HI there, just found your channel and enjoyed your comments about your bikes. I have an XC. Which prefers E5 to E10 petrol, so if you're evaluating these plugs against the old ones, you should stick to E5 or E10 (obviously) for both trials. Same roads, same weather, same speeds inc flat oot! I doubt a Harley engine puts enough stress on a plug to need iridium, but you never know. I would suggest trying them using E5, the slightly higher octane might help them perform better? They might last longer too, but you'll need 20k + miles for that!