So it is my understanding that the OEM Motronic ECU was programmed to run on 87 octane and there is no knock sensor or method for the ECU to advance timing past what are in the ignition tables. So to me that would mean that as long as your ignition system is up to snuff and you aren’t getting any pinging or knocking that 87 octane would be the best fuel to run as increasing octane only has added benefits of reducing knock while the negatives are higher octane fuel has a higher activation energy. The other thing to factor in would be our engines and fuel systems were obviously not designed with the current E10 fuel in mind that we find at most pumps. So my question is what octane fuel do most of you guys run? I have been running E10 93 octane since I didn’t want to take any chances, but now I have found a local PURE gas station that has 0% ethanol 91 octane fuel that I plan to run.
What Octane Fuel Do You Run?
My understanding is also that the engine was designed for 87, and that 93 would seem to be contraindicated as you described. I put in 89 or 91 depending on what’s at the gas station I happen to be at. I don’t have well-thought-out reason for 89-91 octane, but it’s easy to imagine that there was once a meeting at BMW between 3 lawyers, 4 marketing wonks, 6 accountants and 1 engineer. The marketing wonks and the accountants demanded that the car be certified for 87 octane, the lawyers (for once) didn’t care, and although the engineer would have preferred to recommend 89 to be safe for all circumstances, he grudgingly agreed that 87 would probably be ok.
When it’s an issue that would interest lawyers, accountants and marketing wonks, it’s often worthwhile to imagine how those non-technical types tried to influence things. Who thinks that neutral front camber was optimum for balanced handling, that 0W oil is really ideal for modern engines, or that 15k miles is a the best oil change interval?
87 or 91 whatever is available.
I’ll have to talk to the Chemical Engineer in the family, but I think I am right in saying that higher Octane fuels are those with the ignition point of the fuel retarded and reduced potential*** energy. This statement is obviously only applicable to engines that can burn 87 without pre-ignition issues.
MTBE works by forcing the fuel to ignite when the pressure and spark combine to be “hotter”. Therefore useful in high compression engines.
There was something about flame fronts and full air/fuel ignition with 87 Octane in engines designed to burn 87 Octane… I’ll leave this to you to work out - I forget.
potential*** - in reference to the burn time… (not the potential energy as this is always the same)
Sorry I need to be more specific - my generalizations are too general
[quote=“kgobey” post=58173]potential*** - in reference to the burn time… (not the potential energy as this is always the same)
[/quote]
Re. always the same. Why the same? Chemistry is different so why can’t potential energy be different?
runs off to get his ChemE
The explanation of a lower potential energy is specific to the intended use of the fuel type.
87 Octane in a 8.8:1 compression motor like ours has a different potential energy from a 91 octane in the same motor due to flame fronts and complete burns blah blah - where is Chuck to help me with this part?
whereas a 91 octane fuel in a 10:1 compression motor has more potential energy than an 87 Octane due to the same reasons and causes…
(man - I had to google for the numbers here) 1 joule is equal to 0.001 kilojoules, or 7.58955676988E-9 U.S.-gallon of automotive gasoline.
I know of no difference in Joules (or calories I guess) available in Petroleum with either 87 or 91 octane… Of course this is calculated when burned in a beaker not a motor.
YMMV and science without personal research is here-say
[quote=“kgobey” post=58204]runs off to get his ChemE
The explanation of a lower potential energy is specific to the intended use of the fuel type.
87 Octane in a 8.8:1 compression motor like ours has a different potential energy from a 91 octane in the same motor due to flame fronts and complete burns blah blah - where is Chuck to help me with this part?
whereas a 91 octane fuel in a 10:1 compression motor has more potential energy than an 87 Octane due to the same reasons and causes…
(man - I had to google for the numbers here) 1 joule is equal to 0.001 kilojoules, or 7.58955676988E-9 U.S.-gallon of automotive gasoline.
I know of no difference in Joules (or calories I guess) available in Petroleum with either 87 or 91 octane… Of course this is calculated when burned in a beaker not a motor.
YMMV and science without personal research is here-say[/quote]
It’s good that you were able to explain this. Lets see if I have this right:
In engine “A” 87 octane has a different potential energy than 91.
In engine “B” 91 has more potential energy than 87.
In beaker “C” they are the same.
