Who makes them and do they work well ?

gbb exhaust technologies makes inserts and are real good to deal whit respond in a few hrs on email ,they claim no power loss and have some good rewiews .

build customs for me and I lost some rpm s but they take the noise down

I've got a pair of gaffrigs, air actuated for sale, I don't need them, no one bothers you around my area

Thatz a good thing