the knocking, which is not optimum, but is compensated for in modern aluminum, computer controlled, etc engines comes at a rare peak time when all the variables are pushed in the maxed aspect. tracked, 110 degree ambient air, buzzing the motor, lugging the motor, sea level elevation etc. Modern aluminum head design, machining and casting is light years advanced from old sand cast steel heads and hot spots are quite rare. despite all the "if you can't afford it" comments and "if you want it to run like it shoud/get max hp" suggestions, running less than 91 won't kill you or your engine if you are not flirting with all those variables being maxed out.
for many of us that includes winter driving, grocery getting long flat highway trips. the sky won't fall. That said, do what you want and chose to disagree if you like
FWIW, I've got two race jeeps, one dyno tuned to run on 93 pump gas and the other gets a 96 blend due to more timing and boost. When we dyno tuned the L92 (LS3 derivative) it was stoooooopid how much timing we put in it to make more power. The OEM's set them up way tame for warranty considerations.
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2009 silver sport, 6MT touring and nav, 8700 miles when I got it, "almost" a daily driver now... bone stock aside from tint and Gemini CBE, PC'd Rays black stardust and running 275/295 DWS rubber