What octane is the best for a thirdgen. We have 95, 96 and 98.
I use 98 unleaded but im not sure whats the best for the engine. I know you guys in the US have a lot more different octanes. What do you use?

 

Scania,

We may be talking apples and oranges - it might not be a direct comparison. There are a couple common means of measuring fuel octane. One is the Research octane method, the other is the Road actane method. The Research method analyzes the fuel at a molecular level to determine the hydrocarbon content, flash point, burn temperature, and burn rate from a more scienfitic perspective. The presumed volatility and burn rate are associated with a numbered scale to assign an octane rating.

The Road method rates the fuel as tested under actual conditions in a standard test engine. Pressures and spark advance are adjusted until a detonation is detected. At some combination of timing and cylinder pressure, a number is applied to the scale to rate fuel performance.

Through a comparison of these two methods, the research octane method generally produces higher rating numbers. Most states in the U.S. require that octane numbers are posted as an arithmetic mean, or average, of the two rating methods for consistency in advertised numbers. The posted number most be the lowest average octane of the fuel in that tank/pump. From working at service stations (they USED to be called "service stations" because actual work was done there), I know that periodically the actual octane of the fuel delivered is higher than the posted numbers. Station operators find it much easier to update price changes than current octane ratings.

Back to your question, the relatively high numbers you're finding in Scandinavia may be due to the rating method used. Regardless of the actual number, you should use the fuel with an octane rating that allows you to tune for the greatest power output, but none higher. There is no harm in using a higher octane, other than the burn rates tend to be a little slower and fuel efficiency will be reduced. If you can tune your engine to run well and produce good power with no EST spark retard at the lower octane ratings, you should use that fuel. Using a fuel with too low an octane will require spark retard, which costs power, efficiency, and tends to create more waste heat.

Having said that, common octane rating I find in the midwest are 87, 88, 89, 90, 91, 92, 93, and 94. Places that sell dedicated racing fuels go much higher (98 or more) but charge almost twice as much per unit measure.

 

I think 94 is the highest I can get at the gas station around here. I guess you could get higher at the track. I just use the 94.

 

Thank you very much mr Vader for tkaing time to write that answer.

 

no a very direct answer, but you'd want to run the lowest you can with out getting knock (or knock counts if you have a scanner).