RE: noise
Bob,
Yeah, at 9 feet you are most like towards 98db with a standard sized wrap around pitts muffler (also depending on prop). You are going to have a lot of work to do to even get close to 90db at 9 feet and it is going to be a lot more then exhaust.
One of the biggest misconseptions on how to make a muffler quiet is actually doing something like the old glass pack style and drilling holes into the exhaust pipes. You will see some of our competitors do that but it actually doesn't do anything to make it quiet and if you don't do it right you will lose a fair amount of rpm, which yes will make your engine quieter but you probably will not be happy because you can possibly burn up a piston and ring in your engine. When we actually design a muffler we use a formula based on soundwave technology to tackle the problem of noise while working to keep your rpm at the manufactures rpm specs. Thats why on most of our mufflers you will see that we get some of the highest rpm numbers while still being one of the quietest mufflers.
As for adapting anything to your existing muffler the only thing that would really work would be to use the snufflers and remote mount them somewhere inside your airplane. The reason we can not just attach the snufflers directly to your existing muffler is that when we build a muffler for the snufflers we have to build it with supports for the snuffler and a stock wrap around does not have those supports. Even with that you will probably only reach the low 90's range and most people don't really have the room to remote mount the snufflers.
Realistically, you are going need to look at a new muffler and more towards a wrap around pitts muffler that has the snufflers built directly into the muffler or even better one of our large wrap arounds with snufflers built into it. When compared to a standard wrap around the large will knock off about 3db without any powerloss, the biggest downfall is the large will not fit every application because it uses a 2 1/2" diameter can instead of the standard 2" can. Those two options will get you close but you are still going to have to play with props, definitly a 3 blade prop. There are also other contributors to noise, a plane with with a build up wing will generally be a little louder then a plane with a foam sheeted wing. Make sure your covering is tight, any loose covering can contribute to noise. The other thing is carb noise, just as a classic car if you take off the air filter you will get more noise out of your engine. On some of the stuff we have tested we have seen up to 2db just from carb noise.
We are certainly more then willing to work with you and if you can provide us with some more detail about your airframe, prop and overall setup maybe we can help you figure out what will fit your specific application.
Thank you,
Kevin Young
JTEC Radiowave