Christian McArdle wrote:

Increasing the octane actually reduces the energy content of the fuel.
I thought it just made it less susceptible to detonation.
That's obviously the main feature and why you would want higher octane
fuel. However, it has the side effect of reducing the energy content of
the fuel.
What then is the actual reduction in the "energy content" between
petrols with different octane ratings?

That would depend very much on the individual fuel. Obviously, higher octane
fuels from reputable manufacturers will have been formulated to restore the
loss of energy content, so I would be surprised if there is any on general
sale (LPG excepted) that has a lower energy content in practice.

So increasing the octane rating does not reduce the energy content?