Hi, Having a discussion with a friend of mine who has a fiesta st, I was saying that I use high octane fuel (normaly shell) and it does make a difference to the performance - to me the car feels more responsive. I could how ever be thinking this even though there is no effect. My friend was saying there won't be any effect on performance as majority of cars including his st and my s3 are optimised for standard fuel and in order to make any difference the ECU must be mapped to make use of higher octane fuel. What's the truth here, and how does higher octane give more power in standard cars? I don't want to be spending more on a fuel that realistically makes no difference, even though in my mind I think I feel a difference?!