octane?

[JCL]

The best fuel (AKI rating) is the lowest one that will provide full performance.

As others have said, there is no advantage to running higher AKI ratings.

There may or may not be any negatives from running a lower than recommended AKI ratings. The sky won't fall.

That is because AKI is not a precise figure, and engines don't have a specific single requirement, but rather a range over which the engine is designed to operate.

Different driving conditions require different AKI ratings, ie summer and winter. Heavy throttle foot or light. Altitude makes a 2 point difference.

Actual fuel AKI rating also varies in practice. Fuels may be at the high end or low end at your supplier. There are wide regional differences in quality throughout North America.

Considering all of the above, BMW specs 91 because for most conditions in North America, that will be sufficient to never impact performance. Since all engines are different, and fuels range widely, that means that many are buying fuel better than they require. Fuel requirement (and supply) is distributed like a bell curve. 91 is at the far right of the curve; nearly everyone is to the left of it so nearly everyone will be fine on 91. But the most common requirement is something less than 91. Half will need less than that requirement, half will need more.

The test is to run a tank of 89, and see how it does. If you notice increased fuel consumption (measured over a full tank), or lower power, then that is a good indication that you would benefit from running 91. If you notice no difference, there is absolutely no reason to buy 91 over 89. The engine will run on 87, but it is more likely that you will notice a difference.

It is better to focus on how much ethanol is in the fuel, as that will have more impact than the AKI rating most of the time.

Best economy I ever got on my E53 was on 89, which I ran for four years. Same on the 325, which we ran for five or so years. The 535 noticed a difference, and ran best on 94. I used 91 or 94. The X3 runs better on 91 so I use that most of the time. When I travel, and don't know the local fuel quality, I always buy 91.

If all that seems like just too much trouble, just put 91 in it. But don't worry at all if you get a tank of 89 by accident or necessity.

Jeff

[Quicksilver]

Originally Posted by motordavid

There are literally pages of full threads on the topic...OP might use Search Tab for hundreds and hundreds of opins.

Seems to be two camps:
-use nothing but premium, and even pages of debates on 'which brand'
-use premo and occasional 89, brand being besides the point.

My anecdote is: occasional premium, more than occasional ~89, and the 3.0 IL6 hasn't missed a beat in 96k+ miles/almost 13 years.

Run what you choose/like, imo.
GL, mD

There is no holy grail. Different vehicle, different climates, all kinds of stuff
to consider. Like MD said do the research then you decide. It's almost on the
level of change the transmission fluid (or not) What oil to use, (this oil is the greatest)
The whole process is simple. Fill er up and drive......

[Geneticfreak]

It's a 2005 4.8is with 200k miles, and the engine is in great shape. We've had it just long enough to fill it up once, and I used 93. I wouldn't think of using 87, but 91 is a confusing number since it really isn't an option (87, 89 or 93 is all you see) Other than my motorcycles that always got 93, my domestic cars and trucks got 87 since they will run on any sludge you put in them.

[DillonInCO]

They probably said 91 because that's the highest you can get in many parts of the country. I live in Colorado and 91 is "premium". I always run that in my X, as I noticed a bit of a power drop when running 89 (mid grade here). But I have a 3.0, so I'll take all the power I can get Running mid grade shouldn't be damaging at all, but as others have said, just try a little bit and if you notice a serious drop in power or MPG, you know not to use it.

[J.Belknap]

The DME will pull timing if it has to avoid knock, you're not going to lose the engine over it.

[srmmmm]

I run 91 octane during the hot summer months, or when I'm making an extended highway trip. Other than that, I've had no problems with using 89 octane the rest of the time. I notice about one mile per gallon difference between the two octanes, with my lifetime average at 19.2 mpg.

The one point I do consider important is the quality of the fuel itself. I run Shell gasoline whenever possible with Exxon as a second choice. I've had no fuel system issues ever, other than fuel level sending units.

2002 X5 3.0 261,000 miles
2004 325i 115,000 miles

[J.Belknap]

Originally Posted by srmmmm

The one point I do consider important is the quality of the fuel itself. I run Shell gasoline whenever possible with Exxon as a second choice.

[TerminatorX5]

If I remember correctly from the old days in petroleum college (Азербайджанский Институт Нефти и Химии имени Азизбекова - which i never finished!!!)...

AKI - anti knock index...

most BMW engines, (as well as most other sport engines) are high compression engines. High compression engines achieve higher horsepower (power rating) per volume of cylinder displacement.

High compression engines have pistons that travel inside the cylinders a bit further and as a result, compress the air/fuel mixture to higher level of pressure.

