if the car is tuned for 91 ron fuel (like most NA cars are) then you shouldn't be getting any knock running 91 octane fuel, unless the car is leaning out due to a fuel system issue (blocked injectors, lines, filter, or dying fuel pump), or the fuel timing has been advanced or some sort of mechanical issue (such as hot spots in the cylinders). if you were to advance the timing when you put the higher octane fuels in then you will see gains, but if you run it dead stock then you won't see any gains.