First, I'm certainly not trying to hijack this thread. more so I'm trying to understand why you asked the question regarding use of regular or high octane gas. We have a car that recommends no less than 91 octane, my understanding is because it has a twin turbo. I always buy premium and it has always run great. Our truck recommends 87, and that's what we put in it, it has always run great. That's about the limit of my knowledge about octane, but I'm open to learning more.
Being curious about your "regular or high octane" question, I did some googling and I learned that lower octane fuel burns quicker and higher octane burns slower. Engines with higher compression ratios and/or forced induction, requires the slower burn rate of higher-octane fuels to defend against engine knock (which would explain the recommendation for my car -something I didn't know). Knocking or preignition occurs when the air/fuel mixture ignites before intended.
So.....i guess what I'm wondering is if you are asking about the octane because if the Original poster was using high octane, and if the fuel/air mixture is igniting later than it should, could that show the same indications on the spark plugs (black plugs) as a rich mixture condition? Or may be there's another reason?
Thanks in advance.....I'm just to understand.