basically they are lighter weight for reduced rotational mass...and sized to turn accessory pulleys fewer revolutions than stock (% reduction). Slower turning accessories use less hp to turn. Thus the engine has less parasitic drag...freeing up hp.
well... the three piece and two piece are out for me.. .maybe just the crank pulley would work for my needs. I don't mind the lack of power so much as the God aweful stumbling when the A/C is on... It is just plain miserable to drive with the A/C on to me. I actually prefer to drive my 280,000 mile ranger than the focus when the A/C is on....
the 1 pcs replaces just the crank pulley and u will usually need a different sized belt
the 2 pcs replaces the crank and the water pump...you may or may not need a new belt...the fc 2 pcs uses the stock size belt, but the...af/x (i think) uses a smaller belt
the 3 pcs replaces the crank, water pump, and the a/c...i believe u need a new belt for that
personally, i have the fc 2 pcs, and i havent noticed any real problems...no flickering lights, stumbling at idle, etc...also, i refuse to use the a/c on the basis that im cold 7-8 months of the year so i will deal w warmer weather...also, my idle was really bad before, then after they were installed (cams and gears as well) the idle was much better