Hey guys!!!! Stop Bashing the os 91fx!!!!! I have one and after a little work and setting it up properlly it runs great. If your using any other type of muffler besides the stock muffler .....MAKE SURE YOU HAVE THE RIGHT AMOUNT OF BACK PRESSURE OR SHE WILL RUN LEAN AND YOU WILL DEAD STICK!!!!!!. p-51 STANG.14-6 PROP 15%NITRO,20% OIL....SLIMLINE PITTS MUFF. iF YOU LOOK AT THE SLIMLINE PITTS MUFF FOR THE 91FX CAREFULLY YOU WILL NOTICE THE OUTLETS ARE ONLY ABOUT 1/4'' INSIDE DIAMETER. tHIS IS FOR A REASON....BACK PRESSURE.....CK IT OUT..tONY gONZO