This discussion of heavy walled manifolds and the feeling that they do some muffling is interesting. Some of you may remember Andy Venable and his plugs up Christavia. I bought some stuff from him years ago. He used a stock manifold with short pipe shown in the photo and stated he was no louder than a Lycoming. I've got away from my rotary interest in favour of lighter weight stuff but had planned to put one of these on my 13B Bakeng Duce (which is still available: engine or airframe or both). The smaller one is empty inside but the larger one has baffles on the input (one is sitting beside it) and the ourput ( a large round disk). I think the smaller one is 12 lbs but the larger one was quite a bit more.
Peter