You want to use the octane that you’re tuned for. So if that is 91 for you, then yes use 91. I’m sure someone will come in who knows a bit more in-depth about it than me. But I know it will not be safe if you push the car, and can knock if you’re tuned on 91 but run lower. The tune is specifically made for that octane…running lower can cause issues.
In one of my older cars I once had a dumb gas station worker who put regular 87, instead of 93 which I told him. I just drove around like a grandma till the tank was empty and filled back up with 93.
Not only should you use 91, but you should go for higher octane if available. I had knock even with 91 octane back when I had EC P2. If your car is tuned for 91, NEVER EVER use less than 91!
Well, when I read the original question, two things came to mind. First is that you should be using premium fuel as a bare basic even without a tune. The second is that if you're running a tune, use the octane level is was created to run on... and as HaZ noted, never anything less than that octane level.
Octane inhibits early detonation (or knock). Knock is bad. High compression turbo engines are prone to detonation with less than Premium gas because octane makes gas harder to ignite.
More advanced timing means better performance (to a point), more retarded timing poorer performance. ECUs advance timing to optimal performance, and will start retarding it to avoid knock.
You can run lower octane without damaging your engine because the ECU detects knocking and retards the timing to stop it, but this measure also means lower performance.
Personally, I also am not super comfortable with the fact that knock sensors are acoustic devices that detect ping when it happens, so its a reactive system not a preventative one. I have no expert basis for thinking the occasional knock is a serious problem and maybe the ECU is so fast the effect is insignificant, but I don't like the idea of it.
Why would you ask the question, if you took the effort to get the tune?
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