ORIGINAL: Sport_Pilot
They say the Fox Miracle™ is a bit colder, but its lower price is the only reason people buy it.
I buy it because it is a better plug than the OS F for moderate high nitro fuel such as 15%. The OS F is only better with 5% fuel, but that may be different on other brands of engines. I assume it is only lower in price because Fox doesn't chrome plate it, that has nothing to do with its performance.
My experience with the Miracle plug is somewhat different to that.
I threw one in a TT46Pro when the OS#8 failed.
It was obvious that the Miracle was a *lot* colder than an OS #8 because the engine really burbled in the mid-range and if you tried to lean it out it would just quit. Put another OS#8 in and it was good as gold.
In my 4-strokes, the Miracles don't give anywhere near the same idle as an OS-F. Once again, I expect it's the colder design of the Miracle that is responsible for this.
I did find that the Miracle worked as good as anything in my ST90 but it only lasted about two months before it was starting to act "cold" in that engine too which I suspect is due to a reduction in catalytic effect.
I'm wondering of the Miracle is cheaper because there's less platinum in the filament which also makes it a cooler plug.