Are Denso Iridium the best spark plugs? Well, the owner's manual recommends changing these little consumable items every 8000 miles.
And if you do a lot of mileage, the cost of replacing these little beggars soon adds up. I did some research to see if I could find any longer-lasting items, and decided to try out some Denso Iridium Power. Although ordinary spark plugs are not that expensive, the Denso last for ages (I replaced mine after 32000 miles although they still looked perfect). They cost around twice the price of standard plugs, but last roughly four times as long, so they're actually more economical.
Now the owner's manual specifies the type of spark plug as CR8EH-9(NGK) or U24FER-9(DENSO). However, if you're interested in the Denso Iridium, the type you want is IUH24.
The central electrode of these plugs is made of an alloy of two incredibly hard metals: Iridium, and Rhodium. This makes it possible to manufacture this electrode with a much smaller diameter - a miniscule 0.4mm. This, combined with a tapered cut U-groove ground electrode, creates a more efficient spark because the spark is more concentrated at the tiny tip. Not only that, less spark plug voltage is required.
NGK are now also making Iridium spark plugs; their central electrode measures 0.6mm.
When installing the spark plugs, it's important not to over-tighten them. Because the plug holes are recessed on the ST1100, you'll need to use either the tool from the toolkit, or a socket with an extension bar. Tighten the spark plugs by hand (i.e. without using the actual socket wrench. Once they're finger tight, fit the socket wrench to the top of the extension bar, and tighten them an additional quarter turn. No more than that!
I've noticed that since installing the Denso, ignition appears to have improved, and fuel economy is better. I won't go back to standard plugs again!
Update, 2015: I not only continue to use the Denso on the Pan, but I have fitted them to my wife's Hyundai as well. Not only that, as I'm now also operating a Stihl chainsaw, guess which type of spark plug I'm using for that?!
Update, 2017: Oops. I actually completely forgot to check the spark plugs on the Hyundai at the recommended service intervals, with the result that the 'engine check' light eventually came on. I then discovered that the vehicle had done 54 920 miles on the Denso!