octane?

[JCL]

Originally Posted by Qsilver7

There's nothing like empirical data.

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.

[SMOKEY53]

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.

[JCL]

Originally Posted by SMOKEY53

Slightly OT (only because in Australia we have different octane ratings)...

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.

[m5james]

Originally Posted by JCL

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.

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.

[bcredliner]

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.

[ard]

Originally Posted by JCL

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.

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.

[JCL]

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.