Jeff: You are right but aren't the VF's required to use a pipe? A 10X6 on a 46FX on 15% should turn around 14000 give or take a few RPMs. If you piped the engine and ran a 9X8 you would likely see a nice increase in RPM. Do you think running a 9X8 on a stock 46FX would be suitably worthwhile? As a means of comparison there are a few guys I fly with - we all have either Hobbico Vipers or Scats or similar older style Q500 that we race around the field with. I run a 32SX with a 9X6, others run 32F's with a 9X6, 40FX with a 10X5 and 46 FX with a 10X6. The 32 size planes move around quickly but the 40 size are a bit faster - all are stock muffled. However we are all old fart types (50+) so these speeds are fine for us. I guess if Fastsky wants more poop then a Perfomance Specialties muffler or the equivalent Jett version will add another 1500 RPM to his FX engines.