octane?
 

First time BMW owner, picked up our 2005 X5 last week and love it. Although 91+ gas is recommended! the dealer did say that we would be ok with 89, since 91 isn't a real option. Being a high milage engine, does it make a difference if we go with 89 instead of 93?


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no no no no no.................which engine? 3.0 4.4 ,4.6is,4.8is? what year?how many miles?

That's the same thing my sales guy told me when the dealership delivered our brand new X5 about 10 years ago ;)

"It's good to use 91+ all the time to get the maximum &optimum engine performance.
BUT you can also use 87 or 89 only when you have to". At that moment I lost my faith in him (sales dude)

** I always pay the extra .40c per gallon and use Shell or BP as much as possible

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

If the car says to use it, then you use it. BMW gains nothing for whatever fuel you're using it after the fact, so it's not like it's some conspiracy theory to make more money. I accidentally put 87 into my 740iL and the car fell on it's face...bucked and stumbled, felt like someone dropped an anchor when I went WOT, just didn't run right. The fact is the engine was tuned on higher octane for a reason. Just like you can tell the difference between slim milk and whole, your car can tell the difference between 87 and 91 or higher. I explained it like this to my mother...

It's a BMW, so it's higher compression than her Honda. Higher compression means it's runs hotter. Higher octane fuel burns slower, which means it helps cool the engine for that microsecond before it's burned. Can she run 87 octane, yes. Will the engine run hotter, which means it'll ping more, which means the computer pulls timing to make up for predetonation, which means she'll have less power AND lower gas mileage, yes. She can either pay .40 less per gallon and get 20MPG or she can pay .40 more per gallon and get 24MPG (theoretical numbers), but ultimately you're better off following what the manufacturer suggested because it was made for it.

If it recommended 5w30 oil but WalMart had 15w40 on sale, are you going to put in the wrong oil to save money? It'll cost you in the long run, so pay now or pay later. Choose your poison.

What he said.

The engineer's only object is to meet the sweet spot between making the car run at optimum performance at lowest cost. If they could achieve that with lower octane fuel, they would.

There is no vested interest in making the car use more expensive fuel - in fact, if they could deliver the right performance and economy characteristics with a lower octane fuel and deliver lower running costs - they would!

With that said, the car does have the ability to retard timing to cope with a lower octane rated fuel. You will be duly rewarded with lower MPG and less performance.

The choice is yours - but my question would be, who buys a BMW and skimps on fuel?

Sooooooooo don't bother with 93 or 94 and just put 91? I was always told that u can run the lowest octane that the engine will allow. So if you're putting 89 and the engine runs the same then u can run 89 all the time

Engines are tuned for a certain octane fuel. You can run other fuels, but it won't run as well. You can tune a guitar a half step low, but when you try to play with the band, it's not going to sound right if the rest of the band is tuned standard.

I had the opposite problem years ago with a Chevy Blazer. I thought putting higher octane in it was taking better care of it, but I was having issues with bad acceleration, took it to a garage, and the mechanic asked if I had been putting cheap gas in it. I told him that I'd been running 91 and he said that was my problem. The engine was tuned for 88/89 octane. I started putting the cheap stuff in and didn't have any more problems.

Answer:

More than 3/4 of the BMW owners who lease their cars, or who can afford the monthly payments but want to skimp on gas because "super is too expensive".

It's pretty hilarious sometimes because its such a stigma and a negative by some, "ugh I have to put premium in this? whats gonna get hurt if i put regular in it?" , and considering that our gas is such shit nowadays ever since ethanol has wreaked havoc on our fuel supply and our gas tanks, etc, there is a TON wrong with putting regular in, but for the average Joe, they don't care.

Hell, nowadays, not even my Scag, or any of my 2-cycle equipment gets anything less than 93 octane, I'd rather have my equipment for a long time than save a buck or two at the pump.

Depending on which engine you have...the information from the owners manual can be different. As some have stated above, take that information...and the rest is up to you. Do take note of the meaning of "recommended" and "required". Also know that elevation to sea-level, topographical terrain (hills or plains), and temperature can all have an affect on performance or the demand you need from the engine. And the type of driver you are can make a difference as well. If you have a gentle foot or (granny-toe)...your driving demands from the engine is going to be different than those that are lead-footed. ;)

My e30/e32 (m20/m30 engines) only required fuel with an 87 AKI...higher octane fuels would be a waste because it didn't have knock sensors to adjust for different octane.



For my e34, e38 & e39...87 AKI, 89 AKI & 91 AKI is actually mentioned.



The fuel specs for my 06 X5 4.4 w/N62 engine gives no other "recommendation" and says that it "requires" 91 AKI:

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

:iagree:

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......:dunno:

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.

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 :D 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.

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

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

:iagree:

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...

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.

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. :D

That is exactly how I determined what the actual AKI requirement was for each of our vehicles, using multiple tanks. Only applies to my home base fuels, as different fuels will be encountered when travelling. Better than trusting the "lowest common denominator" recommendation from BMW marketing. But if it all seems like too much trouble, just use 91 and be safe.

Slightly OT (only because in Australia we have different octane ratings) but here is a real world test done by running 3 identical vehicles on 3 different fuels - E85, E10 and PULP (98 octane). The vehicle is specified by the manufacturer to run on regular (91 RON), it is the most 'average' car in Australia, but has an engine that can adjust timing and fuel delivery cycles for different fuels - so this shows what happens when you put non-specified fuels in, and how the engine responds in real world conditions.

