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BMW and MINI – Fuel (Gasoline) Octane vs. Economy vs. Quality

March 28, 2011

Over the years, Otto has received many questions regarding what fuel is best to use in our BMWs and MINIs. These questions run from cost of operation to power output to long-term reliability. It’s understandable that the general driving public may not be knowledgeable about gasoline formulation and proper use. The vehicle manufacturers are not educating us (besides a little sticker on the instrument cluster and/or the gas filler) and the petroleum manufacturers are marketing to us in the guise of education, just as Hollister (insert your local young adult clothing & life style boutique, here) is educating us on what is cool and what is not. The fuel companies have done a great job of “educating” the motoring public. They have, for years, implied that your car will run better and even implied more power, by using the premium grades. Why? More profit in it. In fact, one of the major companies was hauled into court for this misrepresentation, a few years ago.

Here are some of the common misconceptions:

* Higher octane equals higher quality fuel (false)
* Higher octane equals more engine power (false)
* Higher octane equals better fuel economy (false)
* Higher octane burns cleaner in my engine (false)
* Higher octane is better for the catalytic converters (false)
* My BMW or MINI must use higher octane fuel because it is built to do so and will be damaged if this is not followed (false)

Ok, so, the above points contradict everything you’ve heard about gasolines. In fact, all of the above points do indeed have some truth behind them (under the right circumstances), but can in no way be applied as blanket statements.

What is octane?

Octane is not a rating of how good (quality) or how powerful a fuel is. It is a rating of the rate of burn for the fuel … how fast it burns. Higher octane fuel burns at a slower more controlled rate than lower octane. The engine management system is programmed to provide optimum engine performance and economy using fuel with a specific octane rating. The big differentiating factor here, is whether your engine management system has knock sensors or not (most BMWs produced from the early ’90s-on, and all MINI models, do have knock sensors).

You may notice that your owner’s manual, the sticker over the fuel gauge or the sticker on the gas cap states a specific octane rating, such as 87 AKI or 91 RON … or both (in this example, the 91 RON is equivalent to the 87 AKI). AKI is the octane rating used in the US. RON has been popular in Europe. AKI is the acronym for Anti Knock Index and is a calculated number that is the average of two different tested octane values (RON & MON). AKI may also be represented as R+M/2 (the average of RON & MON test values).

The explosion of the fuel/air mixture, in each cylinder of the engine, is not an instantaneous event. The burn of the mixture takes a certain amount of time. Due to this, the spark (from the spark plug) occurs at a point prior to the piston being at top-dead-center (TDC) in the cylinder. The spark must be generated in a specific time prior to the piston coming to TDC, so that the fuel/air mixture is at its maximum burn (and corresponding expansion rate) just as the piston passes TDC and is ready to be pushed down by the expanding, burning, mixture. The octane rating of the fuel is one of the determining factors in how long the mixture takes to burn and, hence, how long before TDC the spark must happen (spark timing). Change the octane ….. and you must change the spark timing. If the octane number is raised (higher octane), the burn will be slower (longer burn time) than what is programmed for the spark timing and the maximum burn and expansion will occur later than TDC and power will be lost due to wasted piston travel while waiting for the maximum burn point. If the octane number is lowered (lower octane), the burn rate is increased (shorter burn time) and since the spark timing has not changed, maximum burn is reached prior to the piston reaching TDC. In this case, the expansion of the combustion gasses is trying to push the piston down as it is mechanically being pushed up toward TDC. This is bad. This is the major source of engine knock, or ping (detonation). The engine parts are actually being beat to death and are rattling. Prolonged exposure to this will destroy the engine.

Engines without knock sensors:

Older carbureted and fuel injected engines that do not use knock sensors, have a specific ignition timing curve, or map. The ignition timing determines when the spark will occur to ignite the fuel/air mixture in an individual cylinder, as noted above. In these applications, it is best to use the octane rating that has been recommended by the vehicle manufacturer. Going lower may result in detonation (but not always). Going higher in octane is likely just wasting money, due to the loss of maximum combustion happening at the top of the piston’s travel, again, as noted above. You can certainly experiment with higher and lower octane ratings. Be aware, however, in testing for lower octane use, you may not audibly hear the detonation but it may be happening at a low level. Using higher octane will not hurt anything … except, perhaps, your wallet due to the higher initial cost and potentially lower fuel economy (MPG).

Engines with knock sensors:

Engines and engine management systems that incorporate knock sensors are capable of manipulating, or changing, the spark timing dependent on the presence … or lack of … detonation. The sensors are piezoelectric devices that are secured to the engine (typically, directly to the engine block). The sensors detect the vibration frequencies of the detonation and send this info to the engine management computer. The computer then reduced the spark timing (makes the spark happen later, or closer to TDC) in order to eliminate the detonation. Since the system can detect even the smallest amount of detonation, as well as adjust the spark timing up or down, we can actually use fuels with different octane ratings … without damaging the engine, and possibly even be able to increase engine power and/or economy.

