A few weeks back for a test I filled up with Tesco Momentum 99 just to see if there was any difference (yes, I know it isn't really worth it for a car like this but I was curious!) To be honest there was no change, if anything mpg is slightly worse. The other day as I was getting low I put in a tenner of Morrisons bog standard 95. Since then there has been a noticeable fast knock or what can be better described as a fast farting noise around the 1 - 1.5k rev range, less noticeable at higher revs. Is it a case of the engine getting used to the lower octane fuel again? rubbish petrol, dodgy knock sensor or something worse?

One tank of fuel will not show a noticeable improvement, try at least 5 or more.

As for cheap supermarket fuel my diesel Mondeo hated it and ran rough at idle, all modern engines are tuned to run on the good stuff so that's all I put in my Fiesta.

the ti vct ecoboost will benefit from 99 ron on the old mk2 ti vct i have you can run 95 but optimum performance it states on the fuel filler premium premuim plus or super plus