The reason you use an Iridium plug is easily explained here:
http://spark-plugs.co.uk/index.php/cont ... ts.iridium
General information about Iridium spark plugs.
Iridium is an extremely hard metallic element, related to platinum, which is thought to occur on earth primarily due to an enormous meteor impact millions of years ago. Other than it's hardness, the main property which makes Iridium attractive as an electrode material for spark plugs is its extremely high melting point (almost 2,500 deg C).
Since late in the 20th century Iridium has been used in alloy form as material of choice for construction of Spark plug and industrial igniter electrodes. Pure Iridium is difficult to work with, very brittle and does not have great electrical conductivity hence alloys are used to give the best combination of these properties. An Iridium alloy electrode is able to deliver good conductivity and extreme hardness. The result is that an Iridium electrode can be manufactured with an extremely small diameter, when compared to a traditional copper or even a platinum tipped electrode, without compromising the useful life of the electrode or spark plug. The narrow diameter concentrates the electrical charge and therefore a much lower voltage is required to jump the spark gap. This is beneficial in several ways:
A lower voltage is required, this reduces load on a vehicle's ignition system (less strain = less chance of failure)
A larger gap can be supported without increasing load on the ignition system (Larger gap = larger spark = more efficient combustion)
A more precise spark every time, delivered in the correct position and with the correct timing (More efficient combustion)
Assuming a suitable Iridium plug is available, benefits you are most likely to see are:
Improved throttle response
Improved economy (though this depends largely on how much you enjoy the improved throttle response!)
Increased plug service life (Depending on design, an Iridium plug could last up to ten times the life of a standard plug with little reduction in efficiency over the plug's lifespan)
Increased engine power (Don't expect to double your horsepower but a 1 - 3% increase is possible)
Improved (reduced) emissions.
Will an Iridium spark plug be beneficial for my particular engine?
This is very subjective. The vast majority of vehicles/engines will benefit from replacing a standard spark plug with an Iridium spark plug. Most benefit will result if:
There is a very close Iridium spark plug match for your standard spark plug (i.e. overall design is very similar)
The standard spark plug you are replacing is a basic copper plug (the Iridium plug will then be a more significant upgrade)
The engine has a large cylinder capacity and a large number of cylinders
Least benefit will be realized if:
There is no close match for your standard spark plug, it isn't worth compromising plug design purely to use an Iridium plug type.
The standard plugs are already a performance or enhanced design. Some standard (Original Equipment) plugs are Iridium or fine wire platinum types, these may already provide the ultimate performance and service life for your particular engine. Fitting an aftermarket Iridium plug may actually result in a drop in performance and service life.
I've used them, and continue to use them for the improved throttle response and strong idle which I have experienced in use on various motorcycles. No other reasons other than those two. It's funny, the subject of iridium plugs always sparks......much debate!
