Going back to your original question on HP & pipes. Yes with no back pressure on the valves you will loose horse power . Some pipes cause quite a lot of HP loss like drag pipes while some 2 into 1 do not add hp while others build HP. I did a lot of research in this and two of the biggest names agree the Thunder Header 2 into 1 builds more power than any other exhaust system on the market. Eddie Trodda and Kendall Johnson told me this 4 yrs ago when I built up my 88 EVO motor in my 1996 Ultra Classic. The cherry on top was my 2 into 1 Thunder header. I am Dynoing 98 Hp as of 6 months ago when I upped my injector size!