One thing that is keeping me busy, is that my car seems to react different after I have filled up at the petrol station. I fill it up with 95RON. It seems to be a little more aggressive, but it sounds rawer at high revs. It's the tenth time I filled up this car since I have it, and it happens again every time.
My hypothesis is that the light fractions in petrol evaporate out of the tank with time, leaving petrol consisting of the more heavy fractions. Light fractions would burn earlier and easier, so the octane rating gets higher when they are vaporized out of the fuel.
Light fractions burn easier, causing more agresiveness after I have filled up. At the same time, being light fractions, they cause near-knocking conditions in my engine, hence the rawer sound at higher rpm.(and more engineload) My engine can cope with 95RON, but barely, there's no margin left. I know this because it tends to run on sometimes.(among other symptoms)
What do you guys think of this? True? Plausible? BS? I'd like to hear your opinion.
I have a '76 Triumph TR7, so naturally it doesn't have an EVAP system or something. There's a small opening at the fillercap for the tank to be able to breath. Maybe i'll build in a pressure cap there later, to save fuel from evaporating. I'll also fill it up with 98RON next time, to see if there's any difference. Cheers.