Tim H asked: All gas stations where I live (Massachusetts) use 10%
ethanol in all grades of gas. I want to know if anyone knows if this affects the
octane rating at all. Ethanol has an
octane rating of 110 so if they used 90% 87 octane and 10% 110 octane it would actually bring the octane to around mid grade level. I want to know if this is true or if the gas companies compansate for this and start off with even lower
octane gas before they add the ethanol. If they dont then I should be able to run mid grade gas and get the full effect of premium.
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It’s negligible due to the octane rating being determined is produced after the additive is placed in the gasoline.
The equivallent of 90% 87 octane and 10% Ethanol (a corn-derived alchohol) will yield around 89-89.3 Octane. The major benefit of the ethanol is that it burns much cleaner. I was driving in Iowa for 7 years before Texas, whereas Iowa used the 10% also. I always ran the Ethanol-89 (3-4 cents cheaper than 87 anyway) and never had any problems with dirty valves or spark plugs.
You won’t get the performance of 92/93 Premium because the Octane yield of the premium is still higher. The octanes shown at the pump are the final yield, and if 87 is listed, saying it uses 10% Ethanol, then yes the starting gas-only rating is around 84.5. The same senario for the premium.