Best plugs available, period.
I read a review in which someone complained about these plugs and called them "a waste of money". This person obviously has serious engine problems that it wouldn't matter what spark plugs used, as his problem is not related to or a result of using these.
I am a professional mechanic, and I specialize in Jeeps, mostly the 4.0 or 2.5 Cherokee. I put these in a 93 Cherokee with a 2.5 four cylinder, and my dad also purchased a set for his own Cherokee, a 95 4.0. From the moment we each started our engines after installing these, the improvement was immediately and significantly noticeable. The plugs I took out were in great shape, less than 5000 on them. So this is more than just some little improvement you would get simply by putting in any old new plugs because your old ones were done for. This is one of the most significant improvements I've ever seen a single upgrade make to a gasoline engine. My dad and I agree 100%, both of our Jeeps have a much smoother idle, they start quicker, throttle response is amazingly more predictable and linear. Gas mileage improvement. These are simply the best spark plugs ever designed in the history of automobiles. If anyone tells you otherwise, they are not qualified to make a statement on this subject.
My disclaimer is that I can only attest to the improvements I've observed in 4.0 and 2.5 Jeep engines. Others may respond better, not as much, or produce anything noticeable. In my opinion however, I think that this type of plug used with the correct application would produce noticeable improvements, regardless of the engine. I believe it is one of the most valuable additions if you seek to optimize your engine's performance. I was willing to give these a try, and I am very glad that I made the right decision. These are the only spark plugs I will ever use, nothing but these. Unless they make something even better in the future. But if you are reading this because you are looking for new plugs for your Jeep, these are what you want to get, for sure. I would also recommend getting a CRT, MSD, or similar higher output ignition coil, the 45,000 volt type. To complete optimization of your ignition system, also use 8mm or 8.5 mm plug wires, and your distributor cap should always have the brass terminals. And don't forget a quality rotor along with it. For all the naysayers, you need to address the actual problem your engine has, rather than complain about a set of spark plugs. If your engine is running poorly due to issues unrelated to the ignition system, it will not matter what type of spark plugs you use, none are going to fix the problem.
In summary, no other spark plug comes close to improving the performance of your engine. If you have an XJ Cherokee, Wrangler TJ or YJ, or Grand Cherokee, you must get these. They are so good that I just had to share this with others, I usually don't write reviews, but this was so profound that I felt I would be doing a disservice to fellow enthusiasts by keeping it to myself.
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