TOPIC: Octane rating for old outboards?

With a '73 Rude I'm absolutely going to be using EOH-free gas, but what are your thoughts on premium (93 Octane) vs regular (87 Octane)?

87 octane is fine

87 octane is fine, but around here we can only get non-ethanol fuel in mid-grade (89 octane).

My 73 has been run on both 87 and 89 octane, with no difference in performance or idling.

I seem to only find it in 91 or 92 octane. Is that too high for an older mid 60's 100hp merc 6 cylinder? Would it make it run too hot or?? I don't want to damage my old motor, but do not like the ethanol for older stuff.

Octane only makes the fuel burn in a more stable / controlled manner. Not hotter. The higher the rating, the less chance for premature detonation - which is what eats up a motor. Only high-compression engines (and engines running crazy advanced timing) need the higher rated octane. Will not make a motor run hotter.

Around here all we can get for non-ethanol fuel is the premium. I know with 4 strokes ( cars and bikes at least ) you step up to higher octane if you are experiencing knocking and pinging (premature detonation as you said ) but if you aren't getting that there's no need for the more expensive blends. I've always wondered if it has the same effect on two strokes.

octane will not damage your tower of power Dr. Go. I have burned everything from near Kerosene to nascar racing fuel and it all basically runs the same in all the old motors I have raced and played with.
Back in the 60s, I would drive 40 miles to fill my racing Mercury tanks with "White Gas" by Amoco!!! It was the trend. One race day I was forced to fill with Gulftane which was the cheapest thing ever invented for gasoline and I won the race!!!

Higher octane rating will perform some better, but not that much!!!

The Amoco white gas was just the first "unleaded" fuel to hit the market.

Raced this ole 75 hp 1960 Johnson wide open racing and skiing for 15 years and it ran on whatever I poured in it, all the same. This ole motor was a tuff one. I replaced 1 water pump impeller, one rotor button and one fuel pump in 15 years and this motor ran probably 4-5 days a week as I lived on a lake and ran it many, many miles!!

my present 1969 Merc 1000 loves whatever I pour in it too!!!

Jim Savage

Those old Johnrudes don't have much compression to begin with and would probably run on snake oil!

If you can get non-alcohol 91-octane, your Inline Six would be quite happy running that.

If you can't get straight gas, Merc says don't use Premium 'cause the additives it take to get Corn Squeezin's to 91-octane aren't outboard-friendly.

In that case if you can get a decent 87-octane regular or even a mid-grade, that'll work. But on the 99-c.i. motors you'd better back the timing down from the original factory settings. Like no more than 21 deg BTDC. 18 deg BTDC if your fuel sources are suspect.

The 90-c.i. and smaller Sixes have a different piston design and aren't quite as high-strung.

My old 100hp had the timing stop locknut vibrate loose and the stop screw rattled out.

It ran really good until I figured out the timing was WAY over spec.

"So THAT's why the spark plugs turned blue!!" Fortunately it didn't hurt anything but the performance sure was impressive.

Cheers.......ed

Thanks Ed!! Good info.

Great info. My tower is the smaller block 1965 100. It seems the theme is no ethanol if possible. Motor runs great and has consistent compression across all 6 cylinders.

Last question: My son has some race fuel left over from racing his dirt bike this year. Can that over 100 octane be blended into the non-ethanol 91 octane pump gas and burn ok without damage to my 90cu.in. tower? I don't know if you can burn a full tank of it, so was wondering if it would be "toned down" if mixed?

It is likely obvious to everyone by now that I know next to nothing about fuel and motors other than it takes gas to run a motor (and it should not be old or dirty or full of water).

Thanks for the great info so far!
Mark