Fuel Question about blending 89 and 91 octane gas.

[Frank G]

I know that Yamaha recommends using 89 octane in engines. However, there are a few marinas in the area that carry 87 octane.
I think their main customers are commercial work boats. If I'm in that area and need gas am I OK if I put in 50 gallons to be on the safe side and then add 50 gallons of 91 octane when I hit the neighborhood marina? :?:

 

[Lainie J]

Not sure what engines you have but the book on my 2002 F225's says 87. i use whatever is less expensive at the marina which is usually 89 but i also use some portable tank and use 87, never had a fuel issue in 10 years.

 

[seasick]

What motor. Mine runs fine on regular..

 

[wspitler]

My F250s require 89 Octane, but they have knock sensors that adjust the ignition timing and maybe the mixture as well to prevent/minimize knock. My new F150 specifies 87 octane as the preferred fuel. Mixing octanes is OK as long as the ratios are such that the resulting octane meets the minimums. Even running an engine that specifies 89 on 87 might be OK in the short run depending on operating conditions and ambient temps. The newer engines are pretty smart and although power may be limited, my guess is the engine won't allow itself to knock enough to cause damage in the short run. Remember the older autos that would audibly knock on "lugging" accelerations. It wasn't good for the engine, but they mostly held together OK. If you hear a knock out of the engine then exit that operating range. I'm not sure though if you'd be able to hear it with these more quiet modern four stroke engines. I've seen piston and valve damage from excess knock, but it was only in extreme cases. Knock is essentially fuel that explodes prematurely versus rapidly and smoothly burning. (Think how a diesel sounds) Octane is one way to prevent knock, timing and mixture adjustment are other ways to mitigate the problem. Running an engine on an octane that is higher than required is a waste of money on modern engines.

Sorry! Probably more than you wanted to know. Short answer: Not ideal, but probably OK with the precautions mentioned above.

 

[Frank G]

Thanks for your comments. Motors are four stroke 250's.

 

[Grandpapat]

I routinely mix 87 and 91 (or 93) octane in our GW360 if there is no 89 pump on the dock. It actually saves time on the dock too by filling both tanks at the same time. You have to keep the starting volumes and stopping volumes straight in your head or on paper and it makes you stay focussed on the fueling process, which is a good thing rather than risk an accidental discharge to water. I put a spotter on the dock at the pumps telling me volumes during the process.

The premium grades of gas from the majors like Exxon and Mobil contain a much larger splash of additive package (rust inhibitors, cleaning agents, etc) than regular grade of gas and are made with higher quality components in the refinery blending process. Still not worth the extra money for the extra octane all the time, but mixing them is not an issue.

 

[wrxhoon]

Frank ,
You can use 87, not ideal and don't overload the engine when you do have 87 in the tank . When you can get higher octane fill with 93 if possible that way you achieve the required 91.
You can get octane boosters but they are not ideal either.
When you have the lower octane gas in your engines computer will retard timming as soon as she starts knocking to avoid engine damage , knocking can be very bad for an engine but more so on turbo engines than NA engines.