Originally Posted by kschachn
It is about heat conduction, a small tip is desirable for several reasons and that isn't achievable with a standard nickel electrode. The heat cannot be conducted away fast enough and it will melt, but this won't happen with a platinum group metal.
The one thing it is not about is electrical conductivity. The overall electrical resistance of the plug is far higher than anything going on at the tip. Besides, all the platinum group metals are excellent electrical conductors.
Heat range is all about heat conduction, which is usually related to nose insulator length. For a specific heat range, spark plug manufacturers will offer different choices in the tip material for different costs and performance.
Platinum group metals are not good in terms of electrical conductivity, or thermal conductivity either, which is why modern precious metal plugs use very little of it and alloyed with other metals to improve conductivity. This is why for modern precious metal plugs, the core is still primarily nickel alloy. The precious metals are used for hardness, for the durability, not the thermal or electrical conductivity.
I say "modern" to exclude the old Bosch Platinum plugs which they claimed had a full platinum core.
If they are excellent electrical conductors, why haven't we seen Platinum group metals replace copper when it comes to electrical wiring, circuit boards, etc?
If the EGT's are so high that you're melting spark plugs, something is wrong.