I had a 01 rm 125, stock pipe is good when it comes on the pipe the lower mid hit is just enough to make it fun, but very controlled, engine does it,s best work in the mid, has good over rev but it,s flat. The PC pipe makes it quicker and rushes threw the powerband, have to shift real quick to ride it correctly. The HGS pipe has a low end increase with a much wider and stronger mid that carrys into the topend. Bill,s pipe is a little better than the PC, Factory Suzuki used Bill,s pipes.