We like to follow the general rule of HP levels, and as with any rule, there's going to be exceptions to those rules. However, in our experience, this is the breakdown for which spark plugs you should run in the Coyote 5.0L found in your Mustangs and F150s!
Typically for under 1,000 rwhp, we'd suggest an NGK Iridium (6510s), which is a heat range of 7 (one step colder than stock heat range). Brisk RR14S work as well, but don't seem to last as long as the NGK units do. One thing that we like about the Brisks over the NGKs is that they have a non-projected tip (sunken tip), which seems to be better for all out racing applications. We do check the gap a little more frequently on the NGK 6510s, as we go up in power (and tighter on gap). This is because the tighter on the gap we go, the further we'll be outside of NGK's recommended gap spec, which is only .008" from their "out of the box" gap on the plug. We've used the NGK Ruthenium 95605s in this same grouping of power levels, and they come out of the box with a slightly tighter gap than the 6510s do, so those we end up not having to check the gap as frequently. I just don't have enough time on task with testing them on the 5.0L platform to warrant saying that you should use the 95605s over the 6510s. We do, however, strongly suggest and recommend them over the 6510s in our Ecoboost customer cars, as we do have the testing completed there on that platform.
Once you start heading north of 1,000 rwhp, you really have to be careful on plug selection, as you don't want too hot of a plug, because it'll lead to pre-ignition (which is different than detonation). For these applications, we've been using Denso ITV-24s or Brisk RR12S (heat range 8). And for anything north of the 1,500 rwhp range, our good friends over at Lund Racing suggest using the Brisk RR10S. They do caution that cold starts and putting around the pits can tend to foul those out quicker, so they do get changed often, but other than that, power wise, they handle everything that they throw at them.
We hope this helps you in making a decision on which Spark Plugs you should run in your high performance Coyote 5.0L engines!
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