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      11-27-2010, 05:28 PM   #1
JohnnyT
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89 vs. 93 octane

Here in Central Jersey, from what I see most mid grade gas is 89 rather than 91. Didn't 91 exist years ago??

Anyway, I'd been using 89 up until last week when I put in 93 and I must say, my car runs noticably MUCH smoother with this octane.

I here that 93 is necessary for more high performance cars like the 335 (I could be wrong on that), but I'll never use anything less than 93 again. If I see 91 around I might try it but I doubt it.

Don't know if other 328i owners experience the same thing.
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      11-27-2010, 05:32 PM   #2
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I always use 93/94, unless some boonie gas stop doesn't have it.
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      11-27-2010, 05:43 PM   #3
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91 is super. Some stations like the supermarket or cost club gas is actually 92 for a 93 price.
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      11-27-2010, 05:46 PM   #4
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I never see 91 anywhere. It always 87, 89, and 93. But I can alternate 89 and 93 and end up with 91 I guess.

Does the 335 require 93/94?? or 91??
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      11-27-2010, 05:47 PM   #5
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91+ since 91 is the highest you can normally get in CA.
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      11-27-2010, 05:49 PM   #6
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335 requires 91, which makes it awesome when you're in Northern Texas and they only have 86/88/90.
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      11-27-2010, 05:54 PM   #7
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The engines of these cars are tuned for American 91 octane because "regular unleaded" in Europe is 90-91 octane. Anything but 91 octane (including 93 octane or 87 octane) will cause a reduction in every measurable performance charichtaristic of the car.

Americans use the Anti-Knock Index, and Europeans use the Research Octane Number to measure the octane of their gas. Regular unleaded in Europe is therefore 95 octane because they use a different scale.
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      11-27-2010, 05:55 PM   #8
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Quote:
Originally Posted by Legion5 View Post
The engines of these cars are tuned for American 91 octane because "regular unleaded" in Europe is 90-91 octane. Anything but 91 octane (including 93 octane or 87 octane) will cause a reduction in every measurable performance charichtaristic of the car.

Americans use the Anti-Knock Index, and Europeans use the Research Octane Number to measure the octane of their gas. Regular unleaded in Europe is therefore 95 octane on their scale and 91 on ours. 91/95 is regular in Europe as I said.
Disagree. 93 does not cause a reduction in performance. That's simply um, wrong.
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      11-27-2010, 06:12 PM   #9
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Quote:
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Disagree. 93 does not cause a reduction in performance. That's simply um, wrong.
93 causes a reduction in performance. If you are saying that's "simply" wrong then you don't have a good understanding about octane and it's effects on the engine.

The octane number basically represents the ratio of gas to anti-detonation agent. The higher the octane number, the less gas there actually is in your gas, and the more it's diluted with anti-detonation agents. The reason higher octane can produce more power is because overall running the engine with a more aggressive tune makes more power than diluting the fuel and therefore cutting the amount of energy in it.

The only possible way you'd make more power with 93 octane is if the car had adaptive tuning, which is where the engine tries to make the tune as aggressive as possible by using sensors to constantly adjust the way it runs. Most cars have adaptive tuning that helps them run on worse gas without causing engine detonation, which is called negative adaptive tuning, but very few cars feature positive adaptive tuning due to interfering with the EPA certification process and making it too complicated and uncertain.

Sometimes there is a rare case where running 93 will improve performance of a car without positive adaptive tuning. That case is when you're running in abnormally hot climates (like a desert) or for long periods of time where the car has experienced heat soak which makes your fuel more prone to detonation and so the car will automatically apply a negative adaptive tune (in the past the car would just run pooly when too hot). Running higher octane will cancel out the negative adaptive tune in these rare cases, but really you'll mostly have this issue on hot summer days in the open in places like Las Vegas and rarely anywhere else.

The explenation for why 93 Octane mayhave increased performance is pretty complicated as you can see, and 93 should reduce performance because the engine just sees it as diluted 91.

Last edited by Legion5; 11-27-2010 at 06:20 PM.
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      11-27-2010, 06:19 PM   #10
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Quote:
Originally Posted by Legion5 View Post
The higher the octane number, the less gas there actually is in your gas,
wow

Granted, if your car is designed for 87, and you put 93 in the tank, you're burning $$$ out the tailpipe. But 93 in a 335 does not reduce performance.
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      11-27-2010, 06:32 PM   #11
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So would I be correct in assuming that mixing equal parts of 89 and 93 would get me the optimal 91??

