Spark plugs are all about conducting electricity between a gap to ignite the air/fuel mixture.
The better spark you get, the more complete burn you get, and the more power you get out of the fuel you're using (and less emissions, too).
To get a better spark, you need to use a good conductor.
Copper is one of the best conductors available for spark plugs, and is dirt cheap, though it has pretty short life span.
Nickel and platinum aren't that great, and are only used by manufacturers because of their extended life span.
Because of the metal in the electrode, there is fewer volts required to create a spark, and so for the same amount of power, you should get a hotter spark. A simple copper electrode cannot possibly hope to do that.
Iridium is the best of both worlds. Even though iridium itself isn't the best conductor, it's extremely hard. It'll last the longest of any spark plug tip material (some are rated up to 100K). Being so hard, the tip can be made to a fraction of the size of a normal one. This creates a very concentrated spark in comparison, which easily negates the lower conductivity of the metal. Iridiums are known to ignite lean mixtures that other plugs can't, which would definitely
help fuel efficiency.
If your ignition system is in good shapes (wires, coils), I don't think a plug alone will make a
noticeable difference in miles-per-gallon (MPG).
I think the only difference you'll see between the two is in how long they last.
Platinum and Iridium plugs are mainly there because they last longer, not because of higher performance.
In SOHC engine, I saw an increase when going from copper to Platinum.
In DOHC engines and motorcycle/scooter engines, I have seen an increase by going to NGK Iridium plugs.
In the bikes the electrode doesn't dirty because of how small it is, each spark cleans off the end so they run alot better than standard large electrode plugs.
Service life of some OEM Iridium spark plugs can be as much as 120000 miles with very little degrading of performance in that lifespan. NGK Iridium IX types have a centre electrode tipped with Iridium alloy with a 0.6mm diameter. NGK believe this gives the optimum balance between performance and service life. Service life estimates can be up to 60000 miles for normally
aspirated car engines and up to 20000 miles for motorcycles - this is extremely subjective though, a high revving motorcycle or extremely modified car engine will most likely wear plugs out in a few thousand miles.
I am not promoting any brand here. Just sharing my findings...
