ORIGINAL: the Wasp
the reason I brought-up those heli tuned mufflers is, they are made/tuned to work at a given band and RPM,, enhance the word "tuned",, if they can be made/tuned to work at a high RPM for a heli then they can be made/tuned to work with a wider band for planes,,
now, if one was made to work on a plane I can understand how they may be too large to fit under some cowls..
In my assortment of pictures -note the ZDZ40 with the paint can behind it It WAS a tuned setup but quite fussy
My PIP muffer (shown)was also tuned and worked extremely well but operated in 8000-11000 rpm range.I abondened the whole project when the potential supplier lost interest in the low sales volume project.
Most flyers do not care to learn to match engine prop/exhaust .
It's a buy n fly world .