Therefore the optimum solution is, ah, what?
The lowest octane that you dare run … I’ll stick with 91-93 but I’m sure it’s overkill. Then again, that’s how I roll!
I run 87, so when I race Kish and he’s hopelessly behind me we’ll know that octane doesn’t matter. :lol:
for that to happen you actually have to show up to race instead of chickening out with bogus excuses dontcha? 
[quote=“Ranger” post=58205][quote=“kgobey” post=58204]runs off to get his ChemE
The explanation of a lower potential energy is specific to the intended use of the fuel type.
87 Octane in a 8.8:1 compression motor like ours has a different potential energy from a 91 octane in the same motor due to flame fronts and complete burns blah blah - where is Chuck to help me with this part?
whereas a 91 octane fuel in a 10:1 compression motor has more potential energy than an 87 Octane due to the same reasons and causes…
(man - I had to google for the numbers here) 1 joule is equal to 0.001 kilojoules, or 7.58955676988E-9 U.S.-gallon of automotive gasoline.
I know of no difference in Joules (or calories I guess) available in Petroleum with either 87 or 91 octane… Of course this is calculated when burned in a beaker not a motor.
YMMV and science without personal research is here-say[/quote]
It’s good that you were able to explain this. Lets see if I have this right:
In engine “A” 87 octane has a different potential energy than 91.
In engine “B” 91 has more potential energy than 87.
In beaker “C” they are the same.
Therefore the optimum solution is, ah, what?[/quote] EXACTLY! I hate Chemistry
I went and did some research on this the other day and didn’t find out anything helpful. With that intro, here’s the summary.
-
My gas isn’t your gas. The composition of gas can vary a lot. It varies by brand because they use different additive packages. It can vary by region because of laws that result in different additives, and it can vary by season because apparently different gas chemistries do better in different ambient temps. Therefore it is entirely reasonable that your gas might have better performance potential than my gas.
-
Octane is a lousy measure of performance. Although your gas might be 1% more efficient, looking at the octane #'s is a lousy way to evaluate the liklihood of one gas being a better performer. It would be like deciding which of 10 cars is the most powerful and since you aren’t allowed to look under the hood, you decide that you will try to choose the most powerful car by it’s color. Yes, cars do come in different colors and an arguement could be made that yellow cars tend to be more powerful cars…but that doesn’t make color a valid way to determine a car’s power. If yellow cars seem to be more powerful maybe it’s because teenagers like yellow cars and they are always driving too fast.
-
I couldn’t find anything that talked about octane being related to performance due to flame front or any other similar obscure characteristic of ignition. The flamefront business might be entirely valid, I don’t know. But in the hour I spent looking up articles and reading stuff, I couldn’t find anything authoritative on the issue.
-
There is talk of lower octane being higher performing because it will more easily detonate, but others point out that the heat value of 87 is the same as the heat value of 93 so there seems to be no smoking gun here. Others say that 93 has more potential energy because it’s more dense, but the counter for that is the argument that the dense stuff in 93 isn’t stuff that helps it burn.
-
There is talk of “heat value” being an imprecise measure of a gas’s potential and the real data has to be figured out empirically because there’s obscure things going on during ignition (see flame front) that isn’t easily modeled with HS algebra.
Scott, you are now on the same confused page as me.
Best response to this issue is to say - run what BMW says will work, and if unavailable - run what you can find. (as long as it’s Petrol/Gasoline).
I did some dyno testing in college with different octane rated fuels. Although only slightly more powerful, fuels with lower octane rating performed better than higher rated fuels until detonation occurred. I suspect that the octane boosting additives have less potential energy than pure gasoline and therefore slightly lower the potential energy as you increase the octane.
Bottom line, I don’t think there is measureable benefit (maybe 1 or 2 hp) between 87 to 93 fuels, but I run 93 to be on the safe side. I think 89 would be optimum, but don’t want to risk detonation that I can’t hear.
Disclaimer - Other than the dyno testing in 1996 at college, I’m only speculating…
I know I have read magazine articles that have dyno tested and found the same thing Fish.
I just drove by a gas station with a sign that said “no ethanol marine gas”! I know where I’m filling up my fuel cans before the next race…
I don’t remember having e10 in the southeast. Anyway if you’re not paying a road tax for a car you’re not driving on the road then hell yeah.