Re: 18HP Octane?
Compression ratio is fairly low on these outboards. I don't think I have ever seen a quotable figure as to what it would be, but when I say low I mean in the 7.75:1 area. Regular fuel is just fine, in fact it may make starting easier with lower octane. If the motor has a lot of carbon then the octane requirement climbs. Really though it depends on the refinery and time of the year. Around where I live, Shell uses up to 10% ethanol in the regular 87 octane (Bronze), 5% in mid-grade 89 octane (Silver) and no ethanol in the premium 91 octane (Gold). That would influence my fuel choice as much as the octane.
Try experimenting with different brands and octane ratings. You may be able to feel the difference in how it runs. Just using high octane for no other reason than expecting higher performance is just a waste. Only if your motor requires it should you always use the high octane unless you like to waste money.
