Funny, I just signed on to search on this topic. Im running ACdelco R44T but am trying to track down a gremlin that makes my 454 break up and pop in higher rpms. Ive gone thru the carb, HEI and wires and now planned to get new plugs, (other than AC delcos). Napa counter jockey basically refused to sell me NGKs saying it was a waste of money, but said I could step up to hotter ACDelco R45T. Thoughts?