Here is an article about spark plugs. hope this makes things make sense. I got this from Cars Direct
"Many people come to the point where they are forced to change the spark plugs on their vehicle and face a difficult decision; copper spark plugs or those of another type? You can find spark plugs in a variety of different metal combinations, including aluminum spark plugs, iridium plugs, platinum and more. Copper core spark plugs are still the most commonly seen, however, and many people agree that they are the best. However, the advertising that accompanies the different types of spark plugs is potentially misleading, so it's important to recognize the real difference between the spark plugs in question. In order to do that, it's necessary to first learn about what the metal in the spark plug is used for.
Purpose of the Metal
The metal in a spark plug serves a single basic purpose: to channel the electric energy from the plug wire through the spark plug so that it can be forced to the engine block in the form of a spark. Therefore, any metal that conducts electricity at all could potentially be used for a spark plug. The metal should also not get too hot; one of the main problems with some metals is that they overheat quickly, causing the electric charge itself to be compromised and the spark plug to not operate as smoothly.
Copper Spark Plugs
Copper spark plugs are generally considered to have the best performance of any spark plug type. This is potentially different from what advertising companies suggest, but the other metals are, unfortunately, not as conductive in general as copper is. Platinum and iridium plugs are more likely to overheat, which causes damage to the plug components and can compromise the delivery of the spark to the engine block.
Platinum and Iridium Plugs
Platinum and Iridium plugs perform at a lower level than copper spark plugs, because they are less conductive and they tend to overheat. However, the overall longevity of these two types of metal is better than copper plugs. In reality, copper has the best performance of all three and the worst longevity. Platinum has good longevity and the worst performance. Iridium has good longevity and a performance that is decent, which is why iridium plugs tend to be more expensive than any other type. Still, the difference between these plugs in terms of overall quality is minimal, as there is a trade off for each.
Most copper plugs need to be changed every 20,000 miles or so. Platinum and iridium plugs can often go for twice that before they require changing, but the overall performance will not be as good and you may have to deal with overheating of the plugs. This is potentially not worth the added cost of both platinum and iridium spark plugs, although the decision will depend upon your preference.
If you have any other questions about spark plugs and which are right for your vehicle, consult with a mechanic for more advice."
I give full credit to cars direct for this story and not claiming that I said any of this for copy right issues.
Different engines run at different internal temps when they are going though there cumbustion cycle. The plug goes with the motor and it is always best to use the plug style that came with the motor. The 100k irdiums wont last as long in a V6 becasue they wont fire as hot. I know there are people who know everything on here that will dispute this info so have fun with your theories.
