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Author Topic:   Octane Rating
Al_A_Buy posted 08-29-2002 04:46 PM ET (US)   Profile for Al_A_Buy   Send Email to Al_A_Buy  
I have a 1985 Johnson 185 hp. The other day, a crusty old salt at the ramp observed me having trouble getting started (not related - a power pack had failed) and recommended that if I run premimum (91 octane) instead of the regular, starting and topend performance would be improved.

Any input?

WHALETEX posted 08-29-2002 07:31 PM ET (US)     Profile for WHALETEX    
My expierience in both outboards and other gas engines is the lower octane starts easier and idles better/cleaner than the high octane stuff. The high octane gas is blended to reduce its ability to ignite, thus improving its resistance to predetonation when used in high compression, high spark advance, and/or high load applications. This detonation resistance is not needed at idle or start and actually can make the engine noticably harder to start and idle rougher before the engine reaches operating temp.
David Ratusnik posted 08-29-2002 11:09 PM ET (US)     Profile for David Ratusnik  Send Email to David Ratusnik     
Al- Johnsons run fine on 87 octane. They love the stuff. If you are going to store the boat add higher octane for a few months since the octane degrades. Any problems starting are not related to octane. .03 David
Bigshot posted 08-30-2002 10:20 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
The reason why I used 93 was it had carbon guard in it. Now all grades have it so not an issue. I always run 89 or 93 but if you always ran 87 and it made it 17 years, can't go wrong there. Try some techron in the tank and maybe it will clear out any gunk in your carbs.
tbyrne posted 08-30-2002 04:07 PM ET (US)     Profile for tbyrne    
Here in CT, we have up to 10% methanol in our fuel. As I understand it, the highest octane fuel has more alcohol than the lower grades. More alcohol = more water in fuel. I would stick with the lower octanes and use a fuel dryer such as Zorb if I had water. I also would use a quality Racor fuel filter with a bowl you can drain.

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