I have the same engine in a tower trainer. With the same problem. Must be common to the evo pts engine. I put a one piece Macs muffler on mine, just for the fact of it not seperating and leaking. It also gives a slight power/rpm increase, not that I really need it on a trainer. I am not sure if it will fit on the p51 though due to the cowl and the slant of the engine.

Layne