These Tower mufflers are semi tuned in a way. They have greater internal volume and allows the engine to breathe out more freely. In a test I did the Tower .75 muffler held 222 grams of water and my stock 91 FX muffler held 185 grams of water (could be a little off here, it is a long time ago, but the Tower muffler is definetly bigger)

They are also built very light due to very thin materials, quite opposite of the stock OS muffler which feels like a brick. The stock muffler is very restricted in the outlet and before getting the Tower muffler I just drilled out the stinger to the max and removed the baffle inside, that helped alot in the power departement