What happens if you use 87 octane gas? - AudiForums.com


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Old 03-23-2010, 04:53 PM
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Question What happens if you use 87 octane gas?

Hey guys,

I know we should be using 91 octane gas but I was just asking what happens if you run with 87?

The main reason I'm asking is because right when I got the car I put in 87 for about a month before I realised I was supposed to be using 91... What kind of damage could it have caused and what should I do to try and reverse any of that damage?

I use 91 now but I was just wondering if theres anything I should do to clean it out or something?

Thanks a lot
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Old 03-23-2010, 04:58 PM
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The ecu will retard timing and lower power for low octane gas.



The worst that could have happened was your plugs may have fouled out, but if it drives okay, you're probably fine. Don't worry about it, just keep using 91.
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Old 03-23-2010, 05:04 PM
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Lol one time the station was out of 91 or 93 so I had to use 87 to get me to the next station and My check engine light came on and car drove like pure hell
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Old 03-23-2010, 05:07 PM
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Yeah the car drives fine now. When I was using the 87 octane I noticed that the car sometimes had trouble turning over but ever since I swapped to 91 the car runs super smooth.

My other question is that where I live the 3 types of gas are 87,89 and 91...Is 89 good enough or should I really use 91?
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Old 03-23-2010, 05:09 PM
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use the highest octane possible. 93 would be the best (reasonably).
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Old 03-23-2010, 06:01 PM
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You'd only save about $1.50 for 89 instead of 91, why even bother?
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Old 03-23-2010, 06:02 PM
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Use whatever octane gas the cars engine is built for and the ECU is programmed for.

The octane rating only dictates how hard the fuel is to ignite, it does not dictate the quality of the gas. 87 is easier to ignite than 89, which is easier to ignite than 91, etc.

If you run higher octane than your cars engine and ECU are meant (what it's properly built and tuned for) to use, the engine will have trouble igniting the fuel: your fuel mileage will go down, you will lose power, and it will generally run worse (although it won't be greatly noticeable if the octane difference is small).

If you run lower octane than your cars engine and ECU are meant to use, your engine may pre-ignite the fuel which can severely damage the engine. This happens when pressures inside the cylinder are high enough to ignite the fuel before the spark plug actually fires (ie high rpm or high boost [or both] situations).

The moral of the story is run what the car is built for. Our cars are built for 91 (at least in California, it may be 93 in other states).
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Old 03-23-2010, 06:07 PM
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we have pretty good knock sensors so detonation isn't a huge issue, but yeah, run what it was designed for.

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Old 03-23-2010, 06:34 PM
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Thank you Mr. Education.

Run 93 and you'll be happy.
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Old 03-23-2010, 07:58 PM
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i used 87 for my S4 and it ran fine and my brothers A4 was fine too
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2009, 5spd, 87, 91, a3, audi, gas, level, lower, octane, quality, quattro, required, tt, type

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