Underdrives slow down the alternator, power steering pump, A/C compressor, and the water pump. Translated, the belt spins slower. This is not noticable at about 1000 RPM's and up and frees up horsepower when you stomp down the pedal on the right. The down side is things are sluggish at an idle. The second downside is your case and everyone else with u pullies , the alternator will put out 14V at 1000 RPM or more, but at idle it will not , then your car is asking the battery to make up the difference. Trouble is your battery can only supply 12V. The end result is the your battery goes dead in the course of 1 year instead of 3 or 4 years trying to put out 14V, all depending on how much time your car idles. And of course the lights dim. Everything in the car is designed for 14V. 12V output as per the battery is only for starting the car, not for running a nuclear power plant. Mods are designed for racing applications. Get prepared for a battery. Ask a nascar driver how well his a/c, head lights and stereo are working.