fuel octane

[bowman316]

Hey,
I have an 81 70 hp evinrude, 2 stroke, and i was just wondering what octane fuel i should put in it.
I used to put the 93 in it, but i have been putting 87 in lately.
I think that it only needs 87, i can't really tell much of a diff between the two.
But most ppl seem to think that 93 is the best to put in it.

I hear that only high compression motors need high octane, but is 120 high compression? that is what my compression is.

 

[Gary H NC]

Re: fuel octane

The 87 octane is fine.

 

[JB]

Re: fuel octane

The problem is that people are convinced that "premium" gas is higher quality than "regular" gas. It is not. It is merely more resistant to preignition than regular in high compression engines.

 

[Benny1963]

Re: fuel octane

ditto i ran some 93 this weekend testing gps 53 mph ran some 87 sunday
52 gps mph but was running into head wind

 

[kenmyfam]

Re: fuel octane

87 is just fine for you....and cheaper as well !!!!

 

[Grem]

Re: fuel octane

The octane requirement depends on many factors and the safest route for any engine is to follow the manufacturer's instructions. You won't damage your engine using higher octane than specified, but allowing it to regularly 'pink' or 'ping' (pre-ignite) is very bad news.

Here's a little background info:

http://www.ozebook.com/compendium/suzi/octane.pdf

 

[Dhadley]

Re: fuel octane

Lets not confuse 2 stroke motorcycles with 2 stroke outboards, 2 completely different applications. With a 2 stroke outboard that's designed to use 87 octane, especially a carb'd model, long term use of 93 octane can lead to problems if certain changes are not made. When you deviate from the manufacturers specifications in any way you need to pay attention to combustion temps as well as water temp.

We'll assume your 1981 70 hp outboard is completely stock and as produced. You can run 93 octane for as long as you like but you may have to rejet and you want to make sure you're not even close to lugging the motor. You'll have to avoid anything that might increase the combustion temps beyond their design parameters. Jetting, rpm, timing, cooling etc.

 

[Grem]

Re: fuel octane

^^^^^

"Lets not confuse 2 stroke motorcycles with 2 stroke outboards, 2 completely different applications. With a 2 stroke outboard that's designed to use 87 octane, especially a carb'd model, long term use of 93 octane can lead to problems ..."

Please explain?

 

[Dhadley]

Re: fuel octane

Motorcycles have gearing so the motor should never be lugging and should always be in the proper rpm band for minimum combustion temps. When cruising a bike or car or truck is close to coasting. An outboard is always in "high" gear and never coasts. Like pulling a trailer with your car, it's always working to maintain. It's not uncommon to find outboards at or near lugging. The misused common term is the the motor is "overpropped".

 

[Grem]

Re: fuel octane

Motorcycles have gearing so the motor should never be lugging and should always be in the proper rpm band for minimum combustion temps. When cruising a bike or car or truck is close to coasting. An outboard is always in "high" gear and never coasts. Like pulling a trailer with your car, it's always working to maintain. It's not uncommon to find outboards at or near lugging. The misused common term is the the motor is "overpropped".

Even if that's the case (and I'm not sure it is in the case of small engined 2-stroke bikes which tend to run flat-out and under heavy load), I don't see what the connection is with high octane fuel. As mentioned here by JB, high octane fuel merely resists pre-ignition better and can therefore be safely used in high compression, critically timed engine designs, where lower octane fuels would cause damage. I'm not aware that the calorific value or the temperature of the flame differs significantly between fuels of differing octane levels.

 

[JIGGER]

Re: fuel octane

There is a diff. I run mid-grade in everything I own. Wont run 87 in my weed wacker! The higher the octane the higher the combustion temp. Some motors this would eat up head gaskets let alone valves. Air cooled motors run hotter than water cooled. The hotter the combution chamber the cleaner it burns. Gas is too me the life of your motor! Its what you run through it the most and WILL HAVE the most effect on your motor more so than oil. With 89 you avoid stale additives and cleaner burn with lower temps. I know by running 89 it pays for its self in fuel mileage.

 

[Grem]

Re: fuel octane

There is a diff. I run mid-grade in everything I own. Wont run 87 in my weed wacker! The higher the octane the higher the combustion temp. Some motors this would eat up head gaskets let alone valves. Air cooled motors run hotter than water cooled. The hotter the combution chamber the cleaner it burns. Gas is too me the life of your motor! Its what you run through it the most and WILL HAVE the most effect on your motor more so than oil. With 89 you avoid stale additives and cleaner burn with lower temps. I know by running 89 it pays for its self in fuel mileage.

I think you are confusing combustion temperature with ignition temperature. Since high octane fuel is more difficult to ignite, in low compression engines combustion may not be complete, resulting in a slightly LOWER cylinder temperature. However, these differences are tiny in comparison to all the other variables in your engine.

 

[ezeke]

Re: fuel octane

BRP makes its own recommendation here: http://www.evinrude.com/en-US/About.Us/FAQ/Oil.Fuel.htm

If you read to the end of the text in the response to question 4, you can see the reason for the change away from premium grade.

 

[Grem]

Re: fuel octane

^^^^

Yes, slightly higher octane to prevent pre-ignition is preferred.

Incidentally, didn't I read somewhere here that although Johnyrude recommended 50:1 from 1964 onward, this was a nod to the environmentalists and in reality the motors run better (and longer) on 25:1?

 

[ezeke]

Re: fuel octane

1964 is the year when most OMC motors had stainless bearings.

At a later point the ratio was changed to 100:1 but there was not enough residual oil to protect the engine. It was quickly changed back to 50:1.

Some of the engines' manuals like the TurboJets still recommend 24:1 in high performance use.

 

[JIGGER]

Re: fuel octane

Good read! Cheap gas and mix make for more headache than they are worth! Carbon on one end and higher temps on the other. Isnt that why they are making engines run hotter. To burn cheap fuel. Stick with 89. They better the run the longer they last. Tough to tell when a 2 stroke pins. Plugs tell all! To rich or to lean.

 

[bowman316]

Re: fuel octane

when did they develop tcw-3? What does the evinrude website mean by older engines? I have an 81 engine.
So if i read this website correctly, you only need 89 if you are still using tcw-2 oil, and if you are using tcw-3, 87 is fine.

 

[ezeke]

Re: fuel octane

QUOTE "So if i read this website correctly, you only need 89 if you are still using tcw-2 oil, and if you are using tcw-3, 87 is fine."

Where did you get that? The site says specifically: "Current recommendations are to use a minimum of 87 octane, 89 octane is preferred."

http://www.evinrude.com/en-US/About.Us/FAQ/Oil.Fuel.htm#Q4

 

[bowman316]

Re: fuel octane

so 89 is perferred, but these is no benefit to using 89 compared to 87.
they said:
"There is no additional benefit from using higher-octane fuels, in fact they may cause hard starting conditions in hot weather."
So i take that to mean, 87 if fine, and using 89 is a waste of money.

 

[ezeke]

Re: fuel octane

Well we certainly don't want you to waste any money.