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I am glad someone has decided to have a go at the stock muffler. I think about it every time I open the shed and step over the damn thing. Since I have the replacement installed, there is no burning need to get on it. From the photos of the inside, it appears that improvement will require experimentation. Flow improvement should be as important as increased sound level. Mechanicalmagic, if you haven't already welded yours up, you may want to consider the following:
The primary chamber with packing may impact sound levels only and the latter stages may yield better results. What I envision is something like this:
1) Design some sort of inexpensive flow meter to gauge the restriction and changes. A fairly high volume air sourch is needed. You may have or can borrow a dust collection blower--1hp motor connected to a fan with a 4" outlet. Other possibilities are a leaf blower, A/C blower, or a compressor with a large hose. Neck down to outlet to the size of the muffler inlet. Add a vertical clear plastic tube just before the muffler. The tube dia. should be just large enough for a ping pong ball to easily move inside and be longer than thought necessary to allow fine tuning. The size of the inlet hole and if necessary an outlet hole at the top will be determined by how the system reacts.
2) Take a reading with the stock muffler.
3) Cut and modify the innards.
4) Replace the cut panels and seal with duct tape, 3M putty tape, or whatever you have that works.
5) Take readings, reopen and try different mods, and compare results.
6) If possible, try the modded muffler on the car before welding it up. Use hi temp muffler patch tape, hi temp silicone, or some combination of things. If you warm the car up before switching and only run it long enough afterward to get a good sound recording, you may be able to get away with materials that don't do well at high temps. When satisfied, weld it.
There are some unknowns that might make the whole effort worthless. Will the air source produce enough flow? Will the tube/ball be sensitive enough to indicate the differences? I am guessing that commonly available mechanical pressure gauges and shielding gas flow gauges are not sensitive at the pressures involved. Perhaps a flow bench will work but who has access to one?
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