I agree with the last post. I use an ASP 40 on my very heavy telemaster 70 and it flies just fine at 5280 ft. I own 3 "fun fly" 40 size planes with 40s in them and have not had a problem. I also own a cap 232 40 size with an os 65LA and it has too much power so going too far is easily possible even at higher altitudes. A rule of thumb at the club is to go one step up from the engine you plan on using. i.e. 40 to 46, 46 to 52, etc. I kinda wish I would have put in the 46 size engines in my fun fly planes for a little more performance, but they still fly fine with 40s. Also, I use a 15-18 blend on fuel mixed by guys at my club.

Dave