What octane gas to use

[Jon D]

Hello again fellow water men and women i was curious as to what octane gas to use on my 1995 yamaha 115 2stroke 4 cylinder. I just read something in a car magazine that said that you could burn the 87 octane in new engines even though the manual tells you not to ,they recommend higher octane did not know if this was true or if it mattered with outboard engines any thought is appreciative.thanks

 

[trayscool]

Re: What octane gas to use

I would run 91 octane anyways. outboard motors are under huge stress compared to car engines and the higher octane the better IMO

 

[drewmitch44]

Re: What octane gas to use

I run a 87 yamaha 115 and i was warned by the yamaha dealer not to run any higher than 87. He said that the higher octane fuels can make the motor too hot. That is just what i was told anyways. My motor has 2,600 hours and it has 130 psi on all 4 cylinders. I keep with the regular maintnance though. And use yamalube all the time in the presision blend system.

 

[Silvertip]

Re: What octane gas to use

Higher octane IS NOT always better and in fact is a giant waste of money when the manufacturer recommends 87 octane. Cars can run most any grade of fuel because the ECM and knock sensors detect detonation and can therefore adjust ignition timing accordingly. That is not the case the older carbed outboards. The additives in higher octane fuel may be bad for the engine as well. The long and short or this is to run what the engine manufacturer recommends.

 

[dave11]

Re: What octane gas to use

I have a newer 4 stroke Yamaha. The manual says to use 89 octane. That is what I use. The info you are reading is probably for automotive engines.

 

[marine4003]

Re: What octane gas to use

Higher octane IS NOT always better and in fact is a giant waste of money when the manufacturer recommends 87 octane. Cars can run most any grade of fuel because the ECM and knock sensors detect detonation and can therefore adjust ignition timing accordingly. That is not the case the older carbed outboards. The additives in higher octane fuel may be bad for the engine as well. The long and short or this is to run what the engine manufacturer recommends.

Sooo true.

 

[NelsonQ]

Re: What octane gas to use

I stick to the manufacturers recommendations. I figure they know more about their engines than I do. And, why spend extra $$$ for higher octane if the manufacturer thinks its not req`d?

 

[Chewey]

Re: What octane gas to use

The octane requirement is based on the amount of timing advance and compression of the engine. The reason for additional octane is to prevent premature detonation of the gas in the cylinder due to the compression of the gas. I would never use anything less than the mfr recommendation, and not use any more than that also cause its just a waste of gas. The higher octane gas does not have any more energy in it, it just requires more heat and compression in order to detonate it.

 

[QC]

Re: What octane gas to use

I agree with Silvertip and Chewey here with two clarifications . . .

#1 If you are running in high ambient temperatures and under high load factors you can potentially get slightly better performance using higher octane fuel in electronic engines with knock sensors . . . Think about it this way, if the engine is retarding timing to protect from knock with lower octane fuel then the inverse is true: more spark advance with higher octane . . .

# 2 is that pre-ignition (premature) is detonation before the spark plug fires. But more common detonation (knock) occurs when the spark starts the ignition when it is supposed to, but the burn is actually a bang. Bang is bad. Same phenomenon, just at a different time . . . The latter can be controlled with spark timing. Pre-ignition (rare) cannot be controlled without changing the fuel, or the compression ratio, or the load, or ambient etc. etc. Simply put, if the bang happens before the spark plug fires, how does changing the spark timing do anything?