My .02-for my bobber project we used the stock head pipes and installed thunderboltz-check out www.bigcitythunder.com -incredible sound! similar to an old midget race car with an offy-as for power, the combo of the bella corsa crIIs and the pipes gives very crisp response and my butt dyno tells me improved torque and hp-if you like the drag pipe look this is a very eco way to go-Later!