In using a low octane fuel, with a knock sensor engine, we save money at the pump. However, we may not be running the engine at its maximum efficiency. If the engine and the engine management system are optimized to run with a higher octane, the system will be reducing the spark timing (not all the time, but under conditions where detonation is present), which will reduce power output to some degree and may reduce the fuel economy. Using higher octane, will allow a more aggressive spark curve, which will produce more power and, potentially, better fuel economy.

So, what does all of this mean ….. how do we use this information to determine what octane fuel to use?

Non knock sensor applications -Use the recommended octane fuel, unless modifications have been made that change the spark timing or up the effective compression ratio (performance chip, cylinder head milling, etc.). In these cases, higher octane fuel may be required to prevent detonation. Use lower octane fuel with caution. Even though you may never hear any knock or ping, it may be happening at a low level and, while this is not nearly as destructive as audible knock, it is hard on the engine and is actually reducing your performance and economy.

Knock sensor applications -Here, we can play around a bit. If fuel economy is our main goal, we can run the low octane fuel, but we need to do some testing in order to determine what octane works best for overall fuel economy (cost of the fuel, plus how much we use). Here’s how; Run one to two full tanks of low octane fuel. Track the actual MPG (you can use the On Board Computer, but we prefer a real calculated result). Next, run a tank or two of the high octane fuel and calculate the MPG. Determine the overall fuel cost by dividing the MPG into the gallon cost of the fuel. This gives you the cost per mile. Multiply this x100 so that you can have a clearer picture of the fuel costs. You can now make an educated determination of what fuel you want to run. If performance is our goal, use the recommended octane, or higher. There is a point at which the higher octane would produce no further gains, due to the limitations of the programming in the engine management system. However, the common 91 to 93 octane that we have readily available is well within the typical BMW or MINI engine management system capabilities.

Fuel Quality -

Fuel quality is indeed an issue to contend with. However, it is not directly related to the octane rating. Fuel quality is tied to the refining, storage and transportation of the fuel.
Refining -Certainly, some petroleum companies have higher grade refining processes and additive packages. Just Google “best gas” or other such terms and do some research for yourself. Be aware, however, that you will have to filter through all of the “Internet experts”. Within a given brand of fuel, it is not uncommon for the higher octane fuel to have a fancier additive package as well.

Storage & Transportation -The cleanliness (or lack thereof) of the refinery’s tanks as well as the distribution facility’s tanks and the transportation tanks (trucks, train, etc.), can take a great fuel and contaminate it. There are federal guidelines and regulations on fuel storage facility operation and cleaning, but these are not always followed. Additionally, long-term storage will degrade today’s ethanol laced fuels faster than the pure fuels of years ago. The best we can do in these areas is stick to reputable, high volume, stations and try not to fill-up during or immediately after the station’s tanks are being filled. This can stir up contamination in the tanks (water and other contaminants).

Sum it up?

Remember, the only thing that the octane number gives is a representation of the burn rate of the fuel. The higher the number…. the slower the burn rate. If the engine is not built and optimized for the higher octane, it will only lose power by using it. Fuel quality has nothing, directly, to do with the octane rating. This has more to do with the specific fuel refinery and what they put in it for cleaning additives, etc. The only relationship to octane is that SOME brands will put more quality additives into the premium octane grades. However, if you stick with the major brands, even the regular octane will be high quality.

Typically, installing a performance chip, or programmer (which usually advances the ignition timing as one of the areas that improves power), requires a higher octane fuel (depending on the chip). The advanced spark is occurring earlier (before the piston reaches TDC) and the slower burning fuel is now needed.

Oh, and the octane rating has nothing to do with the function of the catalytic converters.

2 Comments
  1. Jallohos permalink

    Please tell me specifically, what kind of gas should I use on my BMW X5 2003, 3.0. Why. What is the benefit and lost. The above is too complicated for people like us that are not AUTO SAVY. Thanks

    • Your 3.0 X5 is specified to run 91 octane fuel. However, you can safely run 87 or any other octane. If you do not see a noticeable reduction in MPG or undesirable power reduction with 87 octane, you would certainly reduce your fuel cost by running the regular 87 octane fuel. Most of us, here, do run the 87 octane on these late-model 6-cylinder engines. Otto’s personal M3 runs 87 unless it is going to the track.

      Remember, though, on earlier BMWs (not your X5) that do not have knock sensors incorporated into the engine management system, the octane usage should adhere to what is specified by BMW.

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