration.
I also found a Harley wrecking yard and bought a chrome heat shield for about $10. It came with stainless hose clamps that attach to the inside of it so it can be firmly clamped to the exhaust pipe without the clamps showing.
Covering up that Royal Enfield welded tab and silver paint improved the looks of the installation a lot.
The heat shield also keeps me from melting the bottom of my shoe.
https://forum.classicmotorworks.com/index.php/topic,21663.msg245508.html#msg245508https://forum.classicmotorworks.com/index.php/topic,19830.msg217958.html#msg217958I used my 3/8" aircraft length drill to poke a hole thru the baffle that separates the front and rear tubes.
Today on my ride I found that the old torque and acceleration was "almost" back to the pre-swap condition.
Still no sign of any backfiring on deceleration, even when I shift down a few gears to try to force a backfire. 
It still could use just a 'bit' more 'opening it up' so I'll look around and see if I can find a 1/4 or 5/16" diameter drill.
These "aircraft length" drill bits are about 12" long and not every store carries them.
Adding the 3/8" hole to the baffle did not add much noise to the sound at idle speed.
When cruising along at a constant speed of 45-50 mph where the old silencer was almost silent I can now hear the purr of the exhaust. Far from loud but definitely more than the OEM.
During acceleration however the loudness increased probably 20-25 percent.
Still not really loud but definitely louder than the OEM silencer while coming up to speed. 
https://forum.classicmotorworks.com/index.php/topic,14714.msg156857.htmlThe latest news on my new Harley muffler is I rode down to BOBs used Harley place and bought a slightly used heat shield. It's about 8 inches long and covers up the discolored pipe and that ugly welded on RE heat shield retainer. 
The lady at BOBs must think these things are made of Platinum and charged me $20. She said Harley parts are overpriced and a new Harley heat shield would be over $150.
At least I got one with the previous owners screw clamps still installed so I didn't have to buy those.
So far no signs of backfiring so rather than spending money for a 1/4" aircraft drill I got out my 1/8" version and added one 1/8" hole thru the center baffle.
Later this afternoon I'll take another ride and see what the effects are.
So far the total area of the two holes thru the baffle are equal to a single 0.395 hole.
My new used Harley muffler now has one 3/8" (9.5mm) and four 1/8" (3.17mm) holes thru the internal plug that separates the inlet and outlet.
All of the original low end power is restored. In fact, it might have a little more than it did with the OEM torpedo.
The sound now is a quiet heavy thump at an idle and a healthy deep throated moderate roar when accelerating hard.
IMO, none of this loudness would be enough to upset the neighbors or the law and there is no reason for wearing ear plugs at all. At least if your wearing a full face helmet.
There is no sign of backfiring when decelerating from any speed or while shifting gears.
I'm happy with the current situation so I think I'll stop frinkling with it (for now).
If you own a similar Harley muffler and want to modify it to get a similar performance and sound increase you could do as I did or you could save a few bucks by buying only one 12" long drill bit.
If you use a 1/4" (6.35mm) drill you will need to drill 3 holes.
If you use a 3/16" (4.77mm) drill you will need to drill 6 holes.
If you use a 1/8" (3.17mm) drill you will need to drill 13 holes.
I should mention again that this Harley muffler does not have a catalytic converter. It is basically a straight thru design with a centrally located plug that forces the exhaust to pass thru a number of holes in the pipe into an outer cavity. There are a smaller number of holes aft of the plug that allows the exhaust to pass from the outer cavity back into the tailpipe.
I have no idea what the internal structure of the catalytic mufflers looks like so I don't know if this modification will work with those or not."