"UNDERDRIVE" is a definition in itself. It "underdrives" your accessories that is run by associated pulleys. To a certain point that is... e.g. your alternator. The max voltage/amps would be reached at 2000 RPMs. Where as if one had a UD pulley, it may take 2500-3000 RPMS for maximum charging. This mod would not be recommended for people with high amp stereos.