I was wondering, what is the difference in using the different octane levels? And is there a difference in using the higher octane with the ethyl alcohol in it or whatever they put in it and the pure stuff? What will that do to the motor? With gas prices the way they are, it's just insane. And for those of you in Missouri, if you ever go to Illinois, it is about 15-20 cents per gallon more expensive. ARGH. Stupid gas taxes over here. I mean, I want to feed my MINI filet mignon and not McDonald's relatively speaking, but gee whiz, gas companies, give me a break.
Valerie

 

The octane rating is a measure of how easily the fuel with ignite under compression. All gas will ignite on its own if compressed enough. The higher the octane rating the less likely it is to ignite under compression. In a gas engine, you are compressing the air fuel mixture on the up stroke. If the fuel ignites before the piston is at top dead center it will try to force the piston down while the crank is still pushing up. That is what is called knock and very hard on the engine. For proper operation, the air and fuel is compressed, and the spark plug ignites the fuel at the right time and in a more even fashion for smoother operation. The MCS is supercharged, which compresses more air into the cylinder than normal, which is why you need fuel with an octane rating of 91 or higher to avoid knocking. The MC is not supercharged, but has a fairly high natural compression ratio and MINI still recommends 91.

The octane rating is just a measure of combustibility. It has nothing to do with what the gas is made of, so fuel with ethanol does not change that principle. However, on a different subject, ethanol has a lower energy content than gasoline. E85 cars get lower gas mileage, but the fuel is cheaper. A maximum of 10% ethanol can be used in gasoline without effecting other engine parts in a normal engine, so many gas stations add ethanol to reduce the amount of foreign oil used.

On a side note, modern cars have sensors that monitor for engine knock and have the ability to change the timing of the engine to get rid of the knocking. So you will not kill your car if you use a lower grade of fuel. The engine will compensate for it. But you will lose power and efficiency. So it doesn't really benefit you to go cheap on gas.

 

Brian,
Well, I do have an '05 MCS so I should probably stick with the higher level octane. Good to know that the ethanol probably won't bother it though. Sorry I said ethyl whatever, I do medical transcription all day and stuck that in instead of ethanol Gotta keep my terms straight. At the rate I'm reading about all this stuff I'll be an expert in no time. Is there a way to find out about the specs on my car without taking it to a MINI dealer to find out what all is on it? I have no idea and would like to know. I see everybody listing their car's specs and would like to have some idea of what I am driving.
P.S., followed the link from the Mini expert driving teacher link on our main club website to the Minnie Pearl website to see about registration costs in October for the Tennessee driving course, and they didn't have anything up on the Miinnie Pearl site yet that I could find. FYI.

Thanks!
Valerie

 

You can call MINI USA and give them your VIN number and they can give you all the options it has on it. 1-888-ASK-MINI

 

A couple of points of clearification:

- The point of ignition is almost always before the piston reaches TDC. This is because the fuel/air mixture burns at a constant rate, expanding out from the spark plug, and it needs to start burning as early as possible to more completly burn all the charge before the exhaust valves open. You want the burn to start but not build preasure too soon so the piston can reach TDC and be headed down when the big push from combustion kicks in.

Controling the point of spark relative to piston position is hugely critical. Knock is when the burn occurs not from the spark plug and causes preasure to build too fast/high before TDC.

- Ethanol has ~30% less BTU's/"power" than gasoline. 10% ethanol results in 3% lower gas milage. E85 yields ~25% lower milage than gasoline.