Gasoline Grades?

Question

How high octane supergasolines differ from premium mid and regular low octane gasoline in general?

Answer 1

The only difference between grades of gasoline (in general) is the amount of anti-knock compounds they have in the gasoline.

Premium gasoline has more resistance to knock than mid-grade or regular gasolines.

Your gasoline powered engine only needs the grade of fuel required to prevent the engine from knocking.  You find that information in your owner's manual.

If the owner's manual says you can safely run your car on regular gasoline and premium fuel is not required and your engine does not knock or ping on regular fuel, then you do not need mid-grade or premium gasoline and you are wasting your money by purchasing that fuel.

Some stations put additive packages in their premium fuel that do not appear in the lower grade fuel.   That practice isn't as prevalent any more.  Chevron used to have Techron in only their premium fuels, but now provides it in all grades of gasoline.

Answer 2

The term "high octane supergasolines" has no legal or engineering meaning.  If a gas station advertises one, just look at the octane number on the pump.  If it's 91 or 92, the claim is bogus.  It's just premium gas with a fancy name.

Dan H lays it out pretty well.  Octane is a fuel's resistance to preignition or detonation, the tendency to start burning before the spark fires, which would be damaging to your engine.

The number is a percentage of resistance in comparison to a standard fuel, called "octane."  If the fuel resists preignition 10% better than octane, it has an octane number of 110.  If it's 10% worse, its octane number is 90.

One of the ways of improving the efficiency of an internal combustion engine is to raise the compression ratio.  That's because the "expansion ratio" -- the difference between the volume before and after combustion, is higher with higher compression.  That's easy to understand, since obviously the pressure would be higher in the cylinder.

This not only improves power, but since it takes the same amount of gas to cause a higher cylinder pressure, it also takes less gas to give the *same* performance.  So higher compression engines give better fuel economy than low compression engines of the same power.

The result of all this is that while a modern engine will protect itself from damage caused by using a lower octane gas than the manufacturer recommends, it does so by advancing the spark (cause it to occur later in the power stroke) and richening the fuel mixture (more gasoline in the incoming fuel mixture).  Both of these strategies reduce efficiency, burn more gas, and lower performance.

So don't think you save money by using cheaper (lower octane) gas.  It actually lowers your gas mileage.

The other issue, whether higer octane gas will help a car that is designed to use regular, the answer is no.  You are applying a remedy to cure a problem you don't have.  It's like giving PowerAide to a couch potato.  It won't hurt, but it won't turn him into an Olympic sprinter either, because his slow performance is not caused by the problem the sports drink addresses, like dehydration or electrolyte depletion.