Originally Posted by George

There is a germ of truth in what you are saying, but some of the gaps need to be filled to understand the issue completely.

Octane is a hydrocarbon compound with 8 carbon atoms, hence the "Oct" prefix.
Heptane is a hydrocarbon with 7 carbon atoms, hence the "Hept" prefix.

(for petroleum engineers, yes there are more technical names for these, but let's keep it simple!)

You mix these two compounds to get standard fuels of various octane ratings. 50/50 heptane/octane gives 50 octane, 20/80 gives 80 octane, and pure octane is 100 octane.

These chemicals are rather expensive and you wouldn't want to bear the cost of running them in your car.

Fuels are tested in a test engine. The test engine is actually run by an electric motor for consistant speed. The neat thing about the test engine is that the head can be screwed down to vary the compression ratio. The engine is run on the fuel to be evaluated and the head is screwed down until the engine starts to pre-ignite or "ping". The setting is noted, and then the test is repeated with various mixtures of heptane/octane ("standard fuels") until the same point of pre-ignition is reached. The percentage of octane in the standard fuel that produces the same pinging behavior is the octane rating of the fuel.

What is pre-ignition? Simply having the fuel ignite all by itself without the intervention of the spark plug. You don't want this to happen, as the fuel burns all at once, causing very high pressures in the combustion chamber which can break things. The higher the octane rating, the more resistant it is to pre-ignition, and that is really the only difference that octane makes. High-octane fuel has a slightly smaller flame propogation speed than low-octane fuel, but not enough to make any difference in a real-world engine.

Toyota specifies that they have designed the Scion engine to run on 87 octane fuel. If that is truly the case, you will not get any advantage in running 91 octane fuel.

Nevertheless, some claim that they have better performance or mileage with 91 octane. How can that be? Well, one way this could happen is if they had a tankful of "bad" 87 octane that was actually lower in octane. The engine's computer would adjust the engine to run on the lower octane fuel, resulting in lower mileage and performance, but not doing anything permanently bad. When the person fueled up with 91 octane, the computer simply readjusted to the normal 87 octane specs and produced better performance and mileage.

Another scenario is that Toyota pushed the envelope a bit with the engine computer, which means that the engine actually does benefit from higher octane fuel. There's nothing magical about the number 87 and they might well have designed it for 87.5 or 88.

How do we tell for sure? All we need to do is to get a Scion, put fuel of a known, measured 87 octane in it, and run a set schedule on a test track. fuel the same car up with 91 octane and run exactly the same schedule on the same track. The results will be quite clear.

Another way to do it is for someone to keep long-term records of a vehicle in regular commuter service. The quirks in the data will tend to average out, so comparisons can then be made between the fuels.

For those who feel that one brand of gasoline is better than others, be aware that all the refiners freely trade gasoline back and forth with each other to meet the needs of their retailers. That "high quality" Union 76 fuel you are putting in your tank may well have come from an Arco refinery!

George

Originally Posted by doctorcue

I'll pony myself up for this. I drive 150 miles a day for my commute. Generally not as much traffic as I leave later in the day. I'll try to drive conservatively and see what I get with 87 octane & 91 octane. Unfortunately, it is a rough science as any variation in the following can affect results:

Different gas stations
Different pumps
Different weather conditions

In any case, I'll fill up with Chevron 87 then Chevron 91. I'll try to use the same pump so when filling up the automatic stop will give me similar results in volume.

I'll probably be able to reply to this next week. My traveling is going to be all over the map this weekend, so now wouldn't be a good time.

To be honest...I doubt I will see much difference other than about $2 more in fuel costs.

Originally Posted by yanges

as for octane and power, i noticed when i had my 96 Sable [6 cylinder] that the engine felt stronger with a higher octane fuel....i could feel the difference in acceleration between the two....

in my xB, i have been using 89 since i got it thinking i might get a [b]little[/]more power from a higher octane gas.....

i would like to try 87 but i am concerned about starting to use it now since i have been running 89 all along....

any thoughts on this would be appreciated....

Originally Posted by DenZinz

This could be mental but the only thing i have noticed is that when i stopped using 87 and went to 89 the engine idles smoother. At 87 it always seemed like it wanted to stale. After all at 10.5:1 compression seems aweful high to be running cheap gas. Yes I know its tuned for 87, but geez, 10.5:1. Does timing affect it that much that you dont have to run super? Especially under heavy throttle?

I do agree with Darren. Stay away from ARCO, or at least the one by my house. My cars run horrible on it.