It depends on a lot of things. Generally you stay the same or gain a little. A lot depends on thepitts muffler volumn and this is not consistent from on engine to another of the same displacement even for the same manufactor. Generally these mufflers are louder than the OEM's muffler and often have lower back pressure. Some times because of the lower back pressure you have to partially plug one of the outlets and of course this often reduces the power to below even the OEM's muffler. But again in my experience you will stay the same or gain a little. This assumes that the engine is not an engine that comes with a tuned muffler. i.e. Rossi, MVVS etc. In that case you will loose power going to a pitts style.

Denis