Exhaust

Hi Jim

While it may seem cool to make a lot of noise a bit of back pressure is not all bad.  it helps maintain some pressure to smooth out power band of the engine.  While it indeed will make perhaps more peak HP by reduced back pressure it will alter the powerband and can change the output at other ranges (less torque)  Given that we must pull the prop from zero to full without the ability to shift gears we are depending on the engine to have a wide fat band of torque to do this.  Peak power is really only good if you want to go to inflight or constant speed propellers.  Fixed pitch is very dependent on a wide powerband.  

Rotax publishes the max back pressure allowable in the installation manual.  Having a nice free flowing exhaust is fine, but some back pressure will help your midrange in my experience.  

Cheers