Here's some interesting concepts on how to make drag pipes work with "Thunder monsters" from Big City Thunder. Back pressure or the lack of it is what causes the dip in torque on harley motors and here's a way to regain some of that lost power.

http://www.bigcitythunder.com/pages/product-fit.html


http://i149.photobucket.com/albums/s45/bills985/Dragpipesfix.jpg