Proper octane for a 78 merc 115?

what200

Petty Officer 2nd Class
Joined
Jul 12, 2001
Messages
162
Re: Proper octane for a 78 merc 115?

87 octane for any outboard.
No, no, and no.<br /><br />Merc inline sixes are very octane sensitive. In fact, merc switched to lower compression pistons in later years when gas got crappier.<br /><br />93 octane of inlines. Especially the 140 and 150's.<br /><br />Also 93 octane for any merc hipo motors.<br /><br />Also if you let the gas sit for any length of time I would use 93 in anything else as gas can lose octane very quickly in sunlight or high heat conditions. Also the shelf life of gas is only around a month. To me I'd rather spend an extra buck on gas on not worry about it melting my $$$$ motor.
 

JB

Honorary Moderator Emeritus
Joined
Mar 25, 2001
Messages
45,907
Re: Proper octane for a 78 merc 115?

Moving this to Outboards General. It will get long and long-winded if history is any predictor.<br /><br />My position is that unless you have an Owner's Manual that tells you to use premium, use 87.
 

what200

Petty Officer 2nd Class
Joined
Jul 12, 2001
Messages
162
Re: Proper octane for a 78 merc 115?

JB - I agree with you on many outboards or cases. But some OB's need more.<br /><br />I know this has been rehashed many times, but obviously not enough.<br /><br />Anybody that recommends 87 octane for a 70's merc inline doesn't know what they are talking about, unless they are a wiseco salesman.
 
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