Has anyone tried DENSO IW31 plugs? They have the .4 mm iridium center electrode with a tapered U-grove ground electrode. Should be better than the .6 mm center and non-U-groove of the BR10eix. The reach is 19mm vs 21.5 for the IRE01-31 iridiums listed for rotaries but the tip is in about the same place due to the different plug types being measured differently. It doesn't look like there would be any issue with it sticking out too far. They cross reference to a 10 heat range but there are also other versions that are 7, 8, 9, 9.5 and 11 equivalents. It's also a resistor plug so there's no problem with EMI or interference. It's about $19 so it's more than the BR10eix and similar projected nose plugs but it's a lot less than the $30+ of the iridium side gap plugs normally sold for rotaries. You'll still have the issue with socket clearance but it looks like it's got possibilities and even advantages otherwise. There's got to be better options than either going with the $30+ iridium side gap plugs (DENSO, HKS, etc.) or staying with the NGK stock type with the recessed center electrode. Any experience and/or comments relative to these DENSO plugs?

 

I brought a JDM engine that have that Denso IW31 Racing that you are talking about. I never started the engine since I brought it because I was porting and bridgeporting the engine. I dont know if they are good. I need to still waiting to fire the engine and test it to see the performance.