Has anyone run with these plugs? My understanding from searching various spark plug threads on here is that these may have been the original choice for plugs in the X300 but at some point RC12YC was recommended, possibly do to them running hotter.
I was in the process of trying to figure out a misfire issue (since resolved) and I was going to change plugs from the Iridium NGK IX 4469 plugs I put in when I got the car a couple of years ago. I must have read somewhere that they were good plugs.
I went on Rock Auto and ordered Champion copper plugs without noticing they were the RC9YC4 plugs. After I got them I regapped them to .035 buy I haven't put them in yet. Just curious if anyone has experience with these. Should I just toss them and go with the RC12s or maybe stick with the iridiums?

 

Definitely pitch the iridiums. These cars love the cheap copper plugs.

I'm sure the RC9s will do ok, they were the original standard, but I'm also sure there was a good reason to switch to RC12s. I'd get the Champion 71 or the NGK BKR5e and call it a day. Don't waste money on the fancier plugs, there are many posts that seem to believe they were the cause of troubles.

 

I put the RC9s in today. I noticed right away that the threaded portion is 3-4 threads shorter than the ones I took out. Fired right up though. I’ll run with these a while to see if I notice any difference from the Iridium’s. I’ll use the RC12s next go around.