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.

  • How often do you fill-up?
    – tlhIngan
    Nov 2, 2016 at 17:10
  • I don't know why this happens either. Where would the lighter fractions evaporate to? Surely not through that small breathing line at the filler neck/cap?
    – Zaid
    Nov 2, 2016 at 18:26
  • I think what's going on could possibly be due to one of two things (or both): 1) Ethanol water absorption over time. 2) Imagination of the driver. Not trying to be mean with the second part there, but what objective (scientific) evidence do you have to base your claim? Seat of the pants is not a viable testing apparatus. :o) Your TR7 doesn't have a charcoal canister on it? I thought they were introduced in the late '60s? I'd have thought the "shape of things to come" would definitely have one? If it does, it has a closed system, right? Nov 2, 2016 at 19:09
  • @tlhIngan When i finished renovation, I testdrove it for two months, filling it up about each month. Today, I fill up about once a week, since i use it to commute to work.
    – Bart
    Nov 2, 2016 at 22:34
  • @Zaid The lighter fractions would evaporate to the air. The breather hole is not that small. That 'breather hole' is actually a pipe with a constriction in it. about 1mm in diameter. Over night or during driving, a significant amount of gases could escape through it i think, especially during warm weather.
    – Bart
    Nov 2, 2016 at 22:38

2 Answers 2


Addressing the question from the point of view of a chemist: this sounds highly implausible. Evaporative distillation is not terribly efficient to begin with, even with very different chemicals. I.e., even if you have two miscible liquids with very different boiling points, it can take a number of evaporation-condensation cycles to achieve significant separation.

On that basis alone, I think it unlikely that you would find a measurable change in density through a single pass of evaporation of a volatile mix of light hydrocarbons like you get from a petrol pump. (However, that's easy enough to test: Put a sample in a graduated cylinder on a scale, and mark its mass/volume as it evaporates.)

However, even if you found light fractions evaporating significantly faster, you still wouldn't be able to answer the question of whether octane rating is changing in the process: Knock resistance is largely a function of how branched a hydrocarbon is. For example, whereas the heavily-branched iso-octane defines the octane rating of 100, the straight-chain isomer n-octane actually has an octane rating of -20!

So even if you could remove lighter fractions from your pump petrol, you might end up with a denser but lower-octane fuel!

  • Sounds probable to me. The auto-ignition temperature of hydrocarbons don't need to have a link with how fast they evaporate. That was an assumption of me. As @Chris mentioned though, the effect could be caused by water absorbed in the fuel. A comparable effect might be achieved when using a higher octane rated fuel.(but with additional effects like less power) Effectively changing the octane rating; they just compare fuel to a mix of iso-octane and heptane. Using weight to measure won't be of any use then, you don't know how much water is absorbed vs how much fuel is evaporated.
    – Bart
    Nov 23, 2016 at 16:40
  • Great answer though!
    – Bart
    Nov 23, 2016 at 16:40

Over time gas will vent off VoC's absorb water and become less potent not more potent.

After filling your car up it will be heavier, in a small car (awesome and wonderful car) like a triumph this can have a pretty significant effect on handling and feeling during acceleration. Gasoline weighs just over 6 lbs a gallon, in a triumph with a standard 14.5 gallon tank that means around 87 lbs.

My guess is it feels and sounds more aggressive because you're increasing the load on the engine. On the "near-knocking conditions" you may want to reduce the advance on your cam timing higher octane lowers the potential for knock allowing people to run more advance but you can only run so much before you have issues.

  • So it's indeed the lighter fractions evaporating, that causes this effect? How i wish i could adjust my cam timing, but it's equipped with t a standard static cam shaft/timing. I could retard the ignition a bit, but i'd lose power then. It doesn't knock yet, it only runs on sometimes. It's not that of a problem. I do plan to fill it up with 98 octane next time as i said in my question, to see if it makes a difference.
    – Bart
    Nov 3, 2016 at 7:23
  • By the way, in that case building in a valve with a pressure threshold would prevent much watervapor getting into the fuel. At the same time it's creating a higher pressure in the tank, preventing light fractions to evaporate easily.
    – Bart
    Nov 3, 2016 at 7:29

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