This sketch represents the internal structure of the ETA muffler.
Exhaust gases enter from the engine into the area surrounding the internal venturi tube.
Ram air enters the venturi and accelerates toward the Vena Contracta, where the pressure becomes lowest.
There is an array of .100 inch diameter holes around .20 inch downstream of the Vena Contracta where the exhaust gases are drawn in by the low pressure.
The combined exhaust gases and ram air exit at the rear.
I tested this system on my Icarus Junior with the ETA 29 as furnished, with the following results (All tests were done on the same day, calm wind, around 75 degrees ambient):
1) I did not have a sound meter but it was definitely quieter, both on the ground and in the air.
2) Running Brodak 10/23-50/50 fuel and a 10-6 Thunder Tiger prop, the maximum RPM I could get was around 10,600 on the
ground with no muffler.
3) With the muffler, the max RPM increased to around 10,800.
4) For flight testing, the needle was backed out by four clicks from max.
5) With the muffler, the lap time decreased from 3.8 second to 3.5 seconds.
6) Flight time decreased by around twenty seconds with the muffler installed.
The engine is completely stock except for the installation of an ENYA needle valve assembly.
I welcome and suggestions or comments.
Bob Z.