Octane rating?

[_brad_]

I have heard and read conflicting reports as to the octane rating of gasoline I should be buying. I have heard from one person to put regular 87 in and others to use premium. I have also read the same conflicting stories.

Can someone please clear this up? Any benefits to running premium?

1977 70 HP Evinrude

 

[luckyinkentucky]

Re: Octane rating?

Premium tends to be too rich for most 2 cycle engines. From my experience with them you shouldn't go above an 89 Octane rating. Those motors weren't built to run Premium, and you may end up getting a lot of 'pinging' out of them. Premium gasoline is really no different than Regular after you take out the additives. Both Premium and Regular burn at around 111,400 BTUs per gallon. The only difference is the Octane rating. Octane is defined as a fuel's resistance to knocking. There is no benefit if the octane is higher than what the engine needs. If the fuel ignites before it should, as in a too high of an octane fuel for your engine, you will experience knocking. The old belief that "higher octane gasoline burns cleaner, and reduces emissions" is a hoax. All gasoline burns the same, and has the same emissions. Also, people who are brand loyal in saying they only buy XXXX brand gasoline are being fooled as well. Truth be known every major gasoline vendor in the US buys from 1 pipeline. The only thing differently is the additives they add after they get the fuel. Sorry for the rant, but I had an uncle who worked for Exxon for 15 years in the Texas area for pipeline transportation, and this was a common conversation piece at family gatherings.

 

[Darren418]

Re: Octane rating?

I do believe that the higher octance rating actually *increases* resistance to knocking. Knocking is usually an indicator of too *low* of an octane rating. Older engines with carbon buildup (outboards) tend to knock because the carbon displaces air in the combustion chamber leading to higher compression ratios and pre-ignition, or detonation, aka "knocking." Most of the time higher octance gas is a waste unless your engine was designed for it, i.e. high compression, OR it has significant carbon buildup. I run high octane in my old outboard as an insurance policy against detonation which can be very hard on an engine. The higher octance gas will give you more of an operating range before detonation should there be significant carbon buildup or the timing slightly out of spec.

 

[F_R]

Re: Octane rating?

1977 probably is Ok with regular unleaded. It would be nice if somebody can come up with genuine service information from the factory on that. Some earlier OMC motors had a bunch of trouble with preignition because of the changeover to government mandated unleaded gas. There were a number of service bulletins pertining to those. Sure wish I still had them.

Higher octane means greater resistance to detonation, pre-ignition, pinging, or whatever you want to call it. Result is all the same, it's like a little guy with a blowtorch and hammer in there pounding on the pistons.

 

[burroak]

Re: Octane rating?

"If the fuel ignites before it should, as in a too high of an octane fuel for your engine, you will experience knocking."

Just the opposite is true. All gasoline comes through the pipeline as a base product of unknown octane. Using some testing, additives are added to the pipeline product to get a rating of 87, 89 or 91 octane. The additives are added to prevent detonation. The higher the compression the higher the octane.

 

[Dhadley]

Re: Octane rating?

When they "got the lead out" that changed the combustion temps. The addatives they use now to increase the octane rating will cause the combustion temps to actually be higher with high test fuel. All stock recreational outboards produced since the mid 70's will run fine on 87. If you were to install a EGT gage you could see the difference.

If you modify a motor then it's possible it may require a higher octane. IE - putting small bore looper heads on a big bore looper. But stock, recreational outboards will run fine on 87. Sometimes better.

 

[jtexas]

Re: Octane rating?

you might be getting advice to use premium on account of all the hype about better detergents, etc. some oil companies add to their higher octane fuels - personally I doubt the benefits outweigh the cost especially if your area has E-10 or E-15 gas.

 

[iwombat]

Re: Octane rating?

About a decade or so ago it was federally mandated that the same detergents and cleaning additives be in all the octane varieties of a given vendor's gas. These days, 92 has the same cleaning agents that 87 does.

 

[darrklim2]

Re: Octane rating?

If it's not nocking with 87 there is no reason to run 89 or 91/92/93. Higher octane burns slower than lower octane.If you have higher compression engines(generally 150+) you would probably be getting pre-ignition with a lower octane fuel because it is burning too quickly. In this case you would want to run a higher octane. Most engines with compressoion lower than 150 don't need higher octane. With that being said if compression is close to 150 I run mid-grade gasoline just to be sure.