A friend of mine is looking to replace the pipe on his AC650, who make a decent pipe that is not too loud and gives some decent power increase?

 

HMF makes a utility series pipe for the '04 650, not sure if it will fit the 05's.

 

This is just my opinion but almost every pipe I've ever heard on the vtwin has been pretty loud unless you have a quiet core insert. If your friend is noise sensitive or rides in areas that are a quiet core is a must ie hunting or public land. I have a hmf that is powder coated red to match quad made by hmf. I ride HARD and have not had any trouble from mine plus the powder coating still looks great. If you are just looking for power I would STRONGLY recomend the cdi from dynatek. The cdi is about the same price as a pipe but the power difference is not even compariabe. For just power get the dynatek cdi instead!

 

Definitely get the quiet core insert. Wait for the Velocity Devices CDI to come out before ordering the Dynatek. The VDI CDI could be better than the Dynatech. Your friend might as well get the VDI snorkel or mod the airbox. New CDI, pipe, snorkel, jetting... that'll wake it up!