I have an Evolution .46 engine. I can't keep the exhaust end of the muffler tight! Prop is balanced. No matter how tight I get it it still works loose. I have a friend with the same engine and the same problem. HELP KEEP MUFFLER TIGHT!!!!

Me too with the Evo .45. I Locktite everything. I have a .46NT in the box waiting to fly next week.

Tighten it down while the muffler is hot.
If the muffler is a two piece design use high temp RTV at the joint to seal it together. If your muffler has a lock nut holding the assembly together, replace it with two standard nuts tightened against each other. In my experience the plastic or nylon part on lock nuts does not hold very well in high heat applications.
If you are using blue locktite by the way, it won't work on the muffler. Heat is how you remove parts assembled with locktite. Permatex does have a high temp formula but I can't remember the number offhand. You can get it at most good auto parts stores.

Yep, using the blue medium grade stuff. Thanks, I'll look for the hi temp. I'll check if my RTV is hi temp or not. Probably not.