I have A 01 Supercrew 5.4 with a allen S/C kit. Can I ? and will it help if I put an underdrive pulley kit on it? Will it drop the boost because the S/C is spinning slower?

I serched but could not find a thread on this.

 

no, unless you are running everything off of the single belt

this type of setup will not work with the eaton lightning / ati procharger setups

 

I am running a single belt system. so will they add power ??I'm thinking no just because the S/C is spinning slower.. dose that sound right??

 

Yes it dose sound right!
Put a smaller pulley on the blower if you want more power.

Phil