"Christian McArdle" wrote in message
.. .
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.


I wouldn't have thought there would be a significant difference in energy
content. High octane fuel is less susceptible to premature detonation but a
consequence of this is a slightly slower burning fuel. The reason it sounds
"better2 could be a result of that. It's likely that you'll get slightly
worse mpg for a engine tuned for 95RON.

If the ignition timing can be altered to take advantage of the fuel, you'll
get more power and fuel economy than before.