what octane?

[Attwanl]

Hi Everyone
Probabaly a dumb question.... I have 1975 Hp6 Evinrude. What octane should I run in it. I was running low octane non-alcohol , but the station has quit selling it. I can get it in 93 octane non-alcohol fuel. Will it hurt it? Was told the motor was designed for low octane fuel.

 

[Gotem]

The motor was made for low octane fuel. It was not designed for ethanol. I would go with the higher octane nonethanol. Always use a stabilizer, but if you use ethanol use one like marine stabil or one of the other products out there to help handle the ethanol. Our gas today sucks.

 

[Attwanl]

Thanks Gotem
I know ethanol is bad on all the rubber componets, wasn't sure if the higher octane would do any damage. I guess it's than the alcohol.

 

[cgaengineer]

The 93 non ethanol will be fine.

 

[shawnfish]

my .02 cents.... those older OMC motors were designed starting with the plug, in other words the motor is designed around the plug. ive been told by some that the older plugs(factory or the newer peplacements) will work better with the lower octane fuels that was available when they were designed and that them older motors run best with lower octane gas. I have an 1980 Johnson 25hp and it runs and idles better with 89 octane than with the high octane fuel such as 92 octane. either grade will work fine as long as theres no ethanol or a additive is added with it......

 

[cgaengineer]

Octane is added to increase the temps required to ignite fuel air mixture to prevent detonation. I could possibly see where you may notice a very slight difference in performance using higher octane fuel in an engine designed to run on 87 octane, but in all my years I have never noticed it...I have noticed the other way around though...especially in my supercharged Tacoma...if you don't run 93 it will detonate badly.

An outboard could be worse with higher octane especially if its a non thermostat engine...not sure if some of the older outboards were built without thermostats or not.

That being said, your best performance will be achieved with the lowest octane fuel that the engine will digest without detonation.

 

[shawnfish]

cgaengineer » 4 minutes ago[/url]"]Octane is added to increase the temps required to ignite fuel air mixture to prevent detonation. I could possibly see where you may notice a very slight difference in performance using higher octane fuel in an engine designed to run on 87 octane, but in all my years I have never noticed it...I have noticed the other way around though...especially in my supercharged Tacoma...if you don't run 93 it will detonate badly.

An outboard could be worse with higher octane especially if its a non thermostat engine...not sure if some of the older outboards were built without thermostats or not.

That being said, your best performance will be achieved with the lowest octane fuel that the engine will digest without detonation.

your last sentence......spot on!!!

 

[user 6672]

shawnfish » 27 May 2013, 09:00[/url]"]

cgaengineer » 4 minutes ago[/url]"]

That being said, your best performance will be achieved with the lowest octane fuel that the engine will digest without detonation.

your last sentence......spot on!!!

Yes.. but figuring that out can be spendy.. I would stick with the non oxy higher oct to be safe.

 

[Attwanl]

Thanks everyone for thier input.
I'll use 93 octane non-ethanol, it won't hurt it, but I'll get better performance if I can ever find 87 non-ethanol around here.
I just cruise from fishing hole to the next, I probably won't notice the difference. I just didn't want to hurt my old outboard.
Thank again everyone.

 

[PSG-1]

It's my understanding that with ethanol fuel, the ethanol serves the purpose that octane does with non-ethanol fuel. However, because ethanol is hygroscopic (water attractant) the higher the so-called 'octane' rating, the more likely it is to undergo phase separation, where you have a layer of low-octane fuel on the top, and high octane water on the bottom.

It's also my understanding that unless you're running high compression ratios, such as 10:1 or above, you don't need to use high octane. For instance, using high octane in an average vehicle, most all the performance (and the extra $$) is going right out the tailpipe, completely wasted.

 

[chevyrulz]

generally, older motors run best on mid-grade 89 octane, ethanol eats rubber & attracts water, so always prefer non-ethanol fuel

my '05 yamaha 90 is engineered for 91 octane to make it run ok on any type of pump gas

 

[Attwanl]

So ethanol....it eats rubber....attracts water....doesn't sound like it's too good for a boat does it.

 

[cgaengineer]

It's my understanding that with ethanol fuel, the ethanol serves the purpose that octane does with non-ethanol fuel. However, because ethanol is hygroscopic (water attractant) the higher the so-called 'octane' rating, the more likely it is to undergo phase separation, where you have a layer of low-octane fuel on the top, and high octane water on the bottom.

It's also my understanding that unless you're running high compression ratios, such as 10:1 or above, you don't need to use high octane. For instance, using high octane in an average vehicle, most all the performance (and the extra $$) is going right out the tailpipe, completely wasted.

This is correct...

I have to run premium in my supercharged Tacoma and mid in my BMW, I notice pinging in both if I do not.

 

[cgaengineer]

So ethanol....it eats rubber....attracts water....doesn't sound like it's too good for a boat does it.

By now if they can't make rubber that doesn't dissolve from alcohol they should be out of business.

 

[rickybobbybend]

As a practical matter It is tough to find low obtain fuel that does not contain ethanol, whereas higher octane non-ethanol can be found in many locations, though not all. Avoid ethanol fuel if possible, quit worrying about octane and make sure your fuel-oil mixture is correct.

 

[Charger25]

rickybobbybend » Today, 00:11[/url]"]As a practical matter It is tough to find low obtain fuel that does not contain ethanol, whereas higher octane non-ethanol can be found in many locations, though not all. Avoid ethanol fuel if possible, quit worrying about octane and make sure your fuel-oil mixture is correct.

Totally agree.........

 

[Gotem]

One mechanic told me that the Enthanol makes the two stroke oil drier so it does not lubricate as well. I'm not sure how true this is but this gas is garbage.

 

[chevyrulz]

Gotem, it's probably the water attracted by the ethanol that reduces lubrication

and yes ricky & charger, properly mixed 40:1 (or whatever your engine requires) 87 octane is better than 89 or 93 mixed @ the wrong fuel/oil ratio

i run ethanol in my boat all the time, but i don't let it sit there. ethanol doesn't hurt your daily driver vehicle, because it's constantly in motion. ethanol sitting on rubber, causes the rubber to deteriorate more rapidly than normal. air inside your gas tank attracts water, so even ethanol free fuel isn't waterproof. but ethanol itself attracts water, so you've got air & ethanol collecting water if you use ethanol fuel, meaning you attract water more quickly by not using ethanol free fuel.

if you circulate the fuel by using the vehicle or boat or whatever, then the water doesn't pool up or separate from the fuel, & you don't end up with 'bad gas'. the fuel & water have separated, it might still crank up, but then you start moving around that 'bad gas' then you start running pools of water or mostly water into the engine & it spits, sputters, or stalls. the ethanol itself, still runs fine, but it caused water to build up, & your motor runs on gas not water, the ethanol also possiblysent a few small pieces of rubber through your fuel system, possibly clogging up important passageways

point being, if you are going to run ethanol fuels, don't let the fuel sit, especially in the carb. also, if you're going to store your gas powered engine, either drain the tank & fuel system completely (painful), or run it all the way empty, then fill it up w/ ethanol free and fuel stabilizer like Stabil. even "treated" fuel still has a shelf life, so if your planning to let it sit for a year or more, you should complete drain the fuel system, that means the tank, the fuel lines/filter, and the carb. i keep my fuel tanks full, but i also use my toys often.