I was filling up at the pump today and thinking about projects where people have "tuned" their car and necessetating the use of 93 octane fuel. The station I was at only had 87,89, and 90 with no ethanol. Made me think that you'd need to make sure you can get the high-test when you're low.

Anyway, I understand that most fuel with 10% ethanol doesn't have the same energy content as ethanol free gas. I wondered if anyone here knows if the ethanol is also a factor for "tuned" cars. Maybe 90 octane with no ethanol is as good as 93 with 10% ethanol?

 

Pure ethanol has an octane rating of 100. This is higher than any commercially available automotive gasoline. See below:

Ethanol boosts the octane number of fuel, which helps prevent pre-ignition knock. ... The octane rating of pure ethanol is 100. What's interesting is that when ethanol is blended with gasoline, it performs as if its octane rating is 112, making ethanol a very effective octane booster when used in gasoline.