I'll throw in on this discussion. I have an 07 Hammer with a 2into1 exhaust, K&N air filter and Stage 1 map and it runs fine but it is rich, with black soot on the inside of the silencer and the plugs are dry medium to dark brown. Note that I live in a small town with short hops so my situation is pretty unique. In my opinion, if you want to make power on a mostly stock bike, consider a 2 into 1 exhaust. I am of the opinion rich is way better than lean, but I am old 2smoke guy...