Hello
Most older engines will get a lot hotter with a muffler fitted , just have to keep the load down and as Tatone states in their instructions go down one prop size ie. 10x6 to 9x6 and make sure the outlet size is large enough but without going deaf. Most fields we fly at and competitions require we use a muffler which creates some novel solutions, have seen some really nice home made tube mufflers too.
regards Gerald