Originally Posted by MTK

You might consider a couple things to quiet down the large gassers....These will cost some dollars so be forewarned:
First, a soft rubber isolation mount.
Second, a tuned exhaust that is also soft mounted. This doesn't need to be a full blown tuned pipe; a well designed cannister will do.
Third, use a 3 or 4 bladed prop, sized according to the performance you want.

I do these as a matter of course to all my gas engines (for aerobatic models). One of the most interesting set-ups was a piped and soft mounted DLE55 turning a 3 blader 20x12 carbon prop. Incredibly quiet, almost electric like. My OS GT33 equipped aerobats are electric like in their noise footprint.

Having a great prop up front is very important. I have heard some woodies for larger scale models make so much noise they hurt the ears. All that noise comes from only one place, robbing a great deal of thrust in the process