Octane Ratings

RobMcClain

RobMcClain

Member
I was talking with a fellow boater earlier this week and we got on the subject of gasoline. He has a vast boating background, including in-depth boating education, but what he said was new to me. His current boat is powered by a 135HP outboard. As we discussed motors and modern gasoline blends he said when it comes time to fill his boat's gas tank, and if he thinks it might be awhile before he is going to use the boat (like during winter), he will fill the tank with a higher octane gas. Instead of using a normal 87 octane he will use premium gas with an octane rating of 91 or 92. Because new gas does not hold it's octane rating as long as it used to, he fills with premium higher octane so when he finally uses the boat the gas will still have a good octane rating. His concept is that newer gas loses ~1 point octane rating per month, so in 2 months an 87 octane will drop to 85, while a 91 will drop to 89 but still be plenty powerful. I am probably not explaining this very well, but I've never heard this sort of assessment before. I do understand that the new blends deteriorate much quicker than blends made in the past, but is his analysis in the ballpark?

Rob
 
Rob-

I've heard that, too, but without explanation.

Probably is mostly caused by the amount of gas that has been produced by catalytic cracking, where longer molecules are broken down into shorter ones under high pressure and heat in the presence of a catalyst (usually platinum).

Some of the resulting shorter molecules are less stable than regular gasoline molecules that have survived millions of years in the earth (the less stable ones would have deteriorated and re-combined into stable ones over time).

Thus cracked molecules should be less stable to begin with, and their derivatives even more so, at least in the short term.

This deterioration of cracked molecules results in gasoline that cannot be compressed and heated without detonating from the heat of compression compared to a gas with a higher octane rating.

Octane rating is simply a measure of how much a gasoline can be heated and compressed before it detonates.

Higher octane rating gas is intended for use in higher compression engines and those operating at higher temperatures.

I don't have any definite clue as to wether the presence of ethanol in E-10 would effect the stability or octane rating over time.

Some of this is theoretical, and off the top of my head, and I may be wrong, but it's worth a stab in the dark! :lol:

Joe. :teeth :thup
 
Also, I was told when I purchased some Stihl power equipment by our local dealer.... the premium gas at many of the gas stations around Stanwood(he listed some) don't have ethanol at all. Im not sure how widespread that is or if its even true, but I spend a little more for the piece of mind. I still add Sta-bil anyway just in case!
 
For a list of gas stations (regardless of octane) see:
http://pure-gas.org/index.jsp?stateprov=WA

I don't see any gas stations in Stanwood on the list. (but there may be some other city that these stations are in) I would want proof from the station before believing a counter clerk that the high octane does not have ethanol.

I have always used what was the lowest octane at the pump for my outboard. Many of the marine fuel depots will have higher octane, but that is what they have, and so I use it. Never any problem starting an engine after sitting 6 to 8 months if the fuel is treated properly.

I have only had two cars which used higher octane fuel. There is a reason for those specific cars to use it.
 
Thanks for the link Bob! Now saved in my list of favorites. Looks like I have a couple places to fill up on my way to our usual launches.

That clerk must've been feeding me a line :dog
 
The old car restoration web sites are pretty adamant that the ethanol is an octane enhancer but as you get phase separation and the engine is burning more raw fuel, (less ethanol), that the raw fuel has a significantly reduced octane rating. I believe that explanation. My wife's ford mustang convertible will ping rather dramatically after 4 months of winter storage, even with stabilized fuel additives. So, we've been putting it away low on fuel and immediately fuel it with fresh gas, then, no pre-ignition for the rest of the convertible season.
 
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