Gas

[jmsspratlin]

What are the overall effects of using 87 octane as opposed to the suggested 91? Just curious is anyone has proof of damage, or just opinions. Thanks...Jamie

[1stE53]

Just don't do it. A lot of states have ethanol in all but their premium even thought they aren't posted. It will not burn all the fuel and leave massive deposits of carbon in your engine.

[jmsspratlin]

Does this not play a factor for other brands of cars? Why just the German's? Got any research to back that up?

[motordavid]

Hit Search Tab, upper right side of page; there are literally pages of threads
on the topic.

Fwiw, I have run tanks of 87 and, 89 on trips where that tank gets used in
a day, with zero ill effects in 8 1/2 years and 76k miles. Some here are 91/93
addicts, some of us run what we like. YMMV,
GL,mD

[jmsspratlin]

I like your style

[1stE53]

Not just German cars require it. Take a look at ALL luxury cars from Japan and USA. Prem Fuel required. All high end sports cars require premium as well.

[sethro]

Our cars (most modern cars) have knock sensors, the OBC will automatically adjust and compensate for the lower octane fuel.

I know people (sadly) that have leased high end BMW's and Audi's and run nothing but the cheapest gas.

[loobie]

Your mpg will be less negating any positive savings. The compression is the factor requiring the octane. The higher the octane the slower the gas burns and the more power you get.

[1stE53]

Originally Posted by loobie

Your mpg will be less negating any positive savings. The compression is the factor requiring the octane. The higher the octane the slower the gas burns and the more power you get.

Well said!

Case in point: my 2006 nissan altima 4 cylinder got 27 mpg on a 64 mile round trip from Columbia to Jefferson City on 87 octane. I use 91 octane and get 31 or more on good days! That's good enough for me . I have only used premium, even in cars that don't require it. Cheaper than replacing or cleaning engine components. Better mileage offsets the price difference. Now, I have never owned an American car, so I can't speak to them. Japan and German only.

[JCL]

Originally Posted by loobie

Your mpg will be less negating any positive savings. The compression is the factor requiring the octane. The higher the octane the slower the gas burns and the more power you get.

That is simply not true. There is probably more misinformation surrounding octane ratings than any other subject on many car discussion boards.

Your mileage will not necessarily be less; your mileage MAY be less. It may have no impact.

Higher compression is not the only factor requiring a higher AKI (anti-knock index), the combustion chamber design also impacts it significantly. With older engine designs, using high compression pistons, a higher AKI was often the only way to prevent knocking. With modern combustion chamber designs, designers are using much high compression ratios without the same demands on the fuel in terms of AKI.

Higher octane fuel does not burn slower. It burns at exactly the same rate.

Higher octane fuel does not have more energy, and thus does not produce more power in and of itself.

Higher octane fuel is not 'better' for the engine, in that if you have a fuel with sufficient AKI, anything above that is completely wasted. The engine doesn't know the difference.

All a higher AKI means is that the fuel is more resistant to knock, ie explosions or ignition prior to when it should ignite. That may allow the engine designer to use a higher compression ratio, and that design decision is what produces more power, not the fuel.

For the OP, keep in mind that different fuels will not test out at exactly what it says on the pump. It should be that AKI or higher, but sometimes it isn't, just like many other things in life that aren't what they say on the bottle.

Fuel AKI follows a bell curve distribution, in that some fuels will be better than the target, and some will be worse. Also, fuel AKI in North America has traditionally been very variable, creating problems for the manufacturers.

For the above two reasons, BMW designed the engine to run on a certain AKI (let's call it x), and then specified a 'minimum recommended fuel', which for North America, where we use the (R+M)/2 method to calculate AKI, is 91. The engine wasn't designed for 91, it was designed for less than 91. However, because fuel quality varies, and it isn't all exactly as published, BMW recommends 91, which is a good safe practice. Using 91, you shouldn't have any problems. Using less than 91, you MAY have issues. It depends whether your local fuel is better or worse than the average.

When the fuel does not have sufficient AKI to prevent knock, the engine computer adjusts to prevent damage. If that adjustment is enough to reduce power, you may see a noticable power reduction, and you may get less mileage. Or you may not.

In my case, running on 91 (in a moderate climate, near sea level, in a region with good quality fuel), I noticed no difference whatsoever on 89. No reduction in power was apparent, and mileage was very slightly better on 89 when measured over multiple tanks. That makes sense. On 87, the vehicle ran fine, but I noticed a slight degradation at times. I used 87 when I had to (a few times), but nearly always ran on 89. With my newer vehicle, turbocharged, I use 92 because the turbocharged engine can take more advantage of the higher AKI, and using lower AKI is noticable.

Due to new engine designs with advanced combustion chambers, and improved fuel quality in North America (ie Top Tier), BMW is generally more tolerant of lower AKI than they used to be. It used to be 91 was strongly recommended. Now it is just recommended. My 535 manual even says that any fuel down to 87 is fine.

Probably the single biggest factor with different fuel grades is the amount of cleaners/detergents added by the fuel companies. You may want to use a higher priced (higher AKI) fuel just to get the improved cleaners, such as Techron. They likely matter more than the AKI itself.

If the reason to use 87 is to save money, then we can probably all agree that you won't save enough for it to matter. It is essentially irrelevant compared to the total cost of running the car.

For the OP, who asked for proof, PM me for details. My background is 25 years in the engine business (not with BMW), professional mechanical engineer, former mechanic, ex Chevron dealer employee, and I may be one of the few on here who have actually run single cylinder test engines in a lab to measure fuel quality or used labs with bomb calorimeters to measure fuel energy content (although both were a very long time ago).

There are a couple of fuels engineers who post here who can confirm or deny the above.