Gasolines
- Thread starter mkillio
- Start date
WhiningKhan
Regular
Lower octane fuel will autodetonate earlier than a higher octane fuel. In modern engine management systems, knocking does not damage the engine as it is identified early on and fueling is adjusted to suppress knock, but maximum power output won't be reached. How much this affects the power output depends very much of the specifics of the engine and usage conditions, but in practice you won't have issues on this - how often (if ever) do you try to take the maximum power out of the engine?
As for mileage, you may even get a little better figures on lower octane, but in practice it is again negligible, compared to the effect of your driving habits.
As for hard facts - I think I've seen some figures in books of internal combustion engines, but as noted, they change from case to case.
As for mileage, you may even get a little better figures on lower octane, but in practice it is again negligible, compared to the effect of your driving habits.
As for hard facts - I think I've seen some figures in books of internal combustion engines, but as noted, they change from case to case.
WhiningKhan
Regular
ANova said:
You shouldn't get better mileage with higher octane gasoline. Higer octane just means better compression behaviour, so you can inject _more_ stuff into the cylinder each cycle without knocking. If the manufacturer doesn't explicitly recommend higher octane, there's not probably much to gain. On higher performance engines, especially turbocharged ones, the lower octane can limit the absolute peak performance a bit, but even on these the mileage won't get better with higher octane.
And, as I said, the driving style affects your mileage the most, what ever you have in the tank.
You may be subconciously biasing the results - you really need to do a blind or even double-blind test.Powderkeg said:
Get someone else to randomly choose the petrol and fill the tank for you so that you don't know what you've got. Only compare the results after you've done a number of these.
Do you always fill the same amount of fuel each time, or do you compensate for that too?Powderkeg said:
WhiningKhan
Regular
Powderkeg said:
Well, on some conditions it is possible- if you are really doing a lot of full-power accelerations and the engine is very knocking-prone (possibly already damage from knocking, or tightly tuned turbo engine), the mileage will suffer from the longer time you need to accelerate. But you will notice the difference in performance if that is the case.
As Simon F wrote, truth in these matters can be found with blind tests and sufficiently large amount of data. And when numbers turn into statistics, it detaches from reality of a single driver, who fills up what the studies show the most economical and thinks he now can afford to floor it a bit more... negating the gain.
If your EMS can adjust it's ECU spark map table, then higher octane gas will make a difference. IIRC, BMW M3 double VANOS ECU does this. Most vehicles don't have this level of flexibility. Higher octane is required for high compression engines. Unless you have a performance or non-naturally aspirated engine, you won't really see any gains. The flip-side is that fuel quality can be variable, so most EMS have anti-knock to prevent too much pre-detonation (pinging). Once the anti-knock kicks in, your engine is retarded & so preformance falls off. The thoery for lower fuel consumption from high octane is that it burns cleaner thus liberating more power & hence needing less fuel.
WhiningKhan
Regular
Exactly for other parts of the message but this:
If taken literally, I claim precisely the opposite - lower octane rating fuel usually has higher calorific value, as the octane-index enhancers do not burn as well as the basic hydrocarbons of the gasoline. This is obvious if you consider that lower octane fuel detonates more easily - it burns hotter & faster, thus releasing more energy.
Completely another issue is that higher compression ratio means generally higher efficiency along with better burning - the higher octane rating just allows taking advantage of this by reducing the need for dynamically adjusting the timing and boost conditions under heavy engine load.
stevem said:
If taken literally, I claim precisely the opposite - lower octane rating fuel usually has higher calorific value, as the octane-index enhancers do not burn as well as the basic hydrocarbons of the gasoline. This is obvious if you consider that lower octane fuel detonates more easily - it burns hotter & faster, thus releasing more energy.
Completely another issue is that higher compression ratio means generally higher efficiency along with better burning - the higher octane rating just allows taking advantage of this by reducing the need for dynamically adjusting the timing and boost conditions under heavy engine load.
Simon F said:
You are grasping at straws.
How do I "bias" the results of filling my tank with 17 gallons of premium and going 400 miles, and then filling it with 17 gallons of regular, and only being able to go 360 miles? Not just once, but every single time?
In fact, your argument is scientifically unsound. The laws of physics say that if the engine requires a certain time, duration, and combustion rate of the fuel for maximum efficiency, and your fuel doesn't reach those levels, it's not as efficient. Reduced efficiency results in reduced gas milage.
WhiningKhan said:
In my condition, it happens. Of course my car has the same v6 as the 300 ZX, so it is a high compression engine which requires premium unleaded. If I use anything less, the milage drops.
I'm an old man driving a 4 door sedan to and from the same job, day after day, week after week, year after year. I'm not some kid racing around town, and my driving habits don't change just because I used different gas.
WhiningKhan
Regular
Powderkeg said:
And that is what I wrote in the next paragraph. This paragraph was responding to the claim that the fuel itself releases more energy, which I was trying to convey in the words "If taken literally".
I say once again, this is engine-specific talk. You can't just say "premium is always better". There is generally no use for higher octane than what manufacturer recommends, but lower octane stuff than what is recommended can lower performance.
It's too easy to write half-truths or misleading stuff in simplifying posts, so I'll end here. For those interested, information of octane numbers: http://www.madabout-kitcars.com/kitcar/kb.php?aid=124
I'm not grasping at anything. I was only pointing out that what you were doing has often produced invalid results in scientific studies where a human factor is involved. I suggest you do some reading.Powderkeg said:
Because you KNOW what you have put in the tank and may be wishing to justify the additional cost of the premium fuel.
Excuse me! I think you'll find that my suggestion is the sound scientific approach to testing.
