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Discussion Starter · #1 ·
My M3 is at 52,000 now, does it matter which type of fuel I use when I go to the gas station? I used to have a Honda, and my parents used to tell me to always use the highest grade, but I use the lowest one right now. I know it maybe a odd question, but by seeing the mileage I'd just like to maintain my 3. Thanks!
 

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Discussion Starter · #3 ·
Sounds good, Thanks!
 

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old cars can actually run better because they have so much junk built up inside the engine and combustion chamber that they compression ratio is above normal (and higher octane helps this to resist detonation). If your car has less than 150,000 miles, it would not benefit at all from higher octane gasoline.
higher octane is not better, does not increase performance, does not make the car start easier (just the opposite), does not burn hotter or generate more energy...that number only reflects the fuel's ability to resist detonation under pressure and heat (without a spark) and nothing else.
 

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Discussion Starter · #5 ·
Wow! that's pretty good information, now I know the difference. Good thing I've been only using the lowest grade.
 

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ya. u'll be fine with the lowest grade. plus, it is recommended to use 87 octane by mazda on the mazda3s except for mazdaspeed3 which is recommended 91, in terms of a stock car.

[quote author=John_in_the_LBC link=topic=139366.msg3003418#msg3003418 date=1237434102]
old cars can actually run better because they have so much junk built up inside the engine and combustion chamber that they compression ratio is above normal (and higher octane helps this to resist detonation). If your car has less than 150,000 miles, it would not benefit at all from higher octane gasoline.
higher octane is not better, does not increase performance, does not make the car start easier (just the opposite), does not burn hotter or generate more energy...that number only reflects the fuel's ability to resist detonation under pressure and heat (without a spark) and nothing else.
[/quote]


That pretty much sums it up...

also... that means that diesel has a really low octane rating since diesel engines dont use spark plugs... in case you were wondering.
 

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[quote author=as1an_1nvas10n link=topic=139366.msg3003522#msg3003522 date=1237436525]
also... that means that diesel has a really low octane rating since diesel engines dont use spark plugs... in case you were wondering.
[/quote]

That's probably a joke, but I'll say it anyway so no one gets confused: diesel actually has much higher resistance to ignition than gasoline.
 

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it's not a joke really. diesels don't use a spark plug...combustion is triggered by heat and pressure alone - the heat being generated when the air is compressed (although most use a glow plugs to warm things up first if the engine is cold)
 

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[quote author=John_in_the_LBC link=topic=139366.msg3005318#msg3005318 date=1237506903]
it's not a joke really.[/quote]

You're right. Sorry about that, as1an_1nvas10n. Some quick research tells me that diesel does have a low octane rating (15-25). I didn't know they even had octane ratings for diesel; I was only familiar with cetane ratings. So he's right about that, and of course he is correct about the lack of spark plugs in a diesel.

I'm going to have to do some more reading. How can a fuel have a low octane rating yet be so much less volatile than one with a high octane rating? You can blend diesel into gasoline and run it in an old non-emission-controlled engine without damaging anything. Imagine trying to burn gasoline in a 21:1 compression diesel engine!
 
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