Back in September, I did a report on the 97mm engine I built.  The engine ran well but it had a nagging oil leak.  I patched up the leak just to allow me to finish testing.  At the conclusion of the test, I decided to throw a Mac exhaust system on just to see if it made any difference.  To my amazement, that exhaust system made a measurable difference in performance.   The Mac system lobbed about 0.23 seconds off my 3rd gear 4K to 7K time.  Unfortunately, it was way too loud.

I just couldn’t leave horsepower on the table.   I had to find a way to reduce the exhaust restriction without raising the noise to rock concert proportions.

I had been running a Mac 1.79” ID header pipe for a long time.  With that Mac header and a modified stock muffler, my 94mm engine made 41 HP (11/19/20 The tight quench engine with 10.4:1 CR, not the pop-top engine with 9.2:1 CR.  I've never dynoed the pop-top engine.).  I thought the exhaust system was good.  It made lots of power and it was quiet.  But the switch to the full Mac system on the 97mm engine proved that the modified stock muffler wasn’t cutting it.  With the 97mm engine on the workbench for repairs, and the 94mm engine in the bike for learning projects, it was time for some muffler testing.

All of the mufflers were installed on a Mac 1.79” ID header pipe.  I realize that a 1.62” ID header would be a better choice, but the Mac header is off-the-shelf.   So, no messing around with muffler shops trying to get a custom header bent, flange welded on, fitup issues, back & forth, extreme heat coatings, etc., etc.

The Mac header provides an ample cross section to support significantly more exhaust flow than the stock 1.30” ID header.

The Mac header also lends itself to installation of almost any aftermarket motorcycle muffler.  That’s because the pipe runs parallel to the fore/aft centerline of the frame.  See how the Mac pipe runs parallel to the stock muffler bracket.  Changing to a different muffler is a simple bolt on proposition.

I used my poor man’s flow bench as a test stand to measure flow resistance.  I simply pulled on each muffler with two full-size shop vacs (that’s about 10 HP worth of vacuum potential) and measured the depression with a simple U-Tube manometer.  This picture shows a muffler getting the chrome sucked off.

The five six seven mufflers I tested were the stock muffler, a Harley Davidson Dyna muffler, a Mac chrome muffler, a Mac black muffler, a Thrush 1.75” Cherry Bomb glass-pack, and a LCGP 2.50” glass-pack with 1.5" baffle and a LCGP 2.50" glass-pack with 1.75" baffle.

Here you see the modified stock LS650 muffler hangin off a Mac 1.79” ID header pipe.  This setup has served me well.  It makes good power and is quiet.

At 1” ID, the exit tube is also significantly larger.  It transfers the hot gas from the center chamber in the muffler to the atmosphere.  These mods got the stock muffler flowing almost as good as the Dyna.  Close but no cigar.

The Harley muffler is typical Big Twin hardware.  They usually get replaced about one-week after the customer picks up their new motorcycle.  This one has a catalyst.  Excellent quality and it looks good.

This is the catalyst on the inlet side of the Dyna muffler.  If you have a Harley muffler and you see something like this in the inlet, expect it to run major hot (possibly glowing red).  Our carbureted engine simply runs too rich, which causes the catalyst to overheat.  It seems to be a common issue.

Installed on the motorcycle the Dyna looks just right.  It has the classic cruiser look.  With a standard exhaust adapter, it fits up to the Mac header perfectly and aligns with the exhaust mount so that one bolt can be used to attach the muffler to the mount.  No welding, no bending, just bolt it on.