It appears that pure ethanol has an octane rating of 113, but. This gets a little complex because of its vapor pressure and rapid absorption of water. It will enhance the the octane rating of gas, but the other complexities make this benefit change rapidly. All fuel systems today are designed to handle ethanol and octane ratings are what they are. The key is to use the proper octane rating for your application. I am sure harley recommends 91 or better because they can be reasonably assured that the stock bikes will not ping with this rating. If you have changed the A/F ratio, compression, exhaust system and other things your bike may do fine with different octane ratings. Just look out for the pinging, since this is not good for your engine. An octane of 103 would not provide any significant benefit, the lower you can run the better.

 

[Cars have a knock sensor that will retard the timing to prevent pinging so they can handle lower octane, but HD's do not to my knowledge have a knock sensor]



Can you imagine the life that a knock sensor would have on a Harley with all the beatin', bangin', shakin', and rattlin' going on in one of these things!!

 

GREAT point NChogfan!

 

The need for higher octane decreases with increasing altitude. Where I live (at about 400 ft. above sea level) 91 octane gasoline is essentially non-existant, so I use the commonly available 93 octane. The commonly available octanes move from 87, to 89, then 93. As altitude increases, lower octane will suffice and it is commonly available at 91 octane. At really high altitudes (above 10,000 ft) you might see only 89 as the high octane. While, as pointed out by other posters, 91 octane is recommended for H-D engines, that is an average number and lower octane at higher altitudes will suffice.

 

Good point Mr. Miller. Changing altitude less air causes a richer mixture which is less prone to detonation. Can use a lower octane rating.

 

If you look at the octaine reading on the pump, you will see that the label reads MINIMUM OCTAINE RATING. So if a fuel reads 89 octaine, it is 89 octaine at it's lowest level. It is more than likely 90 to 91 octaine. The same is true for the other octaine ratings unless your gas station is using other additives to stretch their ground supply. For example, certain chemicals are used in ground tanks to reduce evaporation, and other additives to encapsulate water (caused from condensation).
Every bike is different amd performs differently based on a number of perameters (heat, humidity, atmospheric pressure, altitide, etc). I use 93 Octaine beacuse it is plentiful here in Atlanta. I was on a trip once where the 93 octaine was not available, so I had to use 89. My bike really ran rough, backfiring and stuttering. When I got to a place that had 93 octaine, I filled up and added an octaine booster and it ran fine after that.
Dwayne

 

Actually, the Delphi ECM does have a knock sensor and will retard timing if/when it detects pinging.

 

07RoadHawg do you know what year the knock sensor was introduced on the Delphi EFI system? I have EFI on two bikes, a 2007 Ultra and a 2008 RG.

 

Cool Hawg, did not know that.

 

I let my brother ride my 09 Classic one day to try it out and he put 87 octane in it as he didnt know any better. When I got it back it was knocking bad every time I accelerated. I thought something was wrong until I thought about he filled it up and asked him what he put in it. Replaced that fuel with premium and the problem went away. Never ran anything but premium since that day.