Ethanol put to the test: E85 v E10 v premium unleaded

You'll have to change your mindset to understand our L/100km measure of economy, and our different octane rating.....and then stop yourself falling off your chair when you see what we pay for fuel. But......overall, if you skip to the summary of the article.....


Even though the car with the cheapest fuel returned the worst economy figures (used most fuel, ran for less kms) it was still the cheapest to run.

But if your main goal is to achieve best performance, fill up less often, and protect your engine (with 'additives') then premium is the best choice for both around town and on the freeway.

AKI is simply an average of RON and MON. Two different single cylinder test engine protocols.

There is around 6 or 7 points difference from RON to AKI, but it isn't linear or exact.

98 RON is pretty close to 91 or 92 AKI
95 RON is what we call midgrade, or 89 AKI
91 RON is pretty close to 87 AKI

And ethanol screws them all up.

This instance happened at a Chevron no less...some say it may not matter, but I try to stay within th Top Tier gasolines, so Chevron, Shell or Texaco. I should note that my 7 isn't stock and has internal performance mods pushing it to around 325hp, but that made it much more reactive to the lower grade fuel than a normal BMW would.

I used to fill my E34 M5 w/ Shell regularily when I was doing construction (we had an account w/ them) and he just happen to sell race gas as well. I only ran the race gas a few times, but in regards to Shell, I could actually feel the difference in using Shell vs any of the Top Tier gasoline brands at the time. I'd spoken to the owner about it, and he went into this lengthly explanation about gasoline qualities, additives, yada yada and while he wasn't trying to say Shell was the best, he said there must be something in it that clearly my engine liked better.

I agree about the whole 87, 89, 91/93 thing...to a point. If it runs like crap on 87, and good on 89, I'd still rather stick w/ 91/93 just for the added cushion and force of habit for when I do random things that'll load the engine more and possibly prevent detonation...WOT, carrying lots of passengers, elevation changes, etc. I can't think of a single manufacturer that suggests actually using 89, it seems mostly it's either 87 or 91/93. The only time I've told people to deviate from using 87 is in a vehicle where they've got clearly audible pinging (poor head design, years of poor maintenance) and going up in fuel grades until the sound disappears, citing that the pinging is not only robbing power, but ruining mileage as well. Once I explained this to my mother who tried cheaping out on gas in her 328ci, telling her she'll pay more now at the pump or pay more later w/ lower mileage, she finally listened. I used the same analogy on dyno vs synthetic oil changes...change more frequently or pay more upfront but go 2-3x as long between changes.

Setting aside the quality of the fuel, the engine will not be hurt with lower octane fuel as long as the variable timing has the range to adapt. Since the engine retards timing for lower octane fuel, performance and mileage will be less.

Conversely, higher octane will improve mileage and performance as long as the variable timing is capable of increasing the timing enough to utilize the benefit of higher octane.

Variable timing eliminates spark knock so the engine is not harmed. If you use lower octane fuel and the result is spark knock that means the engine requires higher octane fuel because the variable timing does not have the capability to adjust to the lower octane.

BMW does not maximize engine performance. That is the reason aftermarket tunes can garner additional performance. A performance tune will usually include advancing the timing and if the engine does not already require high octane fuel it probably will with the performance tune.

That said, if the timing advance is too extreme it will be detrimental and cause performance to decrease and/or damage the engine. As an example, If an engine is turbo- charged, supercharged or has nitrous, a custom tune that retards the timing can be necessary to safely utilize the boost even using 104/106 octane racing fuel.

Agree. with a caveat...

Modern BMW engines have knock sensors- tuned microphones that can 'hear' the microsecond the fuel begins to predetonate...the timing is then pulled, the car makes less power but you dont ket knock. You can run on lower octane or AKI without issues, for the most part. Essentially the car is detuned. (FYI- a bad knock sensor will cause a car to run like crap...as the system goes to full 'detune'.)

IMO THE REASON bmw designs cars for premium is CAFE standards and market forces- simply put they get better performance and mileage with higher octane. The tradeoff isnt engineers, it is bean counters- the downside is market perception of the cost to fuel with premium. You buy a BMW, you can live with this. So they happily tune for premium.

Finally on some cars (two of mine) will in fact develop more power with HIGHER than 91 or 93 octane- WITH a stock (factory) tune. Stock tuning typically has some spread- especially considering the 'norm' in EU is higher than 93 USA octane.

In my experience you can use 95 to 96 octane and gain performance. And further octane improvements will not show performance enhancements. You can remap the DME to use many octanes. I have 100 maps for the E39 and 100 and 116 maps for the 996TT. It is, however, a PITA to make sure the old fuel is flushed and new fuel in before changing maps up or down.

With stock maps I just run 50-50, 91 and 100, and get 95.5 octane. ;)

I had one vehicle (the 535i) that made more power on 94 than the specified 91. The fact that our 94 has zero ethanol made it attractive as well. We are now into the realm of forced induction, though, not something that applies to the E53.

Pulling the timing back isn't an on or off thing. And it isn't going to be doing it all the time, unless you are using 85 or something similar. On 91, it may do it hardly every. On 89, it may do it 1% of the time, just as an example. That would mean that for 99% of the time there would be no difference. It may only do it at a certain ambient air temperature and throttle setting. If an owner can't measure a fuel efficiency difference on a lower AKI fuel, then I submit that for all practical purposes any timing retardation is so negligible as to not matter at all. Increased fuel consumption will show up the same time as lower power does.