I use the Tower 75 muffler on my 91 fx. It has more volum than the stock OS muffler and also boosts power. The 91 fx muffler is long but narrow so it really don`t have as great volume as one might think. It holds about 185 grams of water and the tower 75 muffler holds 222 grams of water. Try to fill up the exhaust system you are going to use and weigh the water on a digit scale, then you`ll know if it is smaller or bigger than the stock muffler