When first activated, the engine management system advances the ignition and shuts the wastegate to find the point at which it senses knock.

This setting stays in it's memory and it will only alter itself to further retard the map if a fault occurs.

This is a safety feature and is common to almost every modern engine. After a fault is repaired (in this case changing to a higher grade fuel), the engine will gradually become better, but unless the memory is wiped, it would take an infinite number of re-sampling cycles for the effect on the average to be overcome.

As an example, WRX cars are designed to run on 100 RON fuel in Japan, their performance on UK 95RON is in the region of 245 to 265 bhp (various unconfirmed reports), purely from this adjustment in timing & boost.

As an example of the difference this makes, a members '96 is running at almost 1.2 bar which it has achieved by 3800 revs. The dealer had found that 95RON would only give 1.0 bar on a '96, as per the manufacturers figures. These figures have also been substantiated by another source in France. An owner in Pretoria reports that the local 91RON fuel allows only 0.5 bar!

The figures for newer cars (post 96) are lower, due to the smaller turbo.