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      01-11-2013, 12:59 PM   #31
vdubturbo
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Drives: 2011 535i
Join Date: Nov 2011
Location: Virginia

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I see these threads pop up from time to time on various forums and they annoy the crap out of me for a couple reasons. Believe it or not, there IS science and engineering behind the recommended octane rating. Because most people don't understand what octane really is, the discussions always revolve around the periphery of the topic. So I'll explain and you can chose to listen or not... I taught internal combustion theory and application at a university, so I feel somewhat qualified to open my mouth (though people rarely care)...

Octane is generally defined as a fuel's resistance to preignition and detonation. To the layman, this is it's resistance to pings and knocks. The higher the octane rating, the more resistant a fuel is to being ignited by the temperature and pressure within the cylinder (and not the spark plug). There is a tradeoff however... the higher the octane rating, the slower the fuel will burn (I'll get to that in a little).

Engines are designed with a certain compression ratio. The higher the compression, the more the fuel must resist igniting on it's own... thus a higher octane requirement. Turbocharges effectively increase the cylinder pressure by forcing additional air charge into the cylinders (duh). So while a turbo car might advertise a static compression ratio of 9.5:1, with the turbo is working that number may actually be considerably higher. This is why turbo cars generally run higher octane fuels.

Modern vehicles have sophisticated sensors to monitor the performance and safety envelope of the engine. Because there's a variety of factors that can induce preignition (other than low octane) like load, temp, ignition and valuve timing, or altitude, the engine's computer will adjust certain variables to keep the engine in a "safe zone"... e.g. no condition where preignition or detonation is present.

So here's why I think using lower than specified octane fuel is a bad idea: By using 87 or 89 instead of 91 or 93, you essentially force the engine into a state where it has to constantly correct for preignition or detonation. For the engine to do this, it must first DETECT the ill-advised condition. This means that some preignition must occur before the ECM can account for it. And because computers are logic based, it doesn't know what is causing the condition, so it will constantly check & adjust, check & adjust...

It was previously stated that no reliability problems are generally identified with short term exposure to this situation (which may be true), but I have to believe that over the long term, those conditions will take it's toll on the internal engine components. If anyone has ever seen what detonation can do, you'll appreciate how catastrophic it can be.

So in my opinion, NO, do not run lower octane fuel unless you're prepared to pay the consequences down the road. It's not a matter of "zippiness" or MPG's, it's a matter of wear. Yes, low octane gas will degrade the performance of your car... but that's a SAFETY mechanism, not a convenience because you are too cheap to purchase the correct fuel or wanted a more expensive car than you really could afford.

One other point... if you have a car that requires 87 and you run 91 because you want to "treat" your car, or get some more power... you're a fool. Running higher than the required octane will possibly degrade performance because it burns slower (as stated in my first paragraph).

Hope this helps. Flame suit on.
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