Or is there a scientific reason why this would not be the case??
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      11-27-2010, 06:37 PM   #12
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Quote:
Originally Posted by John 070 View Post
Quote:
Originally Posted by Legion5 View Post
The higher the octane number, the less gas there actually is in your gas,
wow

Granted, if your car is designed for 87, and you put 93 in the tank, you're burning $$$ out the tailpipe. But 93 in a 335 does not reduce performance.
I know most ecu aftermarket tunes are octane specific, if you use 93 they will tune to 93, if you use 91 they will tune to 91. In such case using 93 has benefit over 91 because they can tune it more aggressively for you.

We only have 91 in Norcal. Recently Costco started to put "10% ethanol" labels on their pumps, I asked a manager he said it was for the 87 only. Hopefully he was correct because ethanol not only reduces the octane rating, it may cause other problems.
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      11-27-2010, 09:37 PM   #13
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Quote:
Originally Posted by John 070 View Post
wow

Granted, if your car is designed for 87, and you put 93 in the tank, you're burning $$$ out the tailpipe. But 93 in a 335 does not reduce performance.

The previous poster just gave a pretty good explanation as to why it does reduce performance.. Do you have any explanation as to why it wouldn't other than just saying it doesn't? Or is just something that is a "gut feeling?"
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      11-27-2010, 10:19 PM   #14
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Long-winded false information is "a pretty good explanation" to you? Welcome to the ignore list.

The only thing octane has anything to do with is the ability to resist pre-detonation. In many places, including where I live, the choices are 87, 89, and 93. To imply that putting 93 in the tank reduces performance is just plain wrong. But someone who buys into it is stupid.
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      11-27-2010, 11:30 PM   #15
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Quote:
Originally Posted by John 070 View Post
Long-winded false information is "a pretty good explanation" to you? Welcome to the ignore list.

The only thing octane has anything to do with is the ability to resist pre-detonation. In many places, including where I live, the choices are 87, 89, and 93. To imply that putting 93 in the tank reduces performance is just plain wrong. But someone who buys into it is stupid.
I think it is safe to say that since BMW tunes our cars to 91, using 93 will not offer any benefit unless you do an aftermarket tune that is tuned to use 93.

Since 93 has less gasoline content than 91, you will get lower gas mileage but gains no other benefit.
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      11-27-2010, 11:40 PM   #16
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The higher the octane the more performance you get...if you have a tune.
Race gas is 100+ octane. I dont think 93 vs 91 will make a huge difference. you are not going to get LESS performance from 93. If anything it will be the same.
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      11-28-2010, 12:14 AM   #17
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Since 93 has less gasoline content than 91, you will get lower gas mileage but gains no other benefit.
In my personal experience, this is not the case for me. By no means do I declare myself to be correct, but I believe that the ecu adjusts timings and such (I don't really know that much about engine intracacies) so that to create the same amount of power with 91 octane, the engine must rev higher compared to 93. As a result, I generally get lower gas mileage with 91 vs 93.

Please go ahead and correct me because I am no expert on this; I just wanted to add my input to this very interesting discussion.
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      11-28-2010, 12:32 AM   #18
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Quote:
Originally Posted by hmmcolor View Post
335 requires 91, which makes it awesome when you're in Northern Texas and they only have 86/88/90.
Actually, 91 octane is HIGHLY recommended by BMW, but they say you can use 89 with no effect on engine life (but you might hear knocking sounds when in warmer weather).

I could probably get away with using 89 where I live since the weather is not going to be getting any warmer than the 50's, but I'm going to stick with 91 just cuz the extra anti-knock give me a warm fuzzy feeling.
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      11-28-2010, 12:33 AM   #19
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I've always found that any of my cars run better with higher quality gas. Just... better. Not faster, not higher mpg's, but better. I'm not really sure what it is, but it's not as sloppy, the engine is more accurate. I can't really describe it.
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      11-28-2010, 12:44 AM   #20
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One of the big tuners mentioned it was good for 5-10 hp stock->91 to 93 octane on an N54.
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      11-28-2010, 12:46 AM   #21
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BMW's ignition system is adaptive. It will advance spark timing until it detects pre-ignition, then back off a tiny bit.

Therefore, it will advance timing until it sees pre-ignition with 91 octane fuel, for a given air pressure and air temperature. Same for 93. Or 89 for that matter.

Cool the ambient air some (e.g. winter), the DME will advance timing a bit.
Increase octane a some (i.e. go to 93 octane), the DME will advance timing a bit.
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      11-28-2010, 12:53 AM   #22
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i think you guys are crazy.

91 vs 93? and youll get reduction in performance?

sir you are wrong.

higher octane=more power. dyno for dyno, youll get more hp on 93 octane rather then 91. thus the reason for using high octane gas(race gas 100+) at the track or high dyno runs.

jb3 has higher octane maps which will give you more power. your not getting maximum performance with 91.
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