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I recently got an 2011 Audi A5 2.0t, and I know the car recommends 91 octane but my local gas stations only have 87, 89, and 93. I know I can run 93 and be fine, but it’s pretty expensive. Could I run 89 octane without any knocking/performance issues?
 

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The car is tuned to run at 91 minimum octane. So if you don't have 91, then go with 93.

My understanding is that when you use a lower octane, the computer can probably compensate by adjusting the timing. However, you'll lose out on power and mpg. Also, you run the risk of having other issues (e.g. carbon buildup/hot spots) that causes so much pre-detonation to the point where the computer can't retard the timing enough.

I haven't seen any real long-term studies done. Anecdotally, I've never heard of anyone saying their engine failed because of lower octane.
 

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Go for 93 mate for peace of mind plus engine care. Also I use some STP or similar every few tanks just to try and keep the injection system clean. 91 is ok from time to time but best mixed with 93.
 
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