Mid-Grade Gasoline (89 Octane)

[tach18k]

My Prius like sthe 87, 89 wasnt as good on mileage. My F150 ran great on 89.

[LegacyGT]

I have to drive 20 miles out of my way to find 93 octane ... unfortunately only two stations carry it here, and 93 is what I tune on when I have a track event.

My wife has a Prius and it runs much worse on mid-grade 89 (with 10% ethanol) than standard straight-up 87. It is a 10MPG difference ... which is a big deal in a hybrid.

[Chase.Barnett]

It is a 10MPG difference ... which is a big deal in a hybrid.

And an even bigger deal in anything else...

10mpg less? Geesh.

[crxman2010]

Our three primary options are:

Unleaded 87 octane $3.55 (guess from gps app)
Super Unleaded 89 octane (10% corn) $3.35 (guess from gps app)
Premium 91 octane $3.75 (paid tonight)

We run 91 in all of our cars even if they do not call for 91. By our calculations, the percentage increase in mpg is greater than the increase in cost. Ex. I think 91 is generally about 5% more expensive then 87 but we get about 7-9% better gas mileage. I don't touch anything with corn in it because I do not appreciate the decreased mpg.

Also, it is my understanding, anything with ethanol in it has a decreased shelf life and during the summer my car can sit for 1-2 months with next to no miles, so it makes premium more of a no brainer.

[BrightonRT]

First to answer the OP's question, Chrysler recommends 89 in both of my Hemis, and like many people have said best bet is to run what the manufacturer recommends or what your engine is tuned for. Second, to clear up a couple myths:

First, octane rating is a measure of a fuels resistance to auto-ignition which causes spark knock. Nothing more, nothing less. It doesn't burn any faster or slower. My understanding of how it is measured is there is a standardized engine set up that the engine is run in, the spark advance on the engine is set at the minimum possible without knock. The engine is than ran with a mixture of octane and some other standardized fuel that I don't remember and the proportion of octane to the other fuel is increased until the same result is achieved and the proportion of octane is what leads to the octane rating.

Second, every engine design has a set spark advance measured in degrees of crankshaft rotation before the piston hits the top of the cylinder head. The optimum point is different for each engine and depends on anything that can change temperature in the combustion chamber. That could be compression ratio, presence of turbocharger or intercooler, intake, exhaust, whatever. Ideally the sooner ignition the better for complete combustion and the most power per event. The downside to that is earlier combustion creates more heat and the potential for spark knock, hence higher octane fuels.

What a knock sensor does in an engine is if it senses knock it retards or delays ignition to protect the engine, but nets reduced performance. Some engines can learn if a higher than recommended octane is used and adapt to take advantage, but not many.

So E85 hurts in 2 ways, it makes the engine run less efficient due to the spark retardation and secondly but ethanol has less energy per gram than regular gasoline. So if you have a Flex Fuel engine you need to watch prices because it may not be an economical advantage.

Ok, end dissertation.