Originally Posted by empilot
If you're putting low octane in your M3's engine to save a few bucks you're in the wrong car and you are damaging your engine
No you're not. Individual cylinder knock sensing is there for a reason. The ECU just pulls timing, netting you less power and worse fuel economy. It does this for 91 fuel as well, just to a lesser extent versus 87.
The ecu pushes for a preset value of maximum advance (which would be achieved using 95+ octane fuel) until it senses knock, and then dials back a predetermined amount. This is where maximum power lives for the given fuel grade.