91 Octane Ethanol Free Fuel

[Rangerdic]

I did look in the search field, there is a chance I overlooked the answer, but I'll ask the question anyways.

Can I run 91 octane ethanol free, and of course, I use the Mercury fuel additives.

1B822149


As always, thanks in advance

[SilverFox579]

Oil, Battery, Spark Plugs, Fuel and Fuel Additive Recommendations

[Macsimus]

Oil, Battery, Spark Plugs, Fuel and Fuel Additive Recommendations

Having carefully read this, would 91 octane be considered "substantially higher octane"?

[hooken203]

Man you don’t even know the size and number of worms in the can you just opened. Read the sticky posted above. Go to the Mercury web site and read there. Contact customer service with your question. There are as many opinions on this as there are fish in the sea. Would I run 91 octane fuel in my engine over a choice of 87 with ethanol yes I would.

[EuropeanAM]

Actually, I'd recommend following the post referenced above (from the stickies).

That is the official answer from Mercury Marine (not an individual's opinion).

For this engine, that would be a High-Traffic, Top-Tier Fuel, 87 Octane, No more than 10% ethanol content. Run the Cocktail (Quickare + Quickleen) in every tank.

[Rangerdic]

Having carefully read this, would 91 octane be considered "substantially higher octane"?

Macsimus... Well stated question, that's how I should posted the question. Since the question wasn't answered, I called Mercury today, they said using the 91 ethanol free, is a good choice.

[EuropeanAM]

You "can" run that fuel. However, you do not need to (and in fact, your engine will perform best on the fuel it was calibrated for).

After some recent "hoopla" brought around on the forums, one of my customers also second-guessed what he's been doing for over a decade (and what I've told him every year), because he "read something different on the internet" (actually it was on BBC).

After emailing Mercury Marine, Mercury indicated to him that he should follow the recommendations he's been receiving all along, SPECIFICALLY (and in the following order):

-Use a Brand-Name, Top-Tier, HIGH TRAFFIC Station
-Correct Octane as specified for your engine (in this case, 87)
-Lowest possible ethanol percentage (your engine can handle up to 10% ethanol content)
-Quickare and Quickleen in EVERY tank of fuel.

He promptly emailed me to apologize... and included a copy of the email from Mercury.

[Rangerdic]

EuropeanAM, Thank you for your time and your knowledge.

[Jim Dunn]

I have a question. I recently purchased a 6cyl Chevrolet. Didn't ask, but, upon opening the gas cover it said 91 octane. I was a little upset as I just sold a supercharged Bonneville that required premium fuel and was hoping for a little cheaper gas fill up. That said I called and was told to use 87,89 or 91 as the computer would recognize the octane.

Now, having said all that, why wouldn't an out board engine, that is almost as expensive as a car do the same? My initial guess is it's a 2cyl?

Just wondering and waiting on my new boat.

[EuropeanAM]

First- lets establish we are discussing the 250 ProXS in the OP above.

All of the Mercury Optimax Gas Engine models are Speed-Density based engines. The PCM does not have the ability to ascertain differences in fuel, Fuel burn rate, etc.

Your Chevrolet is Mass-Airflow based, and through a series of sensors (knock, Upstream & Downstream O2 sensors, etc) can and does adapt to a wider variety of fuels and conditions.

4-stroke models are closer to your Chev... especially the catalyst Mercruiser models.

[OldTimer57]

Car engines have an anti-knock sensor. They can tweak the spark advance based on the fuel being used. You should check out the new dodge challenger demon. It will use anything up through racing fuel, and make more horsepower as the octane ramps up. Or, in your case, the motor will make less horsepower/torque as you go down in octane, because ignition timing gets retarded. Don't know if Mercury does this or not. Most 4 strokes certainly would/should.

[Gamblinman]

Outboard computers can't "learn" like automotive computers can. They are a closed loop system, and all parameters are fixed.

[Jim Dunn]

First- lets establish we are discussing the 250 ProXS in the OP above.

All of the Mercury Optimax Gas Engine models are Speed-Density based engines. The PCM does not have the ability to ascertain differences in fuel, Fuel burn rate, etc.

Your Chevrolet is Mass-Airflow based, and through a series of sensors (knock, Upstream & Downstream O2 sensors, etc) can and does adapt to a wider variety of fuels and conditions.

4-stroke models are closer to your Chev... especially the catalyst Mercruiser models.

I knew it was more than oil

[KCLOST]

Don, I think this question on 91 octane (ethanol free) is asked because, around where I'm located you can only get ethanol free gas in that octane. Ethanol is bad, we all agree... Yes our engines will tolerate it up to 10%, but why put it in there if you have an ethanol free option. Again, around here that is 91 octane... To me, it is a better fuel than any octane with ethanol... And I use the cocktail anyway.... lol..

