Ok, there are a few of these style of articles so I will try to post one a day. Credit goes to the author Ken Lagarec and to NEO. I’m just posting this here for people to take a look at. 
Well, according to the vote, you guys wouldn’t mind having a thing Isa calls Ken’s corner. You ask and I try to answer. Obviously, I need a disclaimer here. I am not an expert and I don’t know everything. However, I have read alot and worked on a few cars so I have a basic idea of what I’m talking about. I also studied physics all my life so I could usually read up on anything related to thermodynamics, engine mechanics etc. and understand it, then relay the information in layman terms. These posts are open for discussion, so if you disagree or think you can explain something differently/better, go ahead. We might not be able to anwser everything here or have all of us understand all the intricacies of cars but I’m pretty sure this is a good way for a lot of you (and me) to learn things you’ve always wanted to know.
Our first topic comes from a man found clueless in the spark plug aisle at Canadian Tire. He was found, after many sleepless nights, scaring customers away by frantically engaging them. “What kind of plugs do you use?”. When the store closed, he was found weeping in the corner screaming out the to 15 year old aisle manager. “Why do you torture me like this?”. Here’s to you Alex (who has officially been banned from CT stores and is being sued by the boy’s parents for causing a traumatic experience).
The question was: what’s the difference between platinum and iridium spark plugs and are they worth the extra cash?
The answer, you will find here: http://www.automotivearticles.com/get.php?action=getarticle&articleid=18. But I’ll condense it for you anyway. A spark is caused by ionizing the air/fuel mixture in the cylinder by applying a large voltage (more than 10000 V) between the electrodes. The best way of generating a spark is by having very thin tips as opposed to flat surfaces. So the best design for a spark plug would be to have needle-like electrodes. Unfortunately, the spark creates a lot of heat that can actually melt the metal of the tip, thereby changing it so that it won’t work the same over time. You’d have to change it very often. So the first thing is to find a material that has a very high melting temperature. Copper melts at 1083 °C, platinum at 1772 °C and iridium at 2410 °C. This means that an iridium tip can be made much sharper without it degrading much over time compared to a copper or platinum tip. So theoretically, you can make a plug that will create a spark with less voltage or that will work with a denser air/fuel mixture (high pressure turbo). The next problem though is heat dissipation. If the tip stays very hot, it might auto-ignite some new air/fuel from the next cycle or just keep getting hotter and eventually melt. Copper dissipates heat much better (4.01 W/cm.K) than iridium (1.47 W/cm.K) and platinum (0.72 W/cm.K) so a copper plug would be best to avoid runnaway melt-down or auto-ignition. A platinum plug would not dissipate heat quickly enough and would probably auto-destruct. So a good spark plug is made of a copper core for heat dissipation, with a tip coated with platinum or iridium. The plug is also cooled by the jacket around it and it’s possible to have the same plug tip with different heat ratings (i.e 5 or 6) but I’m talking only about the tip heat conduction here.
So is it worth it to pay more for an iridium plug vs. platinum? Theoretically, an iridium plug can be made to have a better geometry (thinner tip), leading to a more consistent spark and burn, even under adverse conditions. It should also last very long (comparable to platinum). Will you gain any hp or mpg by using iridium vs platinum? I really doubt it in an NA engine because the conditions aren’t so extreme that a platinum plug won’t do an excellent job. On a high pressure turbo, with tuning, you could probably get better combustion from an iridium plug and it might be worth it. But don’t expect noticeable gains by just dropping them in a driving away.
Of course, the only real way to know is by experimentation and tuning, but who of us as dyno time to waste to tune for spark plug changes. You won’t be able to see 3 hp gains on a dyno anyway.
My .02$. Take it for what it’s worth. Post your questions for next episode of Ken’s corner.
Tell me what you think. Too long, too technical not long enough?
Ken