Gas

[Zulu95]

if this these problems are worth the $5.00 saved to run lower octane for a full tank, then go ahead.

Cheap gas is not that cheap.

[jmsspratlin]

Originally Posted by JCL

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.

Thank you....I like facts rather than opinions for cases like this

[faz]

What I learned from this thread is:

Don't piss JCL off when it comes to oil/gas/transmission fluid!

[JCL]

Originally Posted by killcrap

running lower octane has stalled several engines, decreased fuel enconomy, reduce engine power, caused the check engine lamp to illuminate, and ruin fuel pumps. if this these problems are worth the $5.00 saved to run lower octane for a full tank, then go ahead.

I would suggest that running crap fuel can stall engines, decrease fuel economy, reduce engine power, cause the check engine lamp to illuminate, and ruin fuel pumps. You can get crap fuel out of the 87 pump, the 89 pump, or the 91 pump.

I am interested in when the dealer suspects bad fuel as a root cause, do you test it to see the actual AKI, water content, ethanol content, etc, or do you go by the owner's recollection of what he last put in it, or do you use a process of eliminating other causes?

[markreg]

Originally Posted by JCL

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.

Excellent Post!!

[1stE53]

Originally Posted by JCL

I agree with you on the ethanol issue, it is a sad state of affairs when we are using food crops to make our cars run more poorly.

In reference to your comment that premium doesn't matter, that isn't what the original poster asked. The question was whether 87 or 91 matters. I suggest that it often doesn't, but better fuel sometimes has more cleaning agents, and I agree with you that it sometimes doesn't have ethanol (the 94 AKI I purchased today had zero ethanol, unlike all the other fuels at that particular station). Both the cleaning agents and the reduced ethanol are reasons to choose more expensive fuels, which usually means fuels that have a higher AKI.

. Well said. I learn so much on these posts!!

[JCL]

Originally Posted by faz

What I learned from this thread is:

Don't piss JCL off when it comes to oil/gas/transmission fluid!

I would just like to point out that I am not at all pissed off

There are lots of valid opinions on most issues, and they offen differ. That's cool. I just appreciate accuracy regarding facts, that's all.

And for those who found it all helpful, great. Thanks for the props.

Jeff

[m5james]

All I know is that I learned my lesson when I accidentally put 87 into the tank of my 740iL. Granted, the engine isn't stock by any means, but the power difference was drastic...less power, felt like a brake was hanging, and lots of bucks and stumbles when I got on the pedal. I'm very ritual about using the Top Tier gases. Needed or not, i'll put cheap gas (Maverick, Shitgo, etc) in my Chevy, but the BMW's and motorcycle get nothing but the best. All these varying opinions about what octane is the best, but I'm in no position to question what a BMW engineer recommended is a safe min requirement for a $50k SAV. Maybe its a waste to some, but I'm not gonna risk it to save $5 every two weeks.

[JCL]

Originally Posted by m5james

All I know is that I learned my lesson when I accidentally put 87 into the tank of my 740iL. Granted, the engine isn't stock by any means, but the power difference was drastic...less power, felt like a brake was hanging, and lots of bucks and stumbles when I got on the pedal. I'm very ritual about using the Top Tier gases. Needed or not, i'll put cheap gas (Maverick, Shitgo, etc) in my Chevy, but the BMW's and motorcycle get nothing but the best. All these varying opinions about what octane is the best, but I'm in no position to question what a BMW engineer recommended is a safe min requirement for a $50k SAV. Maybe its a waste to some, but I'm not gonna risk it to save $5 every two weeks.

Given the engine modifications to your 740 that you list in your signature, it appears that you no longer run the compression ratio or combustion chamber design that the BMW engineer originally designed into that vehicle, and so all bets are off on which gas to use. You will need to use the gas (AKI) which was specified by the engineer who designed the modifications.

I fully agree that using lower AKI fuel to save $2.50 a week is not much of a saving. Where I live, though, premium is $0.12 per litre or $0.50 per gallon more than regular, so it is more like $10 or $12 per week for many people. YMMV.

[Fraser]

I have three simple rules with regards petrol grade:

• Use the grade recommended by the vehicle's manufacturer
• When the recommended grade is not available use the closest higher grade
• No ethanol blends