In accordance with isothermic, isobaric and isovolumetric (isometric) laws, in a constant volume (the size of the cylinder), when the pressure builds up, it also changes the temperature of the gas mixture... higher pressure (further travel of the piston), higher temperature... Higher temperature, more chances for the gas/air mixture to self-ignite before the spark from the spark plug is introduced... If the mixture is self-ignited, the resulting combustion expands the gas, and pushes the piston out, as the internal pressure inside the chamber increases and overpowers the external pressure excerted onto the piston (from crank-shaft), thus resulting in mechanical KNOCK, which essentially having two powerful forces working against each other - one is pushing the piston INTO the chamber, the other one is pushing the piston OUT of the chamber... This is the main principle of a diesel engine, that has no spark plugs (glow plugs that provide additional source of heat inside chamber) - the diesel fuel ignites as a result of pressure, and as a side effect, the diesels have huge torque...

The special additives added to the fuel make the gas less "flammable" by increasing the "flash point" of self-igniting to certain level...

when a manufacturer specifies a certain AKI in its fuel, it has tested the engine to perform adequately using that fuel without "knocking" or, having the "knocking" managed by the special hardware/software.

Using a higher octane fuel, than specified, (93, instead of 91) will result in less intervention by the said software, and using minimally qualified AKI rated fuel will provide normal operation with accassional intervention of the "knock" software.

the higher octane fuels do NOT pack more energy per liter/gallon/ounce of weight or volume, they just have higher self-igniting point.

the oil producers charge more for this type of fuel because it geared towards sports and high end vehicles, and the notion is, if you got that car, you gotta pay for it. Besides, since the profit margin is much better on these fuels, the oil producers add as a bonus some detergents and other chemicals (some of them are not added but rather not "removed", thus making it even more profitable) to entice the buyers into buying their products...

so, M5james was right, if it says use minimum of AKI91 - then gotta use that...

[JCL]

Originally Posted by TerminatorX5

When a manufacturer specifies a certain AKI in its fuel, it has tested the engine to perform adequately using that fuel without "knocking" or, having the "knocking" managed by the special hardware/software.

When an engine manufacturer specifies a certain AKI for its engines, it has tested the engine to perform adequately a large percentage of the time, under a variety of operating conditions (temperature, altitude, etc) using fuels that are represented to be 89, 91, etc at point of sale. In reality, fuels range widely across North America in terms of actual delivered properties, as do operating conditions.

The best way to think of it is as a normal distribution. Population on the y axis, required AKI on the x axis. The points making up the shaded blue area represent a variety of owner actual requirements for AKI ,in order to minimize the DME having to retard the timing too often, because of all those variables. BMW needs to make sure that they cover the full range of variables, as much as they can. They don't like getting "low power" complaints. And they don't really care that the 91 fuel costs more. So let's say they specify the requirement (91 AKI) where the red line is, at one standard deviation. That means that for 84% of owners, the DME won't need to intervene, and the owners will get full performance out of their engine. But that also means that most of the owners (by definition.....) are buying a higher AKI fuel than they actually require, to cover all the eventualities.

Your engine doesn't know what AKI it is burning. It has no way of telling. I don't know that you can even measure AKI directly; I recall running test fuels in a single cylinder laboratory test engine at very low speeds and then running a standard fuel, and if they knocked at the same point, then that was the AKI of the fuel (or RON, or MON, or whatever).

All your engine knows is if it needs to retard the timing. That is why 89 will work for most people, because most of them are on the left of the red line. It is also why you buy 85 octane at high altitudes, and it works fine. Thinking that 91 is some sort of magic requirement is wrong. It is a safe operating practice, just as changing the oil at 15,000 miles when the lights say to do so is a safe operating practice. But some would argue it is less than optimal.

Do what M5James did. If it runs crappy on a certain fuel, get a better one. He ran his 740 on 87 and it fell on its face. Good reason not to run 87 in that engine under those conditions. But if it ran fine on 89, then there would be no reason to put 91 in it except to satisfy the original risk management strategy of the BMW marketing department, who spec'd it to account for the widest range of variables that their engines would see in the real world.

[Qsilver7]

Originally Posted by Geneticfreak

It's a 2005 4.8is with 200k miles, and the engine is in great shape. We've had it just long enough to fill it up once, and I used 93. I wouldn't think of using 87, but 91 is a confusing number since it really isn't an option (87, 89 or 93 is all you see) Other than my motorcycles that always got 93, my domestic cars and trucks got 87 since they will run on any sludge you put in them.

Just for grins & giggles...you could do a little experiment for the forum. Just jot down some data so it's not just "seat of the pants" or only subjective. Take note of what your AVG MPG & AVG MPH (speed) is taking the data from the OBC...or doing your own calculations dividing miles/gallons. Do this using the 93 AKI on a couple tanks of gas.

Then do a switch & fill the tank with 89 AKI to see if you can tell the difference in MPG and available power...again jotting down your AVG MPG & MPH...and note if the performance and MPG is slightly different than the 93 AKI you normally use.

Then do some 1/2 & 1/2 of 89 AKI & 93 AKI which will give you a tank of 91 AKI {this really gives you a tank of 91 AKI fuel (89+93=182) & (182÷2=91)}. Again, take note if the 91 AKI (tank of 1/2 & 1/2) is different than using full on 93 AKI.

Then report back the AVG MPG & MPH you got using 89 AKI, 91 AKI, & 93 AKI. There's nothing like empirical data.