More fuel/octane questions.

[oregonducker]

I troll with a '79 evinrude 6 horse. Rarely run it above a fast idle. Should 50:1 mix be fine? I've heard it's best to mix a different ratio for constant low speed operation. True? If so, what ratio?<br /><br />Reading some other posts on octane prompted me to look it up in my manual. It's an original 1979 factory service manual and says that unleaded 86 octane is the minimum. I've read many here that say you shouldn't go over 87 octane for these older motors. Why, when the book says minimum? My main motor is a 2002 Nissan 40 horse two stroke oil injected. The owners manual again says minimum 87 octane. It also lists several bad things about low quality gas and other things to avoid. Seems that if higher octane was bad they'd say so. When they say 87 is the minimum it sounds like higher octane would be better. What's the deal here?

 

[DJ]

Re: More fuel/octane questions.

oregonducker,<br /><br />Use 87 octane at 50:1.<br /><br />Extra octane in a 2-stroke outboard is useless and can be detrimental over a period of time.<br /><br />Octane is the fuels ability to resist detonation. It accomplishes that by slowing down the ignition process thus creating a cooler burn. Outboards (2-stroke) are actually quite low compression ratio engines. Plus, the oil has a cooling/slowing effect to the burn in the combustion chamber thus voiding the need for higher octane.<br /><br />Engines that typically run on fuels containing higher than neded octane fuel turn into octane junkies. They then need the octane to stop the detonation caused by glowing deposits left on piston tops that cause the detonation. You see, it's a vicious circle.

 

[Dhadley]

Re: More fuel/octane questions.

As for the wording in the owners manual, it could get sticky if they told you not to use a certian product. Lawyers have to make a living too ya know.<br /><br />Ford was the first to come closest. In 1997 they stated their 1/2 ton trucks were designed to run on 87. They stated that if it "pinged" to take it to the dealer. Before that everyone had stated that if the motor "pinged" to run one grade higher fuel. <br /><br />Today, fuels burn hotter as the octane goes up. With leaded fuel it used to burn cooler. It helped some when they removed the MTBE but it still burns hotter.<br /><br />Good luck!

 

[ReelDrag]

Re: More fuel/octane questions.

Conflicting posts here. One says higher octane burns cooler, one says it burns hotter. Anyone know for sure?

 

[ZmOz]

Re: More fuel/octane questions.

Higher octane burns cooler, but that is only because it doesn't burn as completely.

 

[DJ]

Re: More fuel/octane questions.

ReelDrag,<br /><br />Cooler vs. hotter. It doesn't really matter. What the octane does is control the SPEED of the combustion flame.

 

[ReelDrag]

Re: More fuel/octane questions.

Ok, I've always used 93 octane Chevron in my 92 Johnson 115 because it starts better when the motor is cold. Problem is after trolling for awhile spark plugs foul. Thought about trying a hotter plug QL82C. If I drop to 89 octane will I get less fouling?

 

[Dhadley]

Re: More fuel/octane questions.

Higher used to burn cooler when there was lead in the fuel. When the lead came out the octane is rasied using oxygen bearing compounds. The added oxygen molecules cause a higher combustion temp. Put a pyrometer or EGT gauge on your outboard and try the different fuels. Its amazing.<br /><br />Try running high test in your fuel injected car or truck thats designed to run on 87 (not something with high compression that needs the octane like a Z28 or Vett). Run it long enough to check the fuel mileage. The O2 sensor will "sense" too much oxygen and think the motor is running lean. It will automatically add fuel to the incoming charge to compensate for the "lean" condition.<br /><br />The added oxygen causes a leaner mix and a hotter combustion temp. Its just that on carb'd motors we dont have the electronics to compensate like the efi motors do. We have to rely on other components like the operator.

 

[rickdb1boat]

Re: More fuel/octane questions.

ReelDrag <br /><br />I think you will foul less due to the more efficient burning of lower octane fuel. Give it a try and see how it goes. 2-Stroke have relatively low compression, compared to cars, and cannot fully take advantage of higher octane and will burn cooler. I don't know how they rate the fuel in your area, but if they have 87 octane, use that. What plug are you using now? Should be QL77JC4 gapped at .040 for better trolling and overall performance.

 

[DJ]

Re: More fuel/octane questions.

Reeldrag,<br /><br />I agree with Rick. And, a good post by Dhadley-Thanks D.<br /><br />See how it runs on 87. You may want to decarb it first.

 

[ReelDrag]

Re: More fuel/octane questions.

Thanks for explaining it. I think I have a better understanding now. I'll try the lower octane and see what happens. By the way, motor was decarbed at 50hrs. 57 on it now ( new powerhead) and I'm useing Pennzoil synthetic blend with QL77JC4 plugs gapped at .035. Thanks again.