Safest thing for running 91 octane?
 

So I'm in a place where 91 octane is all that is commonly available. I'm wondering if running on the stock tune with 91 is doing (albeit minute) damage that will show more wear over time. I'll be here for the next few years at least while my S.O. finishes medical school, and I intend to run this evo till it dies and I cannot find any parts for repairs any more.

I feel like my two choices are leave the stock map and hope any big engine problems happen under warranty, or get an AP and run the OTS 91 octane map and tempt fate that way instead with potential warranty denial.

Local tuner is out of the question, as we don't have any really big internationally known and revered shops like AMS around here, and since that would be a reflash over the stock ECU and 100% detectable (read: guaranteed warranty denial versus only a maybe with the AP).

To my knowledge, the evo x ECU doesn't have multiple maps that it switches between based on detected knock and octane levels like hyundai's and some other companies, so I'm worried and just want opinions on what the best is for engine life. I'm not in it for the performance and looking to extract the most power, just for what is safest.

Are X stock tunes for 91 or 93?

Either way; if the knock control system is anything like the CT9As, yes the ECU will adjust to consistent, heavy (7+) knock. It does so by decreasing a stored flag called the octane flag. The octane flag starts at 100%, which means the ECU will use 100% of the timing value in the high-octane map for it's calculation. As the octane flag decreases, say to 50%, the ECU will interpolate halfway between the high-octane and low octane values, resulting in less timing advance. When there's no/less knock, the octane flag gets increased back up.

In addition to this long-term correction, there is of course the immediate correction as well, where during a pull the ECU will pull roughly 1° of timing advance for 3 counts of knocksum.

If the owner's manual only requires 91 though, I wouldn't worry about it at all.

i have over 120k miles of 91 octane. you'll be fine.

Thanks! Exactly what I want to hear :D

12k miles on 91. Hard hard miles~ And there has been a discussion on this before. You'll be okay.

91 is all we have over here and car still running strong.. buddy of mine has 100k+ miles on evo 9 mr and still runs like new daily driven

The owners manual says to use 93. It also says that 91 is ok, but not ideal.

I would hazard to say the majority of Evo's in the country are sold in states without 93 octane. It's fine.

93 here in ga