No. The octane rating can not change from what is posted on the pump, by state and federal regulations… Alcohol burns a bit slower, so it actually inhibits knocking a bit. And it absorbs water so it will scarf up any water droplets in the fuel tank or system and carry them right on thru the motor. (edit: alcohol does contain only about 70% of the energy of the same volume of gasoline, but octane rating is not about energy content, it is about knock-resistance/ping-resistance. As a result, you are giving up about 3% in energy content (horsepower) in addition to the hot / humid summer airs impact.) It is also a bit harder on various seals that are intended to withstand gasoline, but are less effective with alcohol. newer motors have seals/gaskets/o-rings that work fine with either. Very old motors dont fare as well. I am working on a friends 40 year old outboard right now and am replacing every seal/gasket/o-ring in the carbs as some are soft/gummy, although whether this was caused by ethenol added gas or not is unknown. Weve been using the “oxygenated fuels” (aka as 10% ethenol) in the Summer for years here in Birmingham. I have a 92 merc XR6 that has had zero problems with it. I doubt you will experience any issues. Just dont fill up with E85 as the motor will not run on that at all… If you dont run a racing-division motor, 87 octane will do the trick. Running 89 to 93 is a waste of money. The combustion chambers on outboards are very conservatively set up so that they will run reliably on lower-than-87 gas for safetys sake. The only down-side is that the ethenol-added gas will absorb moisture, and that water-alcohol mix can behave like a corrosive liquid if you let it sit for months. Im not sure of the gas situation where you live, but we lose the ethenol additive once Fall gets here usually, and I always try to run the tank way down before refilling once we get out of the oxygenated fuel time period, to get that stuff out before it sits around for a couple of months with no activity.Last edited by oldtimer57 on May 21st, 2008, 5:14 pm, edited 1 time in total.2008 Pantera Classic2014 Mercury Pro XS 200
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