ive had 2 sets of genuine ford focus rs spark plugs. The 1st set 2 of them cracked (the porcelene) so i bought a new set (one cracked) so changed that, then car had a miss fire, all plugs sparked, but turned out 2 were shite, after help from gup turned out it was brand new spark plugs which just wernt good enough, bought new ones again and is spot on, just think they are shite qualit now and ain impresses