I thought pinking was when the fuel ignites too early. Wasn't sure of the reasons why.....did a quick google and came up with the following........
1) Unleaded fuel, generally being of lower octane rating, will render the car more susceptible to detonation or pinking from sources not directly attributable to the fuel, like oil dilution of the petrol, localised overheating, etc
2) Use of low octane fuel without engine adjustment or octane lifting additives can give rise to detonation in the combustion chamber rather than steady burning. this detonation is heard as pinking and can be very damaging to engines, particularly it can cause piston failure.
3) I understand that the higher the Octane level in the fuel, the more the protection that your car's engine's combustion chamber has from detonating / pre-igniting /' pinking' ... these apparently are the 'tinkling' sounds you might hear from your engine if you go up a hill under full throttle load which may occur from too weak a mixture or too far advanced ignition timing for the grade of petrol being used. Higher Octane levels therefore can protect your engines from these sorts of symptoms. I am informed that Higher Octane levels in the fuel used can prevent the internal 'knocking' in the combustion chamber that can occur from using a lower Octane fuel.
Hope some of that helps.
I used to read the Shell Optimax forums every now and again. Usually some interesting topics on their, even if Shell Moderators do skirt some issues.