It turns out the Black Mac has a larger diameter core.  It’s not evident because the core is stepped.  The part that sticks out is the same size as the chrome Mac, but the long internal section, the section with all the louvers, is much larger.  Good for flow, but how about noise?

A quick test on the bike and I had the answer.  It sounds a bit louder, but the frequency of the noise is much lower than the chrome Mac.  It sounds a lot better, but it’s still way too loud, in the same league as the C-Bomb.

73/82/86 dB

I was able to successfully cut it open and pack it with ceramic wool.  That reduced the noise a bit.  I can ride it around the neighborhood if I take it easy on the throttle.  It’s pretty quiet when its idling.  Still too loud for me but I’m sure it would be acceptable to some of you.  Seems the packing lobbed off about 3 dB and mellowed out the frequency a bunch.  It sounds good, but loud.

70/80/83 dB

It runs great.  With the Black mac installed it pulls the 3rd gear 4K to 7K in 6.72 seconds.  There’s a lot of steam.  Not as speedy as the C-Bomb, but at least you can ride it around the neighborhood.

I will be doing a post to give all the details on cutting it open and filling it with packing.  For those of you that purchased the Mac System from Dennis Kirk, if it’s the Black Mac, the one with the oversize flange, you probably have the same muffler.  If you have the smaller core, the mods I did will probably work good on that one too, it just won’t be as fast.  

To satisfy my curiosity, I threw the modified stock muffler on so I could measure its noise output.  It was the quietest unit at idle, but the C-Bomb with baffle and the LCGP with baffle were quieter at 3000 & 4000 rpm.  That surprised me.

63/80/82 dB

So I will forge ahead with my plan to install a larger diameter baffle in the LCGP and see if I can get the flow closer too or better than the Mac.  More to come.

Edub, I call the pieces at each end of the baffle tube "adapter bushings".  I'm not sure that is the correct terminology but I like it.  They keep the baffle centralized and seal the baffle tube OD to the muffler core ID.  They can be made from any suitable material that can take the heat (not aluminum, found that out the hard way).  I refer to the mid-baffle diverter as a "plug".  Its simply a solid piece of metal.  I lock it in place with a bolt that goes all the way through the baffle tube & plug.  I will be doing a post on constructing the LCGP HiFlow Quiet Muffler.  The techniques would be the same for the Cherry Bomb.

I finished the LCGP with the larger baffle tube.  It worked great.  All the tables I previously posted in this thread have been updated.  The 1.75" baffle tube is a lot less restrictive.  It runs significantly faster which indicates more horsepower.  It is also still very quiet and has a very satisfying, low frequency exhaust note.  

While testing the new LCGP, I noticed I was using a lot less fuel.  Fuel mileage is not at the top of my priority list but it was such a dramtic difference that I was compelled to accurately check it.  Prior to the HiFlow muffler the best mileage I had achieved with this engine was 59 mpg (during the carburetor shootout).  Now it gets 63 mpg.  That sruprised me so I checked it several more times.  It gets an honest 63 mpg riding in a normal fashion.  That means not  trying to milk every last mpg I can.  Just rode it.  I think that's pretty darned good considering the bump stick I'm runnin.

Hi Mike, always look forward to reading of your experiences, I also spent a bit of time modifying mufflers for both looks & sound on the S40 (mostly with unsuccessfull sound reduction) with a 17 inch reverse cone type muffler with a removable cone. One thing which did seem to take the sharp crack out of the note was a piece of curved pipe fitted at the exit of the muffler curving up & pointing the sound down more towards the ground than straight out. I got the idea from a guy who had them fitted to his 850 Norton Which just seemed to whisper along. Thanks for all your very helpfull input.