Stock muffler mods and performance results: when installed on a modified Ryobi 31cc (reed, transfer port inlet, exhaust port) outlined in previous posts in this thread. Tests were done using an APC 18x8W prop.

Test 1) 7415 rpm 14.3 lbs thrust Stock muffler with inlet screen removed.
Test 2) 7630 rpm 15.0 lbs thrust Stock muffler with inlet screen removed and outlet pipe replaced with 5/8" ID pipe.
Test 3) 7680 rpm 15.1 lbs thrust Stock muffler fully gutted with 0.8" ID pipe.

Somewhat surprisingly there was only a very slight improvement between tests 2 and 3. The baffle in the stock muffler appears to be very restrictive and I would have thought that removing it and installing a big outlet pipe would have given a larger effect than it did. This is consistent with what combatpilot found when he covered one of the two outlet pipes on his aftermarket muffler and found no change in performance. I guess the operation of the two stroke engine ain't that simple, as Gordon Jennings professes in his book.

The first photo shows the gutted muffler and all the parts. The baffle and the small outlet pipe (in the upper right corner of the photo), both are internal in the stock muffler, and are discarded. I made the 0.8" id outlet pipe from a thin wall metal (steel not aluminum) broom handle. The hole sizes and spacing of the drillings in the outlet pipe were arbitrary, but plenty greater in total area than that of the pipe. I opened up the outlet hole in the muffler case to fit the larger outlet pipe and braised it in (second photo). When refitting the two halves of the muffler be sure to shorten the bolt standoffs a little to account for the thickness of the baffle which is not present, otherwise the halves will not fully seat together. It was about .030" in my case. I reassembled the halves using RTV to seal the joint and bent the flange back to secure the halves.

In conclusion; all of the work required to gut the stock muffler has not enough pay-off to make it worth while. You can get nearly the same benefit by just cutting out the stock outlet pipe and replacing it with a bigger pipe without having to take apart the muffler.

The gutted muffler is very loud too. Although I haven't measured the noise level of the muffler as configured in test 2 for comparison, the fully gutted one as tested in test 3 measured 104 db at 10 ft distance. Admittedly this was in a test stand resting on saw horses on a concrete slab and no more than 20 feet away from a brick wall. It may have been a little quieter installed in an airplane on a grass field. The AMA guidelines are 96 db for an airplane sitting on a concrete runway and 94 for a grass field.

diceco