i have the one piece UDP but have not installed it yet. from my understanding, the 1 piece is freeing power away from JUST the AC. The 2 and 3 piece ones are freeing power from the AC and i dont know what else. thing is if you use a lot of stuff in ur car but dont really use the AC then i would go with the single. if you just want proformance then i would go 2 or 3 piece