I scrounged up two more mufflers for comparison.  A stock Harley muffler from a 1990 FLSTC (Heritage Classic) and a Supper Trap for a Harley Softail.  The stock Harley unit is pre Catalyst and it did not flow as well as the Dyna unit with a catalytic element.  I tested the SuperTrap with several disc combinations.

Here's a pic of the Heritage unit under test.  It pulled 31" Hg.

Look what I found in my junk pile.  I think this is a perfect chunk of tubing for Stage II.

Dave , Did you notice any difference in high speed performance between the two mufflers? Are we talking about your café bike ? Are we talking about before or after the high comp. piston and cam?

Dave , I didn't mean top speed ,I simply meant ,did you notice any difference in performance generally . did the bike seem to prefer one over the other ? ( just looking  for a " seat of the pants" opinion).

But did your mods remove the whistling sound from the stock exhaust? thats the main downside about the stock ones.

DBM, Is there a point were good flow, may become to much flow ?

For Stage I of the modifications I grabbed the low hanging fruit, the attenuation chamber & tube.  I made that decision because those components had to be removed in order to inspect the interior of the muffler and figure out the flow path, and those components were easiest to restore to the original configuration if things didn't work out as planned.


I mentioned earlier in the post that, in my opinion, the best place to make improvements was in the flow path between chamber "X" & chamber "Z".  Making those two chambers available to hot gas flow and expansion takes advantage of the muffler's large volume.  Using that large volume to store the expanding gasses and allow them to exit gradually between exhaust events keeps the pressure in the exhaust system to a minimum.  There's around 500 degrees of crank rotation between exhaust events.  A muffler with more volume can store the exhaust gas and allow it to bleed off, which prevents the back pressure from increasing.  As the engine RPM increases there is less time between exhaust events so the ability of the exhaust system to store the hot gasses becomes more important as RPMs increase.


The obvious solution was to increase the flow rate between the first and second chambers in the flow path ("X" & "Z").  There really is no practical way to make an internal improvement to that flow path.  So I decided to make an external connection between chambers "X" & "Z".  As it turns out, that was easy to do.


Here is a sketch of the improved flow path.

I attacked the muffler by first using a 1" hole saw to cut through the heat shield at the front portion of the muffler at chamber "X".  Then I used a step drill bit to make a 11/16" hole through the wall of the muffler.

Here's a pic of that portion of the mod.