Four strokes will not benifit significantly power wise from exhaust systems. While technically an exhaust can be tuned for a 4S to help scavange exhaust, the power boost is nothing like what is seen with 2S tuned pipes. So, basically any exhaust systems on the YS91 will reduce power becasue it will add restriction. Pipes are used on 4S engines mainly to reduce noise, not to boost power like a 2S. For this reason, most 4S pipes are designed for pattern competition where there are sound restrictions. Of course, there is nothing stopping anyone from using these quiet pipe setups on a sport plane. A Typical RPM drop from a 4S pipe would be a 100-300 RPM drop depending upon the system. This is noticable, but not critical on most planes. As mentioned in previous posts, most power and cheapest system would be straight pipe, or even just the header. This is loud compared to no muffler, but not obnoxious IMO. I have run just headers on sport planes using YS 4S's before. If you can't run a straight pipe, just use the stock muffler. It will reduce the sound by quite a bit at minimal power loss. If you want a really quiet setup, get a 90 sized 4S pipe.