Octane and ethanol

[clemsonteg]

This is probably a more general outboard question but since I have a Johnson I put it here. I usually fill up everything I own with 93 octane. I recently found a gas station near my house that has ethanol free gas in 87 octane. I am not sure what octane my 78 85 hp requires but should I go with the 93 or 87?

 

[cdn-pk]

Re: Octane and ethanol

Oh boy..... there are some passionate opinions out there. basically the higher the octane the higher the combustion temperature of the gas. so in an outboard situation that's not the greatest. i use 91 because thats the only grade that does not contain ethanol and i notice a difference. but if i could get 87 octane without i would use that instead.

both my 77 140HP and 86 120HP only need 87.

hope that helps

 

[trosky]

Re: Octane and ethanol

i'm using 87-eth-free, on my 95/150 and it's worked fine until now.

 

[78Thundercraft]

Re: Octane and ethanol

I would suggest the 87 ethanol free also. I have the same year and hp engine, 78 Johnson 85hp (javelin version which is simply a 10amp vs 6 amp alt and also w/ tilt n trim).

I found the 87 ethanol free is worth nearly 2mph on top end when compared the the ole' up to 10% ethanol type 87. This experiement to test the two octanes was done with separate 6 gal tanks tried on the same day under the exact same conditions. So your having access to the 87 ethanol free is great. It's hard to find in Columbia SC. My 1977 70hp also likes the 87 better than higher octanes. It's all about timing, compression, and burn speed. So save the money and enjoy!

Also no need for higher octane, the burn speed slows down and the factory timing wont work as well and will also loose about 2mph on top end when going to 93 (yep, tried that too just for the heck of it).

I teach auto engineering so I guess it helps a lil on these motors....
Raymond

 

[Chris1956]

Re: Octane and ethanol

Actually higher octane fuel burns at a lower temperature. It is more resistant to pre-detonation, so you can advance the max spark advance a bit and achieve more power is SOME applications. Stock Outboards are not usually one of those applications that will benefit.

Use the 87 octane. Ethanol is not a problem either.

 

[Silvertip]

Re: Octane and ethanol

Chris is correct. Using 93 octane is of no value on a low compression engine (which you have) or on one that does not have an engine management computer (ECU/ECM). Fuel injected cars for example have a knock sensor that detects preignition in the engine. The sensor signals the ECU/ECM and timing is retarted until knock goes away. The opposite is also true. Timing can be advanced until knock is detected so maximum performance is reached on the fuel being used at the time. Your engine does not have that capability.

 

[rhaser]

Re: Octane and ethanol

Simple when that motor was made it ran on 87 octane leaded fuel. no ethenol. i have a 76 85hp and ran 87 ethenol fuel it worked for a yr. i had to replace both float needles and the fuel lines this yr. the ethenol made them mushy. hope that helps