X or H?

whats the difference between an x pip an H pip (besides shape). i was lookin at the TTI site an noticed the X pipe system costs about $60 or so more. do X pipes improve power? fuel economy? or is it purly a sound thing?

for that matter, how do H pipes an X pipes change anything like power mileage or sound? never understood it

im gonna be building a "mild" 360 for my 70 siwnger an i want as much mileage i can get, hopin for low 20's with some respectable power. hopin to eventually have 3.23's but thats much later