running a higher octane in outboard

[mmadge 2]

Would running 91 octane gas in a 6HP 4 stroke Johnson outboard motor that is recommended to have 87 octane do any damage?

 

Thanks

[Adrian 132]

It won't do any damage, but I would ask why?  If you are just wanting to use the same fuel as another motor you already have, no issue.  There is a little more refining to get the 91 fuel. In this town, there is not a lot of 91 sold, so I would have concerns about the fuel in the ground at your favourite station.  It may be sitting around for quite a while when you buy it.  It will not be as fresh as 87.  The difference between an 87 and 91 octane in the engine is that the 91 octane fuel has a greater resistance to ignition.  It takes more heat, higher compression and hotter spark to ignite.  If your engine was designed to run on 87, the compression ratio, timing, and jetting are adjusted for that fuel.  91 is designed for higher compression engines.  All that being said, you will not do any damage to your engine.  In older engines, it may be a little harder to start cold.

[mmadge 2]

Thanks!