I just bought a brand new 2017 50hp TLDI outboard. The manual say, " Tohatsu engines will operate satisfactorily when using a major brand unleaded gasoline meeting the following specifications: USA and CANADA - having a posted pump Octane Rating 87 (R+M)/2 minimum. Premium gasoline (92 [R=M]/2 Octane) is also acceptable. Do not use leaded gas."


-The Tohatsu website says in their FAQ:

"Do I have to use any special type of gas (super unleaded, regular unleaded, etc) in my Tohatsu outboard?
The minimum octane rating that you should use is 87, however, we recommend using gas with an octane rating of 89 or higher."


So my question to you folks, hopefully a Tohatsu guru will answer, why would they recommend using higher octane fuel and are there any benefits in doing so?

87 oct will work, but 89 oct burns cleaner and will run your motor smoother, thats all its saying. Chevy, Ford, Harley Davidson, Briggs & Straton all have recommended fuel octaine values. But personaly l would run 89 oct when can be purchased

Higher octane doesn't burn cleaner. In fact using a higher octane then called for can cause excessive carbon build up. Octane rating is a resistance to detonation.

So after talking to Tohatsu tech support they said that using 87 octane is fine and that they haven't had any reports from consumers or testing to show the motor to run more efficiently on premium fuel. So then I ask why do you recommend using premium fuel? They said that octane isn't always what the pump states. That 87 octane can sometimes really be 85 octane and using a higher grade ensures there is enough octane. But then went on to say that if you buy fuel from a major brand this most likely won't be the case. Also they sated that gas will lose octane in a vented tank, over the course of time. 100 days shelf life if low humidity and stable temperature or less time in higher humidity and/or fluctuating temperature differences or both. So if you use your boat infrequently or if you store it around high humidity, outdoors were temps fluctuate allot or the combination of both this can cause octane loss. So high grade fuels have a better shelf life when 87 is required.

I go through 18 gallon in three weeks, but always fill my boat to full before storing inside the garage, so I doubt I will lose octane quickly enough to validate buying premium, as the fuel is refreshed with about 6 gallons per week. So I think I will stick with the 87 gas as I see no other benefit then shelf life.

A boater friend who worked in a lab testing refinery gasoline products told me that everything they put in "premium" higher octane gasolines are a problem for outboards. He advised me to only use regular. This was before the ethanol scam, by the way. Now regular with a stabilizer is probably the way to go.
IslandJim

Higher octane fuel burns slower and cooler than low octane. The cooler and slower burn can result on more carbon buildup in addition to a slight loss of performance. Higher octane fuel is meant for engines with higher compression ratios and more advanced timing curves ie; prevents pinging and detonation.
If an engine is designed for 87 octane that's what should be run to get best performance and economy.

I get that but Tohatsu recommends using premium, but then says the engine can also run 87. Normally when a car manufacturer words it this way it's because the computer in the car can detect the premium fuel adjust timing and thus a performance gain.

So I thought maybe the outboard could do the same. But that's not the case as I mentioned in my 2nd post. I find it odd they would recommend premium fuel when the motor isn't designed to run it efficiently as in a higher compression motor.

All on all I'm glad I don't have to run premium fuel as it saves me money.

Computer doesn't really ever know what fuel is in the tank. Ion sensors or knock sensors in the heads that detect the combustion event or detonation will send a bias voltage to the computer to retard the timing is what happens in order to adjust for fuel quality. Some late model performance cars have a driver's manual control to input fuel data that then selects the best preset timing and fuel curves. Corvette is one that I know of. The engines still have ion sensors and O2 sensors to further trim the timing and fuel curves. Big outboards like 7 Marine may have all these features but I doubt anything below a Verado 400 series or any 4 stroke supercharged motor would have.