Pure ethanol has a very high octane, E10 gas is available in all grades (regular -87, premium -91, etc.)...
E10 gas (10% ethanol) that has phase separated from water absorption, will have a substantial drop in octane of 2 to 4 points.
E.G. After water contamination has occurred, a gasoline purchased with 87 octane will drop to about 83 to 84.
An octane number is a measure of gasoline's ability to resist pre-ignition, also known as "knocking" or "pinging".
Octane is a rating, similar to a ratio of measurement, not a substance chemical or product you can add to gas.
Determining octane measurements involves a complex methodology.
Gas is available at different octane measurements - E.G. 87, 89, 91 or 93.
Portable octane analyzers (testing equipment) is available, but their use by the public is limited due to a cost of several thousand dollars.
Contact us for more information on portable octane analyzers for use in E10 fuels.
"Gas Purifying Equipment" also is available, but like octane analyzers their high price tags often limit use by the general public.
While it's true that pure ethanol has a higher octane number, (reason ethanol is ingredient used most often in octane enhancer products),
E10 gas does not have an octane higher than conventional gasoline.
(Chose grade of fuel at pump for octane you prefer).
When fuel absorbs moisture the octane reading can dramatically decrease.
Fuel that has experienced water contamination (W/C) and phase separation (P/S) will decrease about 3 octane points...
Many refer to this as "lean fuel" or "diluted fuel".
If you purchased fuel with an 87 octane level, after W/C and P/S will decrease to only 84, (an unsafe level for most engines).
Proper management when using alcohol-blend fuels will avoid this problem.
Some people chose to purchase a higher level octane blend when purchasing E10 fuel - to prevent octane from reaching a dangerously low level if fuel contaminates.
To avoid wasting money on gas that will contaminate quickly always purchase high quality fuel, test for water and alcohol before purchase, follow necessary precautions, and refill tank often.
Replacing fuel in your tank every 1-2 weeks will usually limit the occurence of PS and WC.
Of course, there are other situations when irregardless of how fresh fuel is, the fuel will go bad - causing decrease in octane and other negative effects on ingredients contained in fuel.
Running on gas with the appropriate octane rating will offer best performance and increase the useful life of the engine.
Many engine owners that have experienced unnecessary and avoidable damage with E10 is often due to sub-normal octane gasoline.
View list of common problems and engine damage from E10 gasoline.
To maintain the octane rating listed at time of purchase, following simple ethanol precautions to protect fuel quality.
Precautions (http://www.fueltestkit.com/ethanol_engine_precautions.html) should be all you need to do to assure engine is running on stable, high quality fuel with an appropriate octane level, recommended by your engine manufacturer.