Thanks Dan, what are you monitoring on your plugs? I saw one of your posts on another thread showing a photo of one of your IKH27 plugs. It looked clean, so that's good. Is there any other data? Have you done a comparison between the IKH27 and any other heat ranges and/or brands?
I keep going back to the fact that the Denso 27 heat range is the same as the NGK 9 heat range. This is colder than the NGK BR8ES plugs (8 heat range) I have run successfully for over 1,500 hours on my last two RVs with IO-375 and IO-360 engines with 8.5-1 compression jugs.
There's also the fact that SDS sells the IKH24 plugs with their electronic ignition for engines with long reach plugs. Therefore I still suspect that the Denso IKH27 plugs actually aren't hot enough for an IO-390 that's not being pushed hard (like racing, etc.) I also suspect that everyone is installing the Denso IKH27 plugs simply because that's what is listed on p. 9 of the PMAG installation manual. But even PMAG writes in their manual that it's just a plug that OTHERS have reported that works. There does not appear to be any test data to support their recommendation. If possible I'd like to get more data to make an informed decision.