Reply With QuoteMy owners manual for my 2000 model 25HP Mercury specifically says to run mid grade 89 octane or higher. I only use ethanol free in it as well.
SeaRay
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Octane
I have heard old timers say not to run anything but 87 octane gas in outboards. I have used 89 with no problem and it runs better. Does anyone know if this is true? I don't see how running higher octane could hurt the engine. I know car motors real well and it doesn't apply. Just asking....
U.S. Air Force Retired
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My owners manual for my 2000 model 25HP Mercury specifically says to run mid grade 89 octane or higher. I only use ethanol free in it as well.
SeaRay
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Using high test when the manufacturer recommends low test is just a waste of money. Most outboards are engineered for low test or 87 octane.
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Bill H. PTC USN Ret
Chesapeake, Va
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A higher octance gas has a different flash point, 89 is faster than 87, almost all motors (there are exceptions) are built to run on 87 octane gas. Most motors are on the verge of melt down to begin with , running 140 - 150 degrees at 5000 rpm's and a digital pyrometer on a spark plug seat at that rpm will read close to 200 degree's. Running a higher octane gas that flashes off quicker can cause the heat range to go up in the combustion chamber, hence making the motor run hotter, throw in a piece of carbon on the piston that is glowing redhot and before you know it you can achive detonation and engine melt down. Just about every motor is built to run on 87 octane gas and running higher grade is not doing anything, and may even cause problems in the long run. JMHO
Tom
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You have it backwards. Octane retards ignition. Higher octane gas is for high compression engines where the fuel tends to ignite from compression alone. When that happens before the plug fires, you get a knock since the piston may still be on its way up.Originally Posted by Archerguy1
Running higher octane fuel in a lower compression engine will result in carbon deposits in the engine. Chunks of carbon flying loose inside a 2-stroke is a bad thing.
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You are correct, the higher the octane the more compression it takes to ignite. However a common misconception is to associate the size of the engine with the compression ratio. In fact most larger engines only require 87 octane, but smaller engines are tuned to get maximine HP from a smaller size so the jack up the compression to achieve that. For instance I have a Motor Home with a 8.4 Liter V8 GMC engine. It is designed for 87 octane. My Acura MDX has a 3.4 liter V6 and it requires 91 octane to achieve the HP rating. The Honda Pilot with same size engine only requires 87 Octane, but it has about 50 less HP than the Acura. A more extreme example is the Mini Cooper. It takes high octane gas because of the output of such a small engine.
That is also the case with my 25HP Mecury. In order to get the HP up, the compression is higher to compensate for the smaller displacement.
Best way to determine what is required is to consult the owners manual. After all, they designed the thing and they know what it requires.
SeaRay
Mark 1:17 ...I will make you fishers of men
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