3037 Views 12 Replies 12 Participants Last post by  JFUSION
I know I read over and over that the levels of Octane used in cars these days doesn't matter. For the last 3 fill-ups I decided to try the mid-grade unleaded which is 89 octane. In my region you can get 87, 89 and 92 grade gasoline. Well folks, my MPG has actually gone up some to 27.2 average from 26.7. The most noticable is the throttle response. I do notice a more reactive throttle response. In order to see if this is just my imagination. I am going to switch back to 87 octane for the next 3 tanks and see.. Anyone else do this? or use a higher grade fuel in their Fusions/Milan? :?
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Modern cars have computers which can figure out what octane you are using. Up to a point, they will advance the timing more on higher octane gas, but only up to a point. You coult put 130 octane av-gas in it, and it would run crappy because the timing couldn't advance enough to start the gas burning.

Remember - higher octane means slower burning so the engine can fire the spark sooner (advance the firing) allowing a longer burn cycle which can (if the car is capable and most new cars are) yield higher power. Beyond the limits of the computer, higher octane is just a waste of money. I would believe 89 or even 92 would yield some better performance and mileage than 87, but somewhere between 89 and 92 the engine reaches its maximum advance capability and the rest, as they say, is wasted.
One thing that hasn't been mentioned here is that the higher octane gasolines frequently contain more of the cleaning agents and detergents than the 87 octane. You can literally "drive your engine clean" by using higher octane for a while. This benefit frequently makes the car run better, and for a while after you switch back to 87, you won't notice any difference.

BTW - in my old flying days we used 130LL avgas in turbocharged engines. LL means "Low lead" which, as posted here, is to lubricate the valves.
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