I also have read your recommendations on fuel and oil, and I read the part about how fuels with substantially higher octane can adversely affect engine performance. Especially the part that says that higher octane fuels burn cooler and slower, and can "potentially" increase carbon buildup...

I would be interested hearing more about this in particular. How much cooler or slower are we talking about? And how much more carbon buildup can possibly occur under normal operating conditions, even when the "cocktail" is being added to the fuel? Is 10% ethanol added in 87 octane fuel, better than 91 octane ethanol free? It sounds like a "Damned if you do, damned if you don't" kind of deal...

Your thoughts are appreciated...

[Macsimus]

KCLOST, you hit the nail on the head. Those are precisely my questions as well, since up here in MN, non-oxy is unfortunately available only as 91 octane.

[bassm1n]

My opinion is changing based on Don's comments. What I am reading and hearing is that using the 87 ethanol with a cocktail will be better for your engine and performance will be better as well than using ethanol free in 91. In the past I always used ethanol free since my boat sits a few weeks between use and for 3-4 months over the winter. I m still concerned with leaving 20-30 gals of ethanol gas sitting in the boat for that long. Wish there was an easy answer but I am leaning to Don's/Mercury's recommendation notwitstanding my cocerns.

[KCLOST]

Maybe so, but is there really a big enough difference between 87 and 91 to rob performance? I understand that higher octane fuels are better in higher compression engines, normally equating to a more powerful engine. So for our stock 3L motors we don't need it.. But we are not talking about using 95 or 98 octane fuel... Just 91 without any ethanol crap in it... Ethanol corrodes, all fuels regardless of octane de-stabilize... I prefer to avoid ethanol when possible (and still use the cocktail)...

Is it really robbing performance.

[Jtreed99]

Without getting into things that I don’t completely understand like motor grade vs research grade octane ratings, if we assume energy content per unit gasoline is identical regardless of octane (it isn’t) a slower more controlled burn would generate a lower maximum temperature. I don’t know the full ramifications of “too high” of an octane in a 2 stroke but I have heard numerous old timers discuss running high octane fuels in old big bore motors with relatively low compression ratios and relatively retarded timing values compared to modern 4 cycle motors. They claimed to see burning and severe carbon deposition on exhaust valves which they inferred was because of the higher octane gas burning too late into the cycle. I have no idea how this correlates to a 2 stroke and I have completely exhausted my knowledge of the subject at this point.

[leo13]

This is a difficult subject for the average person who does not have training in organic chemistry and IC engine design. The composition of the different octane rated pump gasolines is the same. Same amount of hydrogen, carbon and now oxygen per volume. The density and btu/lb are the same within the government's specifications and tolerances. The octane rating is comparing the fuels ability to resist knock relative to the compound octane. Motor is a physical test with a special single cylinder calibrated engine and research is analysis of the fuels composition. R+M/2 is the average of both. Motor is more accurate and research is usually more optimistic.

Think of it as a box of Cheerio's one is smashed into little pieces and the other is whole rings. Both would offer an individual the same amount calories (energy). One might dissolve with less energy then the other hence have a lower stability. There should be no difference in flame speed and btu/lb.

Larger the bore the longer the distance the flame has to propagate from the source of ignition to the outer limits of the combustion area. The larger bore is more prone to detonation because the end gases have more time to prematurely ignite before the spark ignited flame has eliminated the charge.

Without getting into things that I don’t completely understand like motor grade vs research grade octane ratings, if we assume energy content per unit gasoline is identical regardless of octane (it isn’t) a slower more controlled burn would generate a lower maximum temperature. I don’t know the full ramifications of “too high” of an octane in a 2 stroke but I have heard numerous old timers discuss running high octane fuels in old big bore motors with relatively low compression ratios and relatively retarded timing values compared to modern 4 cycle motors. They claimed to see burning and severe carbon deposition on exhaust valves which they inferred was because of the higher octane gas burning too late into the cycle. I have no idea how this correlates to a 2 stroke and I have completely exhausted my knowledge of the subject at this point.

[EuropeanAM]

A relatively simple test is to run two tanks of each fuel, making sure that you limit other variables (such as idle time).

Inspect and document the inside of the prop barrel at the end of each tank.

Inspecting to end of the Direct Injector (Pintle) and checking to see how difficult it is to wipe clean would be another simple method.

Combustion temp testing is a bit difficult unless you have a good budget... EGT is more cost-friendly.