dualing hp
 

is there any dyno-able difference in the hp of true duals (x or h pipe) vs y-pipe?

i think it more depends on the muffler attached to the Ypipe that can cause the difference, and the type of engine it is behind.

on stock or near stock engine, an open ypipe would probably only pull 5hp less than true duals. this would be more pronounced on a heads/cam or boosted car.

Yeah, the wilder you go with heads and cam the more difference you will notice or if you go FI you might notice a difference because you will be trying to force more air out of a smaller hole.