High octane gas

[mkmnc]

I have a 91 Evinrude 48 special and have been using high test fuel. The engine has run well so far with maybe 200 gal of fuel used this summer. I run the high test because no one sells regular ethanol free gas near my house and I have had problems ( carbs needing rebuilt or replaced and fuel lines needing replaced) with other two cycle engines like weed eaters and chain saws which were solved by using non ethanol fuel. Any thoughts on if using high octane gas is bad in the long term for this engine or any thoughts on the use or non use of ethanol added gas in outboard engines. Thanks in advance for any thoughts on these issues.

 

[sutor623]

No, you should be fine. It may change the operating temp a hair, but to be honest, you should have less carbon deposits using high test. This is a good thing that can actually prolong use of your engine. I would still do a seafoam decarb once every season. And good job keeping the ethanol out of that system. I REFUSE to put E-10 into all of my hard work. Plus I have a pretty large under deck fuel tank. 64 gallons. Don't want issues under there!!!

 

[Guest]

I ran a 1992 40, then a 1999 50 Johnson (same engine platform as yours) on a 13 Whaler and used nothing but high test, they both ran flawless and I never had any issues,

Sutor's right, just Sea Foam it every season and run the gas out of the carbs before you long term store.

 

[mkmnc]

Thanks for the info. I will continue to run the high test non ethanol even though I would probably run regular non ethanol if it was available in my area due to the expense of the fuel.

 

[gm280]

One of the main differences between high test (octane) and low is the high octane actually is harder to ignite. And therefore it won't ping as easily because of that issue. Lower octane fuels will actually ignite as the compression gets high enough. And that means the fuel ignites before the piston is at the proper position to utilize the flame front from the pre-ignition explosion. The ping is actually the fuel exploding before the piston is at or over TDC. So the explosion is actually trying to force the piston back down in the cylinder before it is suppose to. And that robs power and really is hard on the rod and crank bearings not to mention the piston pin. So using a high octane fuel really doesn't do anything bad, but chews up a little more money. JMHO!

 

[ondarvr]

Using high octane fuel does nothing to help any aspect of running a 48spl, it was designed to run on regular, and E-10 is fine for it. While you will probably never notice the down side, high octane fuel doesn't quite work as well as regular in a low compression motor like a typical outboard. It does not burn cleaner, add HP, or have any other benefit, real or perceived. It's why in the manual it recommends 87, not 90 or 91 as a fuel.

 

[Tim Frank]

+1 ^^^
Waste of $$$.

 

[bruceb58]

Use regular even if it has ethanol. Won't matter.

Skip the SeaFoam too. Another waste of money.

 

[oldboat1]

One of the main differences between high test (octane) and low is the high octane actually is harder to ignite. And therefore it won't ping as easily because of that issue. Lower octane fuels will actually ignite as the compression gets high enough. And that means the fuel ignites before the piston is at the proper position to utilize the flame front from the pre-ignition explosion. The ping is actually the fuel exploding before the piston is at or over TDC. So the explosion is actually trying to force the piston back down in the cylinder before it is suppose to. And that robs power and really is hard on the rod and crank bearings not to mention the piston pin. So using a high octane fuel really doesn't do anything bad, but chews up a little more money. JMHO!

Thanks, gm280. I really didn't know that -- knew the effects (i.e., higher octane can stop pinging), but assumed that the higher octane ignited easier and burned hotter. Guess that's the usual assumption.

I used to be very selective about grades of gas and sources, along with brands of marine oil. I'm still concerned about stale gas (buy in limited quantities), but less so about the rest. The OP has a newer motor and E-10 isn't an issue, as mentioned earlier, but he also burns through about 200 gallons a season with a 40. So stale gas probably isn't an issue either (unless coming from big permanent tanks, I suppose).

 

[bruceb58]

but assumed that the higher octane ignited easier and burned hotter. Guess that's the usual assumption.

Yes, it's the opposite and another reason you want to run the engine with the octane rating the engine is designed for. Actually likely to get more deposits from high octane fuel along with worse idle.

 

[oldboat1]

Most of my motors were made when gas and lead was cheap and plentiful. Accurate to say high octane does not generate more heat? Actually, maybe less heat? Think that would be important to my choices.

 

[bruceb58]

Accurate to say high octane does not generate more heat? Actually, maybe less heat? Think that would be important to my choices.

Energy content is exactly the same

 

[kmarine]

The only ethanol free gas we can get for our harbor is 91 octane. Runs better in older engines with carbs. have less issues with bad fuel compared to street gas when used in larger boats that dont leave the harbor regularly.

 

[Tim Frank]

Energy content is exactly the same

At least close enough for Government work.
But is a slightly slower burning fuel and so the engines run a bit cooler.So as Bruce mentioned, you may get more deposits in a water-cooled engine.

I run the highest octane i can get in my A/C engines. Especially my old Jonsered 535.

 

[oldboat1]

Thanks. That's a direction I was sort of expecting -- possibly a little cooler running. I use non-ethanol 91 octane in my 1980 Trojan, and seems to work well. I think I run it enough (probably just enough) to keep it running cleanly. As kmarine describes, it's the boater fuel around here too.