We have tested about every brand of spark plug on the market over the last 20 or so years (engine oils, too).
Stitt, Autolite, Bosch, NGK, and who knows what else.
Bottom line is we always come back to Champion. Yes, we occasionally get a couple bad ones. It really hurts when some of our plugs are around $100 a pop.
I've only run Champion in my personal stuff.
We run a bunch of industrial engines (several dozens) that run 24/7/365. We have run Fords, Chevy, Hercules, Cat, even Mazda rotaries in the small stuff. Cat and Waukesha in the bigger high and medium-speed world, and Cooper, Clark, and Superiors. Needless to say we buy hundreds of spark plugs a year.
PS - You should see the exhaust pipe on a full load turbocharged Mazda 13B after it's been up to temp for an hour or so. You can see all the colors of hot steel as it progresses from near white/orange, to orange to red to dull red. Pretty cool. They suck fuel like a pig but they make gobs of power for a tiny little thing
