sounds like you have a start to a very nice little setup. there is most definetly alot to blower motors. to answer your question on the underdrive that means that you are basically not using the full potential of the blower.And how you do this is by changing pully ratios like that of a bicycle sprockets and gears. to be 13% under driven you are likely running lower boost and would result in less lower end. with it being under driven that means the blower is spinning slower. but you better know that bottom end before you start cranking up the boost. and be ready to run race gas.