Is there an advantage of using "underdrive" main crank pulley??

and is there any other disadvantage beside that the alt is underdriven and needed a smaller pulley?.

 

Benefit: Underdrives components, frees a few horsepower, little better rev response.

I have a racing beat underdrive and have no problems with it. No dimming lights or poor charging problems.

Worth money? depends if you really care about 3-4 hp. I don't came with car

 

When a shop rebuilt my motor a couple yrs back, he installed an "unorthodox" racing pulley. I just realized now that it is an underdrive pulley....(i can't even see 13V at idle with my fd alt).

So there is no sense of having underdrive pulley, then replace the alt pulley with smaller pulley. Coz all gonna be the same thing again?

 

The main point of the underdrive pulley is not to free up horsepower- a minimal gain, at best- but rather, to protect the waterpump.
Since race cars spend the majority of the operating time at the high end of the rev range, the waterpump is working harder than need be if a standard pulley is installed. An underdrive pulley will spin the pump in it's optimal range- most of the time- naturally, at idle and low speeds, it's not working as much as necessary and the car will run hotter.

Like many/most real "race parts", the underdrive pulley is designed for use in a very narrow range of conditions- which is just fine (and possibly, absolutely necessary) for a track car, but can be useless and harmful to a street car which must deal with a much more varied environment.