Long-term effects of using < 91 octane fuel
03-26-2010, 10:44 AM
#21
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and that kind of logic, my friend, is why the price of fuel in general, and taxes, are going to keep creeping up and up, cent by cent, for no other reason than they can and guys like you say whats the difference, it's only $3 per tank. Or so what, my taxes are only going to increase by $100 a year - no big deal.
Sorry, one of my pet peeves. I digress.
Sorry, one of my pet peeves. I digress.
and it's also the everyday expenses that robs your wallet worse.
03-26-2010, 10:47 AM
#22
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My Ride: 2008 550, Carbon Black Metallic, Natural Brown Dakota Leather, Sport Package, Sport Automatic Transmission, Cold Weather Package, Power Rear and Manual Side Sun Shades, etc.
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That is not exactly true about the "recommendation" - this is straight from my owners manual (page 170):
Fuel specifications
The engine uses lead-free gasoline only.
Required fuel
Premium Unleaded Gasoline
The minimum octane rating is 91.
If you use gasoline with this minimum octane
rating, the engine may produce knocking
sounds when starting at high outside temperatures.
This has no affect on the engine life.
Minimum octane rating corresponds to the Anti
Knock Index AKI and is determined according
to the so-called (R+M)/2 method.
Do not use leaded gasoline, as
Fuel specifications
The engine uses lead-free gasoline only.
Required fuel
Premium Unleaded Gasoline
The minimum octane rating is 91.
If you use gasoline with this minimum octane
rating, the engine may produce knocking
sounds when starting at high outside temperatures.
This has no affect on the engine life.
Minimum octane rating corresponds to the Anti
Knock Index AKI and is determined according
to the so-called (R+M)/2 method.
Do not use leaded gasoline, as
03-26-2010, 10:55 AM
#23
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The M-technician at my dealer told me that most of the HPFP failures are due to people not using 91 or higher. He knows his stuff, too!
03-26-2010, 10:56 AM
#24
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lower octane will not harm your car. you or your car probably couldnt tell the difference, unless the temperature is high and you load the engine a lot.
lots of times if i know i'm just going to be doing a lot of highway driving, i'll just use the cheaper gas. higher octane gas while just cruising, has no benefits. higher octane gas just prevents pre-ingition under heavier loads and higher temps.
lots of times if i know i'm just going to be doing a lot of highway driving, i'll just use the cheaper gas. higher octane gas while just cruising, has no benefits. higher octane gas just prevents pre-ingition under heavier loads and higher temps.
On a road trip last year I experimented with this. I drove ~2000 miles on regular and averaged 22 mpg. The only real loss was the get up and go, pep, that the car normally had on premium.
03-26-2010, 10:56 AM
#25
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This question isn't about money. I realize the price difference between regular and premium is marginal, I just want to understand the reasoning behind the 91 octane "requirement". It would be easy to assume that since the manual says use 91 that failure to do so would ruin your car, but that doesn't seem to be true.
P.S, you may change your mind when gas hits $6.50 like in Europe!
03-26-2010, 11:23 AM
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91 is garbage. It's all about 93. If you're using 91, your cars are screwed. Glad I don't live in Cali anymore. 
03-26-2010, 11:56 AM
#28
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03-26-2010, 12:00 PM
#29
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and that kind of logic, my friend, is why the price of fuel in general, and taxes, are going to keep creeping up and up, cent by cent, for no other reason than they can and guys like you say whats the difference, it's only $3 per tank. Or so what, my taxes are only going to increase by $100 a year - no big deal.
Sorry, one of my pet peeves. I digress.
Sorry, one of my pet peeves. I digress.
I am not saying I am willing to pay $3 to $5 more for the same exact thing. I am just saying that the difference between putting in regular or premium gas is about $3 to $5 per tank. Not enough in my mind to justify the saving.
03-26-2010, 12:58 PM
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I come from the MDX Board and the question of 87 vs. 91 octane has been debated ad nausea.
I will try to summarize:
Octane is a measure of pre-detonation. The higher the octane the greater the compression of the fuel before pre-ignition, i.e. knock.
Thus by using higher octane in an engine designed for higher octane you will obtain more power.
Now the real question is: ?When will you use that extra power?. Examples are racing, flooring your car at a stop light and towing (applicable to the MDX but not to the 5 series BMW).
Testing, using a dynameter, shows about a 7% decrease in maximum horsepower using 87 vs. 91.
But keep in mind that at 70 mph more than 50% of your horsepower is used over coming wind resistance. You are using between 50 and 70 horsepower, so it will not make any difference whether you use 87 vs. 91.
Other ?wives tales?;
You will get better mileage using 91 vs. 87. Not true. You will get more horsepower but not better mileage.
Your engine will run ?cleaner? using 91 vs. 87. Not true. The detergents in both are identical.
You will void your warranty. Not true. Electronic knock control will retard the spark to avoid knocking.
Also keep in mind, that in western states, Colorado for example, the highest octane available is 89!
One final caveat: You will do no harm using 91 octane other than to your wallet.
At 15,000 miles per year, 18 mpg average that is 833 gallons per year. At a price difference of $.30 a gallon, that is $1,000 extra after only four years.
To me it is simple: I would rather have $1,000 than Exxon Mobile, so I use regular and use regular in my Boxster and in my prior cars: 1997 BMW 745Li, 2000 and 2002 Boxster, 2004 Audi 8L and my 2006 BMW Z4, all with no ill effect.
JeffK
I will try to summarize:
Octane is a measure of pre-detonation. The higher the octane the greater the compression of the fuel before pre-ignition, i.e. knock.
Thus by using higher octane in an engine designed for higher octane you will obtain more power.
Now the real question is: ?When will you use that extra power?. Examples are racing, flooring your car at a stop light and towing (applicable to the MDX but not to the 5 series BMW).
Testing, using a dynameter, shows about a 7% decrease in maximum horsepower using 87 vs. 91.
But keep in mind that at 70 mph more than 50% of your horsepower is used over coming wind resistance. You are using between 50 and 70 horsepower, so it will not make any difference whether you use 87 vs. 91.
Other ?wives tales?;
You will get better mileage using 91 vs. 87. Not true. You will get more horsepower but not better mileage.
Your engine will run ?cleaner? using 91 vs. 87. Not true. The detergents in both are identical.
You will void your warranty. Not true. Electronic knock control will retard the spark to avoid knocking.
Also keep in mind, that in western states, Colorado for example, the highest octane available is 89!
One final caveat: You will do no harm using 91 octane other than to your wallet.
At 15,000 miles per year, 18 mpg average that is 833 gallons per year. At a price difference of $.30 a gallon, that is $1,000 extra after only four years.
To me it is simple: I would rather have $1,000 than Exxon Mobile, so I use regular and use regular in my Boxster and in my prior cars: 1997 BMW 745Li, 2000 and 2002 Boxster, 2004 Audi 8L and my 2006 BMW Z4, all with no ill effect.
JeffK