I had those plugs in mine and I know that when I booted my saf it would feel like it was kangerooing/surging unless I feathered the throttle if you get what I mean (causing me to lose a few straight liners ) I now have Champion C59's and they haven't let ME down yet and mine's running 350bhp(been told their good for 400 bhp) NOW some people will say that Champions are shit and you should use X Y or Z think it could be a case of trial and error if your car's all set up correctly then up to u m8 I speak from MY personel findings they cost about £30-£40 a set