the spit back can be used to build some extra hp by utilizing whats called a hemholz resonator my tests have shown up to a 1200 rpm increase. i dont have a dyno but this also equates to a tork gain also. these have to be adjusted to case volume, port timing and rpm range. the example shown below is for a boat engine and was made from a simple plastic container. the shape isnt important other than you dont want it to flex or "oil can" one was made from a stock aircleaner housing and one other fabricated from a small extinguisher container