bla bla bla.. all i know is my car runs better with the tti's then it did with the headmans... different combos require different headders.. big deal.. i do know i have yet to scrape my tti's on the street, yet to burn a plug wire on #5 or #7 cyl., can get #5 and #7 plugs out with eas even when the motor is hot. can get my starter out with no problem and don't hvae aby leaks.... so i'll keep my tti's.