Mid grade gasoline question?

Basscat 1 · Apr 22, 2005

I have heard two different theories on which gas to run in older outboard engines. My local OMC mechanic recommends using a mid-grade fuel in my 1974 outboard. However, I read numerous times on this forum to use 87 octane fuel. Does this really make a difference? If so, why? Thanks.

JB · Apr 22, 2005

Re: Mid grade gasoline question?

87 octane unleaded is adequate for most outboards, Basscat. Mid-grade (91) will make no measurable difference except to your pocketbook.

beau123 · Apr 22, 2005

Re: Mid grade gasoline question?

I have always wondered what octane to run and have asked a few people and they have always told me to run high octane because it has been refined more and it burns clearner leaving less carbon, but I always run mid-octane. Now that I know there is no difference I will run the cheaper gas. Thanks hoss123

JB · Apr 22, 2005

Re: Mid grade gasoline question?

Sorry, Hoss. Higher octane gasoline has not been refined more and it does not burn cleaner. It simply has more and different additives to prevent knock in high compression (4 stroke) engines.

chris in va · Apr 22, 2005

Re: Mid grade gasoline question?

High octane fuel actually has LESS energy, with a burn-retardant added to reduce knock. 87 just burns a lot quicker...same base gasoline. Now if you had a turbo...that's a different story.

Walker · Apr 22, 2005

Re: Mid grade gasoline question?

Back when the government made the oil companies phase out leaded gas the new unleaded gas had different additives per grade of gasoline. Premium gas had a carbon dispersant additive that regular grade gas didn't have. 20 or so years ago the oil companies started using the same additives in all grades of fuel.

rolmops · Apr 22, 2005

Re: Mid grade gasoline question?

I asked my outboard guru the same question and he came up with a different answer. He showed me a lot of blown pistons(he has a few buckets full just for kicks.He then pointed out the place where most of the burn damage was both on loopers and cross flows.Most of the damage was always on the exhaust side of the piston,with both types. His theory is that 87 octane ignites as a flame in the cylinder and spreads as a flame in the cylinder head. It only explodes during the down ward stroke and the shock wave aims more towards the exhaust side.He claims that 89 octane explodes almost instantly and because of this has a more evenly divided shock wave. What do you think about this theory???

Walker · Apr 22, 2005

Re: Mid grade gasoline question?

Cool air comes in the intake ports and hot air goes out the exhaust ports. The exhaust port side gets much hotter so naturally thats where your risk of meltdown is greater in a properly tuned 2 stroke.

rayjay · Apr 22, 2005

Re: Mid grade gasoline question?

When the piston crown uncovers the upper edge of the ex port hot gas jets down the face of the piston heating up the surface and potentially burning off the oil [ depending on the temp of the gasses]. That is why the ex side of the piston gets burned up first. Run the motor lean and/or hot and the oil gets burned off and the piston gets too hot and expands until there is no more clearance. Screech !!!!

moderator1 · Apr 22, 2005

Re: Mid grade gasoline question?

He claims that 89 octane explodes almost instantly and because of this has a more evenly divided shock wave.

The lower the octane, the faster the burn...he has it backwards and higher octane is still burning when the exhaust ports are exposed creating a loss of energy but higher temps at the ports.

Basscat 1 · Apr 22, 2005

Re: Mid grade gasoline question?

I found an article that explains the difference in a little more detail. web page I think I will stick with the 89 octane, I dont mind paying the extra $ 1.00 when I fill up.

moderator1 · Apr 22, 2005

Re: Mid grade gasoline question?

89 octane is the most common carrier of ethanol...make sure you're not exceeding 10%. In my parts, we have 85, 87, 89 and 91 fuel...no 93 here due to higher altitude.

Mid grade gasoline question?

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