> Obviously, because of less potential detonation problems. These are
> exacerbated by hot temperatures as the intercoolers in the early 1.8T are
> somewhat underengineered considering the demands of chipped cars. The net
> result is hotter air intake, which means the fuel mixture shows a tendency
> to self-ignite at the wrong times for the engine - read detonation
> translating into less power per time unit.
>
> The chips adjust the fuel-air-boost map to provide maximum efficiency, but
> they also advance ignition timing to the maximum bearable by the engine,
> given a particular temperature. The engine is possessed with two
> detonation sensors, and the moment the ECU detects any instance or this
> happening, it will delay ignition, thus providing less energy release per
> fuel unit - read power. The problem with this is that even if the ECU and
> sensors do a fine job, there always needs to be detonation for the system
> to delay timing - read the fewer the instances of detonation before the
> system acts the less stress on the engine in the long term.
>
> The Octane number is just a measure of how easily the fuel will
> self-ignite, therefore, the higher this number, the better the fuel in
> terms of lower detonation, which translates into lower consumption as the
> fuel is more efficiently burned because it is not being burned at the
> wrong times.
>
> JP Roberts

In this case it's a 2005 engine so the problem with early 1.8T intercoolers
probably does not apply.

The ECU may only be calibrated for 95 so putting in 98 would not cause the
ignition to be advance any further.

I doubt anyone could tell the difference between 95 and 98 in a blind test
in this particular car.