1. There are a few different ones.
2. Depends where and what you buy. Doesnīt come cheap. Someone has to install it too.
3. KVG (Ask the f.cking Google). Hint http://www.newspeedmaster.com/ or http://www.bonnevilleperformance.com/
4. Some (the few that have it) say itīs well worth it. I`m thinking of buying one when I feel like the motor needs big enough service. Not that I have money for one now anyway.