[OhondaU]

Ryker, if you want to delve into the specifics of detonation and octane ratings on high compression high rpm engines I can dig up the equations.

But before that nitty gritty embarassment ensues where the numbers prove you wrong, I will direct your attention back to the original question of this thread: will running 87 octane in the engine cause damage to the engine? The short answer from the manual: NO.

Now, can the combination of "low" octane (needs to be quantified) and "high" comression (needs to be quantified) cause detonation under the right conditions? Yes, no one argued that. HOWEVER, in this application the engine will be just fine on 87 octane instead of 91, or are you arguing with the vehicle's owner's manual?

Edit: As a case in point that the scenario of knock is not as straight-forward as you suggest: I have one motorcycle in my garage that has 12.4:1 compression ratio and runs on the manufaturer suggested 87 octane. Never a single knock....