[Hugh Keir]

Just to stir it up a bit,

There are two effects that fall under the category of knock.

One is Pre-ignition generally caused by carbon deposits, exposed and glowing spark plug threads, thin edges on your head / pistons glowing etc. This causes the fuel to ignite before the ignition spark.

The other is Detonation caused by Lean fuel mixture, Fuel octane too low, Improper ignition timing and too high boost this causes secondary spontaneous combustion in addition to the spark induced flame.

In both detonation and pre-ignition, it is the collision of the flame fronts that causes the classic pinking sound.

Not much difference between the two, but you can deal with pre-ignition by re-building your engine to a higher specification, whereas detonation requires an adjustment to the management of one of the following - Boost, ignition timing, Octane rating or combustion chamber temperature.

You could of course use water injection to control the combustion temperatures - might even be cheaper in the long run when compared to a lifetime of octane enhancer.

You might also consider that the higher the octane the slower the burn of the flame front and more often than not less energy available from the higher octane fuel.

You can of course run more boost and create more power that way, but it would produce more power if you could run the same boost as the high octane example with low octane fuel + water injection. Just depends if you want the absolute maximum power at any cost which is then high octane fuel + high boost + water injection.

That is unless you use some oxygenated fuel like nitrous which allows the argument to go on and on.

All of the above assume you can tune your engine to take advantage of the different fuels - no trivial task.