2012 Tohatsu 6hp?


Oct 21, 2008
oday 222 niagara
I am not a petrochemical engineer, nor do I repair outboard motors for a living. But my understanding (and firm belief) is that there is no benefit to running higher octane in engines of this type (with comparatively low compression ratio). I do have a friend who has owned a gas station for decades. He was the first to tell me this because I used to run 93 octane in everything.

I have no firm understanding or belief as far as downsides, other than cost.

I would absolutely run high octane if I had to in order to NOT run ethanol. In any small engine. Downsides or no.
In the small motors, most of us use we want the spark plug to ignite the air-fuel mixture at the proper time. If the compression heats the air-fuel mixture to a "flash" point and the mixture ignites before the spark plug sparks, the motor knocks. Fuel w/ a higher octane rating will ignite at a higher temp. allowing the spark of the spark plug to do the job r.ather than the heat of the compression. The higher the compression the higher the heat.
So Gene's friend is correct. You do not need a high octane fuel in a low compression motor. But high octane (within reason) used in a low compression engine has few drawbacks. The burn temp. of high octane fuel once ignited are within the boundaries of our motors and added deposits in are minimal.
I agree with Gene. The potential of ethanol to cause problems is well documented. I want gasoline without ethanol it in.