Knocking when using 89 octane gasoline?
 

I realize that the recommended gasoline for an '86 951 is 91 octane. Has anyone tried and gotten by using 89 without any engine knocking sounds or any other problems?

You know, I would dare not recommend it to anyone, but once I ran out of gas, and had to use 87 octane (thanks to the safety crew). My MBC was set to ~14psi, and it ran fine without knock. Ambient was around 50 degrees F. If the car is bone stock, and everything works properly, the car should run fine, perhaps w/a bit less power (as the DME back off on boost, and pull back timing). EGTs are sure to rise (and fast), if your stock boost control system is not in place however.
Ahmet