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X3 F25 (2011 - current)
The latest X3 brings some added style and some new features to the BMW SUV family. Talk about the new F25 now!

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  #1  
Old 07-01-2014, 12:44 PM
radarlover radarlover is offline
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Octane at high altitude

On a normally aspirated engine one can run lower octane the higher altitude. That is why in Denver regular gas might be 85 but at lower altitudes it might be 87. Question is, the X3 3.5 requires 91 octane. If the car is run normally above 5,000 ft. could one drop down to mid grade 89 octane? The manual says a grade below 91 is acceptable on occasion. Would the turbo affect this argument?
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  #2  
Old 07-01-2014, 01:25 PM
Coder Coder is offline
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We're at 6500 ft here in Colorado and, by mistake, I once filled with regular (87 octane). No perceptible effect, no pinging or any other problems. Perhaps down on power but the engine management seems to mask any problems very effectively.
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  #3  
Old 07-01-2014, 04:42 PM
kevink4 kevink4 is offline
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In my previous car (which I still own), which recommended 91 but required at least 87, I just went with the safe choice of 87 or more. My 2015 owners manual recommends 91, minimum of 89. (I have diesel). So I would probably just go with 89 or higher in the mountains.
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  #4  
Old 07-01-2014, 05:17 PM
RhoXS RhoXS is offline
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I suspect, with reasonably street driving habits, using lower than recommended octane will mostly not be noticeable at any altitude because the computer will sense on the onset of detonation and silently adjust spark timing accordingly. When this happens the engine will probably run less efficiently and there will be a power hit to one degree or another, negating some of the already very minimal saving of buying the cheaper gasoline. The harder the engine is pushed the more the spark must be retarded. At some point the computer will not be able to retard the spark any more and I believe this is where one starts risking big time engine damage if detonation is allowed to occur. In other words, under most driving conditions one would have to monitor spark timing to see when the car is compensating for the lower octane fuel.

My opinion? Saving a few pennies a gallon to use a lower octane than specified by the manufacturer is silly after spending approximately $50k for a car.

Last edited by RhoXS; 07-02-2014 at 02:12 AM.
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  #5  
Old 07-01-2014, 05:46 PM
suvguy suvguy is offline
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Amen.
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  #6  
Old 07-01-2014, 07:23 PM
todd92 todd92 is online now
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Quote:
Originally Posted by RhoXS View Post
I suspect, with reasonably street driving habits, using lower than recommended octane will mostly not be noticeable at any altitude because the computer will sense on the onset of detonation and silently adjust spark timing accordingly. When this happens the engine will probably run less efficiently and there will be a power hit to one degree or another, negating some of the already very minimal saving of buying the cheaper gasoline. The harder the engine is pushed the more the spark must be retarded. At some point there the computer will not be able to retard the spark any more and I believe this is where one starts risking big time engine damage if detonation is allowed to occur. In other words, under most driving conditions one would have to monitor spark timing to see when the car is compensating for the lower octane fuel.

My opinion? Saving a few pennies a gallon to use a lower octane than specified by the manufacturer is silly after spending approximately $50k for a car.
The question was about lower octane at altitude because lower octane is all that is available. It was not a question about saving money.

For the OP, use the highest octane in the mountains (90?) and don't lose any sleep over any potential harm.
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  #7  
Old 07-02-2014, 06:59 AM
radarlover radarlover is offline
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Correct. The question was not about saving money but what is the affect of lower octane than specified in the manual when the car is operated above 5,000 ft and we know that lower octane is acceptable at higher altitude. I'm sure BMW did not determine their octane rating for those relatively few drivers who live and drive above 5,000 ft. So it would seem driving with mid-grade (87-89 octane) would be fine. But the turbo does force a higher air pressure so not sure the effect of that. Thanks for comments.
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  #8  
Old 07-02-2014, 07:06 AM
tlm999 tlm999 is offline
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Quote:
Originally Posted by radarlover View Post
Correct. The question was not about saving money but what is the affect of lower octane than specified in the manual when the car is operated above 5,000 ft and we know that lower octane is acceptable at higher altitude. I'm sure BMW did not determine their octane rating for those relatively few drivers who live and drive above 5,000 ft. So it would seem driving with mid-grade (87-89 octane) would be fine. But the turbo does force a higher air pressure so not sure the effect of that. Thanks for comments.
It's not as if builders of German cars haven't encountered high elevations in their own country. I expect they took elevation effect into prime consideration.
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Old 07-02-2014, 07:10 AM
radarlover radarlover is offline
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I'm sure they did but the % of drivers above 5,000 ft would be relatively small and therefor you have to design for the most common conditions/requirements knowing you have over designed for others. Same as for suspensions, etc.
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  #10  
Old 07-02-2014, 07:46 AM
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kyfdx kyfdx is offline
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Quote:
Originally Posted by radarlover View Post
Correct. The question was not about saving money but what is the affect of lower octane than specified in the manual when the car is operated above 5,000 ft and we know that lower octane is acceptable at higher altitude. I'm sure BMW did not determine their octane rating for those relatively few drivers who live and drive above 5,000 ft. So it would seem driving with mid-grade (87-89 octane) would be fine. But the turbo does force a higher air pressure so not sure the effect of that. Thanks for comments.

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  #11  
Old 07-11-2014, 08:24 AM
BM2W BM2W is offline
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I didn't buy an expensive car to put cheap gas in it. That said, I did put 3/4 tank of 89 in our '11 X3 3.5 coming home from Denver because Svc station was out of 91. Ran fine, tho seemed a bit down on power, DME did its job, no problems. I'm in Santa Fe @ 7000'.
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Old 07-11-2014, 09:16 AM
RhoXS RhoXS is offline
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Quote:
Originally Posted by BM2W View Post
I didn't buy an expensive car to put cheap gas in it. That said, I did put 3/4 tank of 89 in our '11 X3 3.5 coming home from Denver because Svc station was out of 91. Ran fine, tho seemed a bit down on power, DME did its job, no problems. I'm in Santa Fe @ 7000'.
I guess there is big difference in the capability of the fuel injection electronics now in 2014 compared to 1988 when I had a brand new E30 M3. Minimum 91 octane was specified for the E30 M3. As a result, I told my soon to be wife to be sure she filled it with "Premium". In those days "Super Premium" was 92 octane and "Premium" was 89 octane. Since I said "Premium" that is exactly what she did and I shortly discovered I had a full tank of 89 octane gas.

In those days BMW had very good customer support on their 800 number. I called BMW to ask if they saw any issue with 89 octane gas and would it be prudent to avoid heavy throttle applications. I was put on hold for a few minutes and then, to my surprise, was very firmly advised to not drive the car but have it towed to a mechanic to drain the tank. I guess now that it had no detonation sensor although, at the time, I thought it did. Nevertheless, I did drive it 20 miles to a mechanic that thought I was nuts and had the tank drained.
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  #13  
Old 07-11-2014, 05:18 PM
missedbass missedbass is offline
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just went to check what the gas cap says- min 89. also just found out that the if the car is locked you cant open the door to get to the cap. learn something new everyday!
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  #14  
Old 07-12-2014, 07:46 AM
radarlover radarlover is offline
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An MB 550 I had had the same gas cap feature - nice to know someone can't get to cap with car locked, at least without a tool.
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  #15  
Old 07-12-2014, 02:28 PM
kevink4 kevink4 is offline
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I remember once buying gas many years ago in Denver. Back when driving a car that only needed 87.

Worst tank of gas ever. I couldn't even drive the speed limit out of Denver (65) unless I was headed down hill. As soon as the needle dropped enough, I put in new gas.

Never had such bad gas since.
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  #16  
Old 11-07-2014, 12:59 PM
philbake1 philbake1 is offline
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I live near Denver (actually at 6,000 ft). The SA at my BMW dealer told me they use mid grade fuel in their cars. Mid grade here is 87 octane. (regular is 85/premium is 91) I have a 2010 X3 (6cyl N52inline non-turbo) and a 2013 328xi (4cyl Turbo). I'd always used 91 octane (premium here) in both cars. I tried a few tanks of mid-grade in the '10 X3, and I found no performance difference and about 1-2mpg better gas mileage. Go figure! I'm not inclined to run 87 in the 328xi because it's a turbo, but I'll continue to experiment with 87 in the X3 to see if that increase in MPG is real. I always use top tier from either Conoco or Shell however.
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