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Re: 91 octane gas in a small engine?


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Posted by MarkB_MI on May 17, 2013 at 18:59:05 from (75.198.21.166):

In Reply to: 91 octane gas in a small engine? posted by Stick welding on May 17, 2013 at 17:44:59:

Many small engine manufacturers specify "91 octane" fuel. What most folks don't notice is that the spec is actually 91 Research Octane Number (RON). The pump rating in the US in Canada is NOT RON but rather the average of the RON and Motor Octane Number (MON). 91 RON is more or less equivalent to 87 octane in the US. In other words, regular unleaded gas.

Note that octane has ABSOLUTELY NOTHING to do with the long-term stability of gasoline. It is strictly a rating for knock resistance. It's true that 100LL avgas is quite stable, but that has nothing to do with its octane rating. Rather, avgas must have a Reid Vapor Pressure rating of no more than 7 psi. Auto fuel, on the other hand, may be as high as (I think) 14 psi RVP, but that would be unusual. Refineries raise RVP in the winter and lower it in the summer.

There are a few ways to get low RVP fuel. The surest way is to use avgas. You can also purchase the ready-mixed power equipment fuel in a can. Lastly, if you happen to live in a state or metro area with specific RVP requirements (such as California), your pump gas will comply with the local requirements. You can find those local requirements at